The 16th International Workshop on Tau Lepton Physics (TAU2021) (Virtual Edition)

27 Sept 2021, 07:55 → 1 Oct 2021, 16:50 America/Indiana/Indianapolis

Virtual (Indiana University)

                  30 years of Tau Lepton International Workshops 

The 16th International Workshop on Tau Lepton Physics (TAU2021) will be held online

 

TAU2021 is the 16th International Workshop on Tau Lepton Physics in the series devoted to the physics of the tau lepton and its associated neutrino. The goal of the workshop is to bring theorists and experimentalists together to discuss recent progress in the field. Latest results from e+e- colliders (Belle II, BESIII experiments), the proton-proton colliders (ATLAS, CMS and LHCb experiments), g-2 and the neutrino experiments (DUNE, MINOS, etc.) will be presented. Properties of the tau lepton, as well as methods of using tau leptons as a probe for new physics, will be discussed, along with a review of future experimental projects. 

The TAU2021 Workshop is dedicated to Simon Eidelman and Olga Igonkina.

Topics:

• Hadronic decays
• CP and T violation
• CKM matrix and Flavour Mixing
• Rare decays
• Precision SM tests
• CPT and Quantum Mechanics
• Lepton universality and flavour violation
• Anomalous Magnetic Moment of the muon
• Neutrino Physics
• Chiral perturbation theory
• Physics beyond the Standard Model
• Future opportunities in Tau Physics

Tau2021_poster.pdf

Monday, 27 September

07:55 → 08:00 Welcome to the 16th International Workshop on Tau Lepton Physics (TAU2021)

Convener: Emilie Passemar

Welcome_TAU2021.pdf

08:00 → 08:10 Welcome and introductory remarks from Prof. Rick van Kooten, Dean, Indiana University College of Arts and Sciences

08:00 Welcome speech

Speaker: Prof. Rick Van Kooten (Indiana University)

08:10 → 10:20 Session 1: Properties of the tau lepton

Convener: Toru Iijima (Nagoya University)

08:10 Historical overview of 30 years of tau lepton physics work

Speaker: Antonio Pich (IFIC, U. Valencia -.)

30YearsTauPhysics_Pich.pdf

VideoTau2021_Pich.mp4

08:40 Electron-Muon-Tau Universality Theory Overview

Speaker: Dr Bill Marciano (Brookhaven National Lab)

Tau2021 Presentation.pdf

VideoTau2021_Marciano.mp4

09:05 Measurement of τ lepton mass at BESIII

τ lepton is a fundamental particle in the standard model, and its mass is a very sensitive quantity in testing the lepton-universality.
BESIII gave the most precise τ mass value in a single measurement in 2014, but the accuracy is still much lower than those of electron and muon.
In order to improve the accuracy of τ mass measurement, more than 130 pb-1 τ mass scan data were collected in April 2018, and the uncertainty of mτis expected to be reduced as the level of 100 keV.

Speaker: Tao Luo (Fudan University)

TaumassmeasurementatBESIII_v02.pdf

VideoTau2021_Luo.mp4

09:30 Status and prospects for tau property measurements at Belle II

The Belle II experiment is a substantial upgrade of the Belle detector, operating at the SuperKEKB energy-asymmetric e+e− collider. The design luminosity of the machine is 8 × 10^35 cm−2s−1, and the Belle II experiment aims to record 50 ab−1 of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run and the main operation of SuperKEKB has started in March 2019. Belle II has a broad tau physics program, from high-precision measurements of SM parameters to searches of new physics via experimental observation of BSM processes, such as lepton flavor universality (LFU) violation or lepton flavor violation (LFV) decays. In this talk we review the status of the Belle II experiment, and the prospects for the measurement of the tau lepton mass and lifetime, fundamental inputs in tests of LFU violation.

Speaker: Dr Michel Hernandez Villanueva (DESY)

TAU2021_tauProperties_atBelleII.pdf

VideoTau2021_HernandezVillanueva.mp4

09:55 Third order correction to the muon lifetime

In this talk I will present the calculation of the QED third order correction to the muon lifetime. This result is obtained in the so-called heavy daughter approximation, i.e. the decay rate is computed in the limit of equal muon and electron masses which yields crucial simplifications in the evaluation of multi-loop Feynman diagrams. Despite the electron being about 207 times lighter that the muon, our result allows to determine the such third order correction at physical values of the muon and electron masses, with about a 15% uncertainty. Finally I will discuss the impact in future measurements of the Fermi constant.3s

Speaker: Dr Matteo Fael (KIT Karlsruhe)

TAU2021.pdf

VideoTau2021_Fael.mp4

10:20 → 10:35 Break

10:35 → 11:45 Session 2a: Test of fundamental symmetries with tau lepton

Convener: Swagato Banerjee (University of Louisville (US))

10:35 Breaking of Standard Model symmetries with the tau lepton

I will provide a short overview on deviations of the Standard Model symmetries within tau lepton processes. I will consider from dynamic dependent symmetries such as universality to the breaking of global symmetries, namely lepton flavor, lepton number or baryon number violations.

Speaker: Jorge Portoles (Instituto de Fisica Corpuscular)

BSM_symmetry_TAU2021_J_Portoles.pdf

VideoTau2021_Portoles.mp4

11:00 Lepton Flavor Violation Searches at ATLAS and CMS

Speaker: Luca Fiorini (Univ. of Valencia and CSIC (ES))

fiorini-lfv-27_09_2021_v2.pdf

VideoTau2021_Fiorini.mp4

11:25 Tests of Relativity in the Tau Sector

In the last couple of decades, there has been a tremendous growth of interest in testing fundamental symmetries, such as CPT and the Lorentz invariance of special relativity. Using effective field theory, it has been possible to parameterize a much broader range of symmetry violations that had previously been envisioned or tested. The new possibilities include different patterns of symmetry breaking in different sectors of the theory. However, for short-lived elementary particles like the tau, precision tests of Lorentz symmetry are very challenging, and the best constraints on Lorentz and CPT violations in the tau sector actually come from the observation of extremely high energy cosmic-ray photons and hadrons.

Speaker: Dr Brett Altschul (University of South Carolina)

Altschul_TAU21.pdf

Altschul_TAU21.pdf

Altschul_TAU21.pptx

11:45 → 12:20 Lunch

12:20 → 13:25 Session 2a: Test of fundamental symmetries with tau lepton

Convener: Bertrand Echenard (California Institute of Technology)

12:20 Charged Lepton Flavor violation at the EIC

In the quest to map out the structure of the proton in exquisite detail, the Electron Ion Collider (EIC) will collide electrons and protons at high energy with unprecedented luminosity.
In this talk I will discuss how to exploit the features of the EIC to explore physics beyond the Standard Model. I will focus on charged-lepton-flavor-violating (CLFV) interactions in which an electron is converted into a tau lepton, which, in several BSM models, are intimately connected with the generation of neutrino masses. I will discuss the luminosity and efficiency requirements for competitive CLFV searches at the EIC. I will then compare the EIC sensitivity with existing bounds from LEP and the LHC, and discuss the complementarity of the EIC with the next generation of B factories.
I will identify the most promising directions in parameter space for the EIC to explore, and discuss the theoretical improvements needed to take full advantage of the EIC potential.

Speaker: Dr Emanuele Mereghetti (Los Alamos National Lab)

Mereghetti_LFV_TAU.pdf

VideoTau2021_Mereghetti.mp4

12:45 Lepton-flavour violation in hadronic tau decays and $\mu$--$\tau$ conversion in nuclei

Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and $\ell$-$\tau$ conversion in nuclei, with $\ell = e,\mu$. Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. The translation of these constraints into the most general leptoquark framework is also considered. Our work paves the way to extract the related information from Belle II and foreseen future experiments.

Speaker: Kevin Monsalvez Pozo (Instituto de Fisica Corpuscular (IFIC))

ModelindependentanalysisCLFV.pdf

VideoTau2021_MonsalvezPozo.mp4

13:05 Charged-lepton-flavor violation from Lorentz violation

Lorentz invariance is among the most fundamental and tested symmetries in physics. Measurements from over two decades of experiments place stringent constraints on many Lorentz-violating interactions affecting the particles of the Standard Model. However, a large class of interactions inducing charged-lepton-flavor violation remains largely unexamined. In this talk, we discuss dominant Lorentz- and CPT-violating operators initiating flavor-changing tau and muon decays. Branching-ratio measurements from the MEG and BaBar collaborations allow several first constraints to be placed on flavor off-diagonal tau, muon, and electron coefficients for Lorentz violation. The outlook for improved constraints in future experiments is also discussed.

Speaker: Dr Nathaniel Lawrence Sherrill (University of Sussex)

CLFV_LV_Tau2021.pdf

13:25 → 13:40 Break

13:40 → 15:00 Session 2a: Test of fundamental symmetries with tau lepton

Convener: Alexey Petrov (Wayne State University)

13:40 𝜏→𝜇𝜇𝜇 at a rate of one out of 10^14 tau decays?

We present in full analytic form the partial widths for the lepton flavor violating decays $L^\pm \to \ell^\pm \ell'^{+} \ell'^{-}$, with $L = \tau, \mu$ and $\ell^{(\prime)} = \mu, e$, mediated by neutrino oscillations in the one-loop diagrams. Compared to the first result by Petcov:1976ff, which was obtained in the nonphysical zero momentum limit $\mathcal{P} \ll m_{\nu} \ll M_W$, we retain full dependence on external scales $\mathcal{P}$ and determine the branching ratios in the physical limit $m_\nu \ll \mathcal{P} \ll M_W$. We show that in this limit the conclusion by Pham:1998fq that $\tau \to \ell \ell' \ell'$ could be as large as $10^{-14}$ is flawed. In this talk we will describe the details of our calculation, present our results, and motivate some of the peculiarities of this calculation from the viewpoint of effective field theory.

Speaker: Patrick Blackstone (Indiana University)

Tau 2021 Blackstone Presentation.pdf

VideoTau2021_Blackstone.mp4

14:00 Search for lepton flavor violation in Y(3S)→eμ at BABAR

We report on the first search for electron-muon flavor violation in the decay of a b quark and anti-b quark bound state. A search for the LFV decay Υ(3S) → e± μ∓ in a sample of 118 million Υ(3S) mesons from 27 fb−1 of data collected with the BABAR detector at the SLAC PEP-II e+e− collider operating with a 10.36 GeV center-of-mass energy revealed no signal. We set a limit on the branching fraction B(Υ(3S) → e± μ∓) < 3.6×10^−7 at 90% CL. This can be interpreted as a limit on the energy scale divided by coupling of relevant new physics (NP) processes of Λ_NP/g_NP^2 > 80 TeV.

Speaker: Nafisa Tasneem (University of Victoria)

ntasneem_Tau_2021.pdf

VideoTau2021_Tasneem.mp4

14:20 Searches for new physics with leptons using the ATLAS detector

Many theories beyond the Standard Model predict new phenomena, such as Z’, W’ bosons, or heavy leptons, in final states with isolated, high-pt leptons (e/mu/tau). Searches for new physics with such signatures, produced either resonantly or non-resonantly, are performed using the ATLAS experiment at the LHC. This includes a novel search that exploits the lepton-charge asymmetry in events with an electron and muon pair. Lepton flavor violation (LVF) is a striking signature of potential beyond the Standard Model physics. The search for LFV with the ATLAS detector focuses on the decay of the Z boson into different flavour leptons (e/mu/tau). The recent 13 TeV pp results will be reported.

Speaker: Matteo Franchini (University of Bologna and INFN (IT))

27-09-21 - Tau 2021 - Search new physics with leptons ATLAS - Franchini.pdf

VideoTau2021_Franchini.mp4

14:40 Precision tau physics: Challenge for Theory, on & off the lattice

In the past half a dozen years or so, the tau lepton has become the central focus for many reasons. Foremost in this is the fact that there are strong experimental hints that the tau is intimately involved in strong indications of lepton universality violations (LUV). Moreover, it is no longer just one type of experiments but rather three different types are involved and in each case the deviations from the SM is over 3 sigma. Chances of survival of one of these therefore is rather high. Possible theoretical scenarios that may be relevant are extremely intriguing. Then there is BELLE-II data set on the horizon which should be able to move tau precision to an unprecedented level. And of course LHCb with more upcoming data due to RUN-3 and beyond will also be very potent. This should open many avenues for searching new phenomena esp BSM-CP violation. Furthermore, its mass around 1.8 GeV renders it readily amenable to precision studies on the lattice and can prove very useful for more precise tests of the SM and for search of new phenomena.

Speaker: Amarjit Soni (Brookhaven National Lab)

Soni_Precision tau physics- final for tau2021.pdf

VideoTau2021_Soni.mp4

Tuesday, 28 September

08:00 → 09:35 Session 2b: Test of fundamental symmetries with tau lepton

Convener: Sacha Davidson (Centre National de la Recherche Scientifique (FR))

08:00 Search for charged lepton flavor violation at BESIII

The charged Lepton Flavor Violation (cLFV) is highly suppressed in the Standard Model (SM) by the finite but tiny neutrino masses. Its branching fraction is calculated to be at a negligible level and so far none has been found in all the historical experiments, including searches in lepton ($\mu$,$\tau$) decays, pseudoscalar meson (K,$\pi$) decays, vector meson ($\phi$,$J/\psi$,$\Upsilon$) decays, Higgs decays etc. This talk reviews the charged Lepton Flavor Violation process searches at BESIII experiement. Besides the result for the decay of $J/\psi\to e\mu$ published earlier, the decay of Jpsi-> e tau, with
tau->pi- pi0 nv_tau is searched with the 10 Billion $J/\psi$ events
collected by BESIII and the result improves the previously published limit by two orders of magnitude. Future perspectives will also be discussed.

Speaker: Dayong Wang (Peking University)

VideoTau2021_Wang.mp4

Wang_20210928_Tau2021_BES3CLFV.pdf

08:25 Search for tau LFV/LNV decays at Belle

We report the result of a search for \tau \to \ell \gamma (\ell = e, \mu) using the full data sample at Belle. Charged lepton flavor violation (CLFV) is forbidden in the Standard Model but possible in several new physics scenarios.
In many of these models, the radiative decays $\tau \to \ell\gamma$ are predicted to have a sizeable probability and are thus particularly interesting CLFV channels. Consequently, we have obtained the most stringent limit on the branching fraction of \tau \to \mu \gamma. In addition, we report the result of a search for \tau \to p \ell \ell at Belle. Any observation of processes involving Lepton number and baryon number violation would be a clear signature of new physics. Consequently, we set the most stringent limits on the branching fraction of $\tau \to p \ell \ell$ .

Speaker: Kenta Uno

Tau2021_Kuno_28Sep21.pdf

VideoTau2021_Uno.mp4

08:50 Measurement of the tau EDM at Belle

We have measured the electric dipole moment (EDM) of the tau lepton using an 833 fb^-1 data sample collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. Using an optimal observable method, we obtain the EDM result with O(10^-17) ecm uncertainty. The result is consistent with no EDM at the present level of experimental sensitivity and improves the sensitivity by about a factor of three compared with the previous result.

Speaker: Kenji Inami (Nagoya university)

Inami_202109TauEDMv3.pdf

VideoTau2021_Inami.mp4

09:15 Probing charged lepton flavor violation with axion-like particles at Belle II

We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), $X$, at the $B$-factory experiment Belle II.
We focus on production of the ALP in the tau decays $\tau \to X l$ with $l=e,\mu$, followed by its decay via $X\to l^- l^+$.
The ALP can be either promptly decaying or long-lived.
We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating $\tau$ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle~II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime.
The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.

Speaker: Yu-Heng Wu (National Tsing Hua University)

Probing Charged Lepton Flavor Violation with Axion-like Particles at Belle II.pdf

VideoTau2021_Wu.mp4

09:35 → 09:50 Break

09:50 → 10:55 Session 2b: Test of fundamental symmetries with tau lepton

Convener: B. Lee Roberts (Boston University)

09:50 A next-generation rare pion decay experiment to study LFUV and CKM unitarity

I will briefly review the interesting collection of flavor anomalies that continue to accumulate from various experiment campaigns. What are they trying to tell us and, might there be connections? To add evidence to the experimental findings, a group of us is forming to develop a next-generation rare pion decay experiment. We aim to improve on the lepton flavor universality violation (LFUV) test in the electron – muon sector with a 10-fold or higher precision measurement of the ratio 𝜋+→𝑒+𝜈𝑒𝛾/𝜋+→𝜇+𝜈𝜇𝛾; the Standard Model theory for this process is already known extremely well. We further aim to improve the measurement of pion beta decay 𝜋+→𝑒+𝜈𝑒𝜋0 by a factor of 10. Even with a 3-fold improvement, the ratio of this process to the related decay 𝐾→𝜋𝑒𝜈 will already shed light on the question of 1st-row CKM unitarity. A 10-times better measurement will produce the cleanest stand-alone determination of Vud. I will describe our experimental concept, which is based on state-of-the-art detector and electronics concepts, and lessons learned from previous rare pion decay experiments.

Speaker: David Hertzog

Hertzog-TauLepton-2021.pdf

Hertzog-TauLepton-2021.pptx

VideoTau2021_Hertzog.mp4

10:15 Search for Muon to Electron Conversion at J-PARC - COMET Experiment

COMET is an experiment at J-PARC, Japan, which will search for neutrinoless conversion of muons into electrons in the field of a nucleus (μ− + N → e− + N); a lepton flavor violating process. The experimental sensitivity goal for this process is order of 10^{−15} for Phase-I and 10^{−17} for Phase-II experiment, which is a factor of 100–10,000 improvements correspondingly over existing limits. Recent progresses in facility and detector development are presented, along with COMET Phase-I and Phase-II experimental schedule. The physics and feasibility of searching lepton number violation in the COMET experiment will be also discussed.

Speaker: MyeongJae Lee (Institute for Basic Science (Korea))

TAU2021-MJLee-COMET.pdf

VideoTau2021_Lee.mp4

10:35 Status of the Mu2e experiment

The Mu2e experiment will measure the charged lepton flavor violating (CLFV), neutrino-less conversion of a negative muon into an electron in the field of a nucleus. The process has never been observed, but is predicted to occur in many Beyond the Standard Model scenarios at rates within the reach of the Mu2e experiment. The goal of the experiment is to improve the previous upper limit by four orders of magnitude and reach a single event sensitivity of $3\times10^{−17}$. Mu2e will operate in a dedicated hall at the Muon Campus of Fermilab; installation has already begun, and detector and beamline commissioning are expected to begin in 2023. A number of years of running will be required to reach our target sensitivity. Research and development tasks have already begun for a proposed extension called Mu2e-II.

Speaker: Nam Tran (Boston University)

2021-09-28 - mu2e - Status of the Mu2e experiment (Tau2021).pdf

VideoTau2021_Tran.mp4

10:55 → 11:10 Break

11:10 → 12:20 Session 2b: Test of fundamental symmetries with tau lepton

Convener: Enrico Lunghi (Indiana University)

11:10 Muonic Force Behind Flavor Anomalies

An economical theoretical framework for combined explanations of the flavor physics anomalies involving muons, (g-2)_mu, RK(*) and b->s mu mu supplements the Standard Model (SM) with a lepton-flavored U(1)_X gauge group where the auge boson has mass of O(0.1) GeV and a TeV-scale leptoquark. We explore the theory space of the chiral, anomaly-free U(1)_X gauge extensions and carry out a comprehensive phenomenological study of the muonic force in representative benchmark models

Speaker: Jure Zupan (University of Cincinnati)

VideoTau2021_Zupan.mp4

Zupan_Tau_2021.pdf

11:35 Lepton Flavour Violation searches and Lepton Universality tests at LHCb

Speaker: Sara Celani (EPFL - Ecole Polytechnique Federale Lausanne (CH))

TAU2021.pdf

VideoTau2021_Celani.mp4

12:00 Searches for violation of Lepton Flavor Universality at Belle II

Searches for the violation of lepton flavor universality (LFU) are critical precision tests of the standard model (SM) motivated by the growing number of anomalies reported in several measurements in the flavor sector (quarks and leptons) in the last decades. At the Belle II experiment, thanks to the large amount of tau-lepton pairs produced in electron-positron annihilation, it is possible to perform a full set of LFU tests with unprecedented precision using tau-lepton decays. Such an approach allows not only to test the SM to high accuracy, but it provides a unique and complementary way to understand and eventually establish or rule out the new physics nature of the so-called flavor anomalies. We will discuss the status of the ongoing precision tests of LFU in both hadronic and leptonic tau decays and, using both 3x1 and 1x1 decay topologies, we will present the expected sensitivities to LFU parameters.

Speaker: Alberto Martini (DESY)

Martini_16th International Workshop_on_Tau_Lepton_Physics.pdf

VideoTau2021_Martini.mp4

12:20 → 13:00 Lunch

13:00 → 14:00 Session 2c: Test of fundamental symmetries with tau lepton

Convener: Radovan Dermisek

13:00 A data-directed search for electron-muon asymmetry in events containing taus

Lepton flavor is not an exact symmetry of nature as indicated by neutrino oscillations. Moreover, the hints for discrepancy between the measured data and the theoretical prediction of numerous observables related to the ratio between events containing electrons to those containing muons, e.g. R_D(*) and R_K, points towards new physics exhibiting lepton non-universality. These examples also demonstrate the role that e/mu asymmetry could play in search for new physics. In this talk, I will present a novel, data-directed paradigm (DDP) to search for new physics. The DDP is complimentary to traditional theory-directed searches that follow the blind-analysis paradigm and could open the door to regions in the data that will otherwise remain unexplored. While the paradigm is generic, I will show that it can be effectively exploited in search for e/mu asymmetries and in particular in events containing in addition to the light lepton also tau leptons.

Speaker: Shikma Bressler (Weizmann Institute of Science (IL))

2021_09_28_ShikmaBressler_Tau21.pdf

VideoTau2021_Bressler.mp4

13:20 Searches for leptoquarks with the ATLAS detector

Leptoquarks (LQ) are predicted by many new physics theories to describe the similarities between the lepton and quark sectors of the Standard Model and offer an attractive potential explanation for the lepton flavour anomalies observed at LHCb and flavour factories. The ATLAS experiment has a broad program of direct searches for leptoquarks, coupling to the first-, second- or third-generation particles. This talk will present the most recent 13 TeV results on the searches for leptoquarks and contact interactions with the ATLAS detector, covering flavour-diagonal and cross-generational final states.

Speaker: Zhiyuan Jordan Li (University of Liverpool)

Li_LeptoQuark Tau2021.pdf

VideoTau2021_Li.mp4

13:40 Precision measurement of the B[Υ(3S) → τ+τ−) / B(Υ(3S) → μ+μ−] ratio at BABAR

We report on a precision measurement of the ratio R^Υ(3S)_τμ = B[Υ(3S) → τ+τ−] / B[Υ(3S) → μ+μ−] using data collected with the BaBar detector at the SLAC PEP-II e+e− collider. The measurement is based on a 28 fb−1 data sample collected at a center-of-mass energy of 10.355 GeV corresponding to a sample of 122 million Υ(3S) mesons. The ratio is measured to be R^Υ(3S)_τμ = 0.966 ± 0.008 (stat) ± 0.014 (syst) and is in agreement with the Standard Model prediction of 0.9948 within 2 standard deviations. The uncertainty in R^Υ(3S)_τμ is almost an order of magnitude smaller than the only previous measurement.

Speaker: Alexei Sibidanov (University of Victoria)

Sibidanov_TAU20210928.pdf

VideoTau2021_Sibidanov.mp4

14:00 → 14:15 Break

14:15 → 15:20 Session 2c: Test of fundamental symmetries with tau lepton

Convener: Matheus Hostert (University of Minnesota / Perimeter Institute)

14:15 Lepton flavor violating tau decays with a light gauge boson

$L\to\ell\chi$ decays (with $\chi$ a boson associated to this lepton flavor violation, LFV) have not been described satisfactorily so far for light spin-one $m_\chi$. In particular, observables exhibited an unphysical divergence in the limit of massless $\chi$, associated to its longitudinal polarizations. Based on gauge symmetry, we show how to correct this issue. To this end, we consider two general models realizing the effective field theory description. Being the LFV generated either at tree level or at one loop, these processes are well behaved for light $m_\chi$. We discuss the most salient phenomenological consequences and its relevance in the searches for this kind of decays.

Speaker: Marcela Marín (Cinvestav)

Marcecla-Marin-Tau2021.pdf

VideoTau2021_Marin.mp4

14:35 First results and prospects for tau LFV decay tau -> e + alpha(invisible) at Belle II

The Belle II experiment at SuperKEKB, an asymmetric e+e- collider, aims at a total integrated luminosity of 50 ab-1, to pursue a rich program of Standard Model and Beyond the Standard Model physics. In its first year of operation, approximately 10 fb-1 were collected at the Upsilon(4S) resonance, with about 100 fb-1 expected by the end of 2020. This results in a sizeable sample of tau pairs, enabling detailed studies of Standard and Beyond the Standard Model measurements, including searches for Lepton Flavor Violating (LFV) decays. One of the first channels where competitive limits are expected is the tau -> e + alpha(invisible) process, where alpha is a Goldstone boson. Here, the currently best limit has been obtained by ARGUS with an integrated luminosity of 475 pb-1. Belle II is expected to be able to improve on this result already with the data recorded. This contribution will discuss selected analysis details and present first preliminary results and the prospects for future larger datasets.

Speaker: Alejandro De Yta Hernandez (Cinvestav)

AHernandez_tau2021_adh.pdf

VideoTau2021_DeYtaHernandez.mp4

14:55 Search for dark sector at BABAR

Many models of dark matter and hidden sectors predict new particles with masses below the electroweak scale. Low-energy electron-positron colliders such as BABAR are ideally suited to discover these hidden-sector particles. We present two recent BABAR searches for low-mass hidden-sector particles, including new searches for prompt and long-lived leptonically decaying hidden scalars produced in association with tau leptons. This search is sensitive to viable models that could account for the muon 𝑔−2 excess. We also present results of a search for dark matter bound states (darkonium). These examples show the importance of 𝐵-factories in constraining and discovering new hidden-sector physics beyond the Standard Model.

Speaker: Yunxuan Li (California Institute of Technology)

TAU2021_YunxuanLi.pdf

VideoTau2021_YLi.mp4

Wednesday, 29 September

08:00 → 10:05 Session 3: Exclusive and inclusive hadronic tau decays

Convener: Sergi Gonzalez-Solis (Indiana University)

08:00 Exclusive hadronic tau decays within and beyond the Standard Model

After a brief review of semileptonic tau decays in the Standard Model, I will discuss the usefulness of these channels in several new physics searches.

Speaker: Pablo Roig Garcés (Cinvestav, Mexico)

TAU2021.pdf

VideoTau2021_Roig.mp4

08:25 Radiative correccions to 𝜏→𝑃𝜈 and its consequences for tests of the SM and New Physics

The calculation of radiative corrections (radcorr) in semileptonic decays requires a good description of photon-hadron interactions at all energies. We present a calculation of the radcorr to $\tau^- \to P^- \nu_{\tau}$ decays ($P$ a pseudoscalar meson) where photon-hadron form factors obeys the expected QCD constraints at very small and very large virtual photon momenta. The implications for tests of lepton flavor universality, CKM unitarity and new physics constraints are discussed.

Speaker: Gabriel Lopez Castro (Cinvestav, Mexico)

RadCorr-GLC-Tau2021.pdf

VideoTau2021_LopezCastro.mp4

08:45 Radiative two-pion tau decays and the T-odd asymmetries

The radiative two-pion tau decay process is studied in this work. The resonance chiral theory is used to calculate the relevant form factors. Different light-flavor vector resonance dynamics are revealed in several different two-particle invariant mass distributions. Interesting studies on the T-odd asymmetry distributions arising from the radiative two-pion tau decay process are explored as well. Our results could provide useful guides for future measurements conducted at Belle-II and super tau-charm facilities.

Speaker: Zhi-Hui Guo (Southeast University)

guo-2021-tau.pdf

VideoTau2021_Guo.mp4

09:05 Deriving experimental constraints on the scalar form factor in the second-class 𝜏→𝜂𝜋𝜈 mode

The rare second-class decay mode of the $\tau$ into $\eta\pi\nu$ could be
observed for the first time at Belle II. It is important to try to derive
a reliable evaluation of the branching fraction and of the energy
distribution of this mode within the standard-model. Many predictions
exist already in the literature which, unfortunately, can differ by one to
two orders of magnitude. In this talk I discuss an approach based on a
systematic use of the property of analyticity (of form factors and
amplitudes) in QCD. In particular, I will show that the scalar form factor
in the $\tau$ decay can be related to photon-photon scattering and
radiative $\phi$ decay amplitudes for which precise experimental
measurements (by Belle and Kloe) exist.

Speaker: Bachir Moussallam (Universite Paris-Saclay)

tau21secondclass.pdf

VideoTau2021_Moussallam.mp4

09:25 On the scalar $\pi K$ form factor beyond the elastic region

Pion-kaon ($\pi K$) pairs occur frequently as final states in heavy-particle decays.
A consistent treatment of $\pi K$ scattering and production amplitudes over a wide energy range is therefore mandatory for multiple applications:
in Standard Model tests; to describe crossed channels in the quest for exotic hadronic states; and for an improved spectroscopy of excited kaon resonances.

In the elastic region, the phase shifts of $\pi K$ scattering in a given partial wave are related to the phases of the respective $\pi K$ form factors by Watson's theorem.
Going beyond that, we constructed in Ref.[1] a representation of the scalar $\pi K$ form factor that includes inelastic effects via resonance exchange, while fulfilling all constraints from $\pi K$ scattering and maintaining the correct analytic structure.

As a first application, we considered the decay ${\tau\to K_S\pi\nu_\tau}$, in particular, we studied to which extent the $S$-wave $K_0^*(1430)$ and the $P$-wave $K^*(1410)$ resonances can be differentiated and provide an improved estimate of the $CP$ asymmetry produced by a tensor operator.

[1] Von Detten, L. and Noël, F. and Hanhart, C. and Hoferichter, M. and Kubis, B.
Eur. Phys. J. C 81, 420 (2021); DOI: 10.1140/epjc/s10052-021-09169-7

Speaker: Frederic Noël (Universität Bern)

Tau2021_Noel_27092021.pdf

VideoTau2021_Noel.mp4

09:45 Updated determinations of |Vus| with tau decays using the HFLAV fit

We present updated determinations of |Vus| using tau decays that include a preliminary updated HFLAV global fit of the tau branching fractions, up-to-date lattice hadronic form factors and decay constants, and novel determinations published in 2019 of the electromagnetic and strong isospin-breaking corrections to the π+→μ+ν[γ] and K+→μ+ν[γ] leptonic decay rates obtained with lattice QCD+QED.

Speaker: Alberto Lusiani (Scuola Normale Superiore and INFN, sezione di Pisa)

alusiani-tau21.pdf

VideoTau2021_Lusiani.mp4

10:05 → 10:20 Break

10:20 → 12:00 Session 3: Exclusive and inclusive hadronic tau decays

Convener: Marc Knecht (CNRS)

10:20 Precision measurements on dipole moments of the tau and hadronic multi-body final states

In the framework of precision experiments, the search for electric dipole moments and the precise determination of magnetic dipole moments (g-2)
have since long be of prime interest. Tau leptons play a particular role owing to the large mass as compared to the electron and muon.
The precision of these measurements is, at the one hand, dominated by radiative corrections, which govern the production of tau leptons in e+e- colliders.
The search for electric dipole moments on the other hand relies on the determination of spin correlations of the tau pairs and thus on the analyzing
power of its decay. Three body hadronic decays play a significant role but require correct modeling. They can only be studied precisely through partial wave analyses, which
may reveal even small contributions of partial waves, mostly still ignored today. We will enlighten on both the precision experiments and the technique of analyzing three body hadronic decays of tau leptons
and demonstrate the impact of precision.

Speaker: Dr Fabian Krinner (Max Planck Institute for Physics, Munich)

talk_TAU21.pdf

VideoTau2021_Krinner.mp4

10:40 Resonances in hadronic three-body decays of 𝜏

Hadronic decays of $\tau$ lepton provide a unique possibility to study dynamics in a three-body hadronic system. Particularly, $\tau^-\to \pi^+\pi^-\pi^-\,\nu$ and $\tau^-\to K^-K^+\pi^-\,\nu$ gives a clear sample of the $a_1(1260)$ decays. While the pionic system is dominated by the $\rho$ intermediate resonance and the system with kaons shows mostly $K^*$ resonances, the two final states are strongly coupled via the scalar sector and $\pi^+\pi^- \leftrightarrow K^+K^-$ transitions. In the talk, I will discuss the consequences of this coupling as a modification of the resonance lineshapes and an appearance of the observable triangle-singularity cusps.

Speaker: Mikhail Mikhasenko (University of Bonn (DE))

a1.pdf

VideoTau2021_Mikhasenko.mp4

11:00 Reconciling the FOPT and CIPT predictions for the hadronic tau decay rate

The discrepancy between the FOPT and CIPT predictions for the hadronic tau decay rate (and other spectral function moments) has been a subject of intense investigations for many years and constitutes a major theoretical uncertainty for strong coupling determinations from hadronic tau decay spectral data. We demonstrate that the discrepancy may be understood since the Borel representations (which have been assumed to be identical for both approaches until now) are not equivalent in the presence of infrared renormalons. The difference, called asymptotic separation, can be calculated analytically for any concrete model of the Borel transform of the euclidean Adler function. Theoretically this implies that the OPE condensate corrections are, as a matter of principle, different for the FOPT and CIPT approach, and one can show that only the OPE corrections within FOPT have the standard form assumed in previous phenomenological analyses. This opens up the possibility that OPE predictions based on FOPT and CIPT perturbation theory can be reconciled reducing the theoretical uncertainty in future strong coupling determinations. In practice, the asymptotic separation is only sizeable (and thus phenomenologically relevant) if the known perturbative coefficients of the Adler function are dominated by the gluon condensate renormalon. The analytic knowledge of the asymptotic separation allows the dedicated construction of spectral function moments, where the discrepancy between the FOPT and CIPT predictions is suppressed, and it predicts that FOPT and CIPT lead to compatible predictions for any moment in the presence of infrared subtractions in the loop computations. In this talk we present why the Borel representations of the FOPT and CIPT series in general differ, we discuss the conceptual aspects of the asymptotic separation, and we demonstrate that the asymptotic separation correctly describes the different large-order asymptotic behaviour of the FOPT and CIPT spectral function moments for concrete Borel models. We also show first results for FOPT and CIPT spectral function moment predictions based on infrared-subtracted perturbation theory.

Speaker: Andre Hoang (University of Vienna)

Hoang-Tau-CIPT-FOPT.pdf

VideoTau2021_Hoang.mp4

11:20 The strong coupling from an improved tau vector isovector spectral function

We use a new, more precise, non-strange, inclusive vector
isovector spectral function to determine the strong coupling at the
tau mass scale employing finite energy sum rules. The new spectral
function is obtained from a combination of (i) ALEPH and OPAL results
for the 2 pion and 4 pion tau decay channels, (ii) recent BaBar
results for the tau K Kbar decay distribution, and (iii)
estimates of the contributions from other hadronic tau decay modes
from recent electroproduction data, related using CVC, for subleading
contributions. This new inclusive spectral function is fully data
based and does not rely on Monte Carlo simulated data. Using the
fixed-order perturbation theory (FOPT) prescription, we find for the
strong coupling at the tau mass the value 0.3077(75), which
corresponds to the five-flavor result 0.1171(10) at the Z mass.
Additional experimental input on the dominant 2 pion and 4 pion tau
decay modes would allow for further improvements to the current
analysis.

Speaker: Diogo Boito (Universidade de São Paulo)

Tau2021_TalkBoito.pdf

VideoTau2021_Boito.mp4

11:40 New results on the use of the operator product expansion in finite-energy sum rules for light-quark correlators

Tau-based finite-energy sum rule (FESR) analyses often assume that scales
s_0~m_tau^2 are large enough that (i) integrated duality violations (DVs)
can be neglected, and (ii) contributions from non-perturbative OPE
condensates of dimension D scale as ~(Lambda_{QCD}/m_tau )^D, allowing the
OPE series to be truncated at low dimension. The latter assumption is not
true in general since the OPE series is not convergent, while the former
is open to question given experimental results for the electromagnetic,
I=1 vector (V), I=1 axial vector (A) and I=1 V+A current spectral functions,
which show clear DV oscillations with amplitudes comparable in size to the
corresponding alpha_s-dependent perturbative contributions at hadronic
invariant mass-squareds s~2-3 GeV^2. In this talk, we (1) introduce, and
illustrate the utility of, a new strategy for assessing the numerical
relevance of omitted higher-D OPE and/or residual DV contributions, (2) use
large N_c and analyticity arguments to derive the expected large-s form for
DV contribution to the I=1, V spectral function, under the assumption that
the leading behavior is Regge-like at large s, and (3) use this form to
explore the level of suppression of residual integrated DV contributions
in I=1, V channel FESRs.

Speaker: Maarten Golterman (San Francisco State University)

TAU2021_Golterman.pdf

VideoTau2021_Golterman.mp4

12:00 → 12:30 Lunch

12:30 → 14:00 Session 4a: Neutrinos and Dark Matter

Convener: Alexander Friedland

12:30 The search for sterile neutrinos with short-baseline neutrino experiments

In the past two decades, several results from short-baseline neutrino experiments have hinted that sterile neutrinos could be present. If confirmed, these new particles would be a definite proof of physics beyond the Standard Model and would offer attractive solutions to many unanswered questions in our field. After briefly reviewing selected results, I will present the current experimental landscape and what the prospects are to resolve this important open question on the existence of sterile neutrinos. I will also discuss some of the implications of having additional neutrinos.

Speaker: Roxanne Guenette (Harvard University)

TAU2021_SBL_Guenette.pdf

VideoTau2021_Guenette.mp4

12:55 Sterile neutrino theory review

In this talk I will review the short baseline anomalies and recent developments relevant to this topic. I will discuss theoretical interpretations of these anomalies, including sterile neutrinos, decaying neutrinos, and novel gauge forces. Limitations of the theoretical interpretations will be highlighted.

Speaker: Pedro Machado (Fermilab)

Machado-Sterile neutrino theory.pdf

VideoTau2021_Machado.mp4

13:20 Status of long-baseline neutrino experiments

I will give a summary talk on long baseline neutrino measurements, with attention to topics and searches related to tau neutrinos. I will cover the latest results from T2K, NOvA, and OPERA, and future prospects from DUNE and Hyper-K.

Speaker: Morgan Wascko (Imperial College (GB))

VideoTau2021_Wascko.mp4

wascko_20210929.pdf

13:45 Discussion

12:30 → 13:50 Session 5a: Proton-proton and e+e- colliders

Convener: Sridhara Dasu (University of Wisconsin Madison (US))

12:30 Higgs boson measurements in couplings to tau leptons with the ATLAS experiment

Testing the Yukawa couplings of the Higgs boson to quarks and leptons is important to understand the origin of fermion masses. ATLAS has recently measured the properties of the Higgs boson decays to two tau leptons, with an analysis based on the full dataset of pp collisions collected at 13 TeV at the LHC. The talk will present details on the analysis techniques used to measure the inclusive cross sections in the four dominant Higgs production modes, as well as in exclusive regions of the phase space using the simplified template cross-section framework.

Speaker: Christopher Young (Albert Ludwigs Universitaet Freiburg (DE))

ChrisYoung.mp4

Presentation.pdf

12:50 Tau leptons as a tool to investigate the CP properties of the Higgs boson at CMS

Among the Higgs boson decay channels, the one to tau leptons can offer insight into the properties of the Higgs boson. The structure under CP symmetry of the Yukawa coupling between the Higgs boson and tau leptons was investigated in CMS by reconstructing the decay planes of the two tau leptons and measuring their angular separation. Tau decay planes are reconstructed depending on the studied decay channel to take advantage of the correlation between the tau lepton spin and the momenta of its decay products. Using the data collected during the LHC Run 2 data-taking period, the study revealed that the Yukawa coupling is largely dominated by a pure CP-even component. A pure CP-odd Yukawa coupling is excluded with a 99.7% allowing to constrain the allowed phase space for possible BSM scenarios.

Speaker: Andrea Cardini (Deutsches Elektronen-Synchrotron (DE))

AndreaCardini-HiggsCPintaus.pdf

AndreaCardini.mp4

13:10 Tau identification in CMS during LHC Run 2

The LHC Run 2 data-taking period was characterized by an increase in instantaneous luminosity and center-of-mass energy. Several techniques have been deployed in the CMS experiment to reconstruct and identify tau leptons in this environment. The DeepTau identification algorithm is used to identify hadronically decaying tau leptons from quark and gluon induced jets, electrons, and muons. Compared to previously used MVA identification algorithms, the use of deep-learning techniques brought a noticeable improvement in the tau identification and rejection of contaminating sources. Low transverse momentum topologies were addressed separately with a dedicated identification algorithm, while machine learning techniques were implemented to improve the identification of the tau hadronic decay channels. These algorithms have been already used for several published physics analyses in CMS. The algorithms are presented together with their measured performances.

Speaker: Mohammad Hassan Hassanshahi

20210929-TAU2021-TauInCMS-MohammadHassanshahi.pdf

MohammadHassan.mp4

13:30 Tau as probe for ATLAS experiment

Speaker: Christian Grefe (University of Bonn (DE))

2021-09-29_TAU21_Grefe.pdf

ChristianGrefe.mp4

13:50 → 14:05 Break

14:00 → 14:15 Break

14:05 → 15:05 Session 5a: Proton-proton and e+e- colliders

Convener: Olga Shekhovtsova (Kharkov)

14:05 Muon g-2 to Δα

The Muon 𝑔−2 experiment at Fermilab has recently confirmed Brookhaven's earlier measurement of the muon anomalous magnetic moment. This new result increases the muon g-2 discrepancy with the Standard Model prediction and strengthens its "new physics" interpretation, as well as the quest for its underlying origin. I will discuss the connection of the muon g-2 discrepancy to precision electroweak predictions via their common dependence on hadronic vacuum polarization effects. This is particularly relevant for the ongoing comparison between results for hadronic vacuum polarization effects as calculated from hadronic cross section data and from lattice QCD.

Speaker: Alex Keshavarzi (U. Manchester)

AKeshavarzi_TAU21_29-09-21-compressed.pdf

AlexKeshavarzi.mp4

14:25 The Strong2020 and RadioMonteCarlow activities

During the last 15 years the "Radio MontecarLow (“Radiative Corrections and Monte Carlo Generators for Low Energies”) Working Group, see www.lnf.infn.it/wg/sighad/, has been providing valuable support to
the development of radiative corrections and Monte Carlo generators for low energy e+e- data and tau-lepton decays. Its operation which started
in 2006 proceeded until the last few years bringing together at 20 meetings both theorists and experimentalists, experts working in the field of e+e- physics and partly also the tau community and produced the report
“Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data” S. Actis et al. Eur. Phys. J. C 66, 585-686 (2010) (https://arxiv.org/abs/0912.0749), which has more than 300 citations.
While the working group has been operating for more than 15 years without a formal basis for funding, parts of our program have recently been included as a Joint Research Initiative in the group application of the European hadron physics community, STRONG2020, to the European Union, with a more specific goal of creating an annotated database for low-energy hadronic cross sections in e+e- collisions. The database will contain information about the reliability of the data sets, their systematic errors, and the treatment of RC.
We will report on both these initiatives.

Speaker: Graziano Venanzoni (Universita & INFN Pisa (IT))

GrazianoVenanzoni.mp4

gv_tau21_290921_s.pdf

14:45 Monte Carlo Event Generator updates with tau pair events at Belle II energies

The Monte Carlo for lepton pair production and tau decays consist
of KKMC for lepton pair production, Tauola for tau lepton decays and
Photos for radiative corrections in decays.

An effort for adaptation of the system for precision data to be collected
at Belle II experiment lead to extension of phase space generation modules
both in Photos and Tauola to enable decays and/or radiative corrections
with emission of additional light lepton pairs. The phase-space and matrix
element parts are separated, that is why extension is useful for
processes where lepton pair is produced through narrow resonances,
like dark photon or dark scalar candidates.

List of tau decays is enriched with multitude of exotic decay channels useful for
new physics searches. The hadronic currents parameterizations of main decay channels
is prepared for basic simulation in the experiment. The basis for future
work on precise fits of hadronic currents including Machine Learning
is retained, but development of necessary software solutions is left for
the forthcoming years.

Programs are now available in stand-alone format or through the Basf2 system of
Belle II software as well.

Speaker: Zbigniew Andrzej Was (Polish Academy of Sciences (PL))

Indi21.pdf

14:15 → 15:30 Session 4a: Neutrinos and Dark Matter

Convener: Vittorio Paolone (University of Pittsburgh (US))

14:15 Tau Neutrino Physics with IceCube

Kilometer-scale neutrino detectors, like the IceCube Neutrino Observatory deployed in the ice cap at the South Pole, are uniquely capable of detecting energetic tau neutrinos and tau leptons. IceCube has sensitivity to tau neutrinos with energies at and above the threshold for tau lepton production, and has sufficiently large volume to contain tau leptons that travel hundreds of meters. The experiment has world-leading acceptance for atmospheric tau neutrinos at energies above roughly 10 GeV, and to astrophysical tau neutrinos at energies above roughly 100 TeV. Atmospheric tau neutrinos are primarily created by the oscillation of muon neutrinos as they pass through the earth after production in the northern hemisphere, and IceCube detects them and their tau lepton daughters inclusively, as an excess of shower-like events in its DeepCore sub-array. Astrophysical tau neutrinos are likely produced by neutrino oscillations over cosmic baselines, and can be detected exclusively, through the distinctive signatures created by the tau neutrino interaction vertex and the subsequent tau lepton decay vertex. We present results of IceCube's atmospheric and astrophysical tau neutrino measurements, and provide projections for future improvements and possible new channels for tau neutrino and tau lepton detection.

Speaker: Douglas Cowen (Penn State)

TauNeutrinosWithIceCube-Tau2021-DCowen-RS.pdf

VideoTau2021_Cowen.mp4

14:40 Theoretical perspectives on dark matter

This talk will provide of overview on searches for particle dark matter, including direct, indirect, and accelerator searches that cover different energy and mass scales. I will highlight the future directions in particle dark matter searches, and how experiments can be complementary in maximizing the sensitivity to a broad range of viable models.

Speaker: Louis Strigari (Texas A&M)

tau_2021.pdf

VideoTau2021_Strigari.mp4

15:10 Tau (and Muon ) Airshower on Earth (and Space)

The neutrino astronomy is polluted by the atmospheric noise up TeVs energy. Anyway Icecube claimed an astrophysical signature (just above a few tens TeV energy), since the 2013. They discovered a sudden increase in the cascade event rate, surprisingly more abundant than the previous TeVs muon neutrino tracks. Such a flavor switch might be also mimic by an atmospheric charm rising presence. Absent correlations with optical, X, gamma sky of Icecube events it is still puzzling. Nevertheless the eventual tau neutrino appearence, almost absent in any atmospheric noise,( even the charm one), should confirm a neutrino astronomy above PeVs energy. The tau signals are expected by flavor mixing in cosmic flights; they may offer a definitive fingerprint of the astrophysical neutrino nature. These tau neutrino ruling role might rise in ice (IceCube) or water (Antares) as a double bang ( first interaction, second tau decay cascades) or by a tau airshower: an internal tau neutrino interaction in the rock of a mountain or of the Earth crust and the later tau decay outside, while in flight in air. The physics of tau airshowers had been noted since 1999 and became today the main road map in widest experimental projects. We discuss and update the tau airshower signature from Earth and planets,by array on top Mountains, Balloons or satellites. Very new ad hoc instruments and novel theoretical expectation will be shown for the first time.

Speaker: Prof. Daniele Fargion (Rome University)

Tau (and Muon) Airshower.pdf

Tau (and Muon) Airshower.pptx

Thursday, 30 September

08:00 → 09:45 Session 4b: Neutrino and Dark Matter

Convener: Mark Messier (IU)

08:00 Searches for Neutrinoless Double Beta Decay

Observation of neutrinoless double beta decay would profoundly impact our understanding of the neutrino. This lepton-number violating process requires new beyond Standard Model physics, would imply that neutrinos are Majorana particles, and provide insight into the nature of neutrino mass. It is thus a highly-sensitive and promising probe of new physics. In this talk, I will present the status of the experimental field, particularly highlighting the recent achievements. The future is bright, with next-generation experimental concepts proposed to leverage these advances and drive the discovery sensitivity of the field down to the $m_{\beta\beta} \sim 10$ meV scale.

Speaker: Walter Pettus (Indiana University)

pettus_tau.pdf

session4-nudm-01-pettus.mp4

08:25 Latest results from the CUORE experiment

The Cryogenic Underground Observatory for Rare Events (CUORE) is the first bolometric experiment searching for 0νββ decay that has been able to reach the one-tonne mass scale. The detector, located at the LNGS in Italy, consists of an array of 988 TeO2 crystals arranged in a compact cylindrical structure of 19 towers. CUORE began its first physics data run in 2017 at a base temperature of about 10 mK and in April 2021 released its 3rd result of the search for 0νββ, corresponding to a tonne-year of TeO2 exposure. This is the largest amount of data ever acquired with a solid state detector and the most sensitive measurement of 0νββ decay in 130Te ever conducted, with a median exclusion sensitivity of 2.8×10^25 yr. We find no evidence of 0νββ decay and set a lower bound of 2.2 ×10^25 yr at a 90% credibility interval on the 130Te half-life for this process. In this talk, we present the current status of CUORE search for 0νββ with the updated statistics of one tonne-yr. We finally give an update of the CUORE background model and the measurement of the 130Te 2νββ decay half-life, study performed using an exposure of 300.7 kg⋅yr.

Speaker: Miriam Olmi (INFN (LNGS))

session4-nudm-02-olmi.mp4

TAU2021_OlmiM_def.pdf

08:45 Neutrino oscillation results from the MINOS/MINOS+ Experiment

The MINOS/MINOS+ experiment, which consisted of two magnetised steel-scintillator tracking calorimeters, observed neutrino and antineutrino flavour change over a baseline of 735 km in the NuMI beam from Fermilab.
With 11 years of collected data in configurations with peak energies at 3 GeV and 6 GeV, the experiment, which has performed precision measurements of neutrino oscillation parameters and searches for new physics in the neutrino sector, it is now moving toward the final results with complete dataset.
This talk will present the latest update on three-flavour oscillations parameters and sterile neutrino searches from the MINOS/MINOS+ experiment.

Speaker: Stefano Germani (Perugia University)

MINOS_TAU2021.pdf

session4-nudm-03-germani.mp4

09:05 Measuring three-flavor neutrinos with FASERnu at the LHC

FASER$\nu$ at the LHC is designed to directly detect collider neutrinos for the first time and study their properties at TeV energies, where no such measurements currently exist. The detector will be located 480 m downstream of the ATLAS interaction point. With FASERnu, the three-flavor neutrino cross-sections will be measured in the currently unexplored energy range between 360 GeV and 5 TeV. In particular, tau-neutrino and electron-neutrino cross sections will be measured at the highest energy ever. In 2018 we performed a pilot run with the aims of measuring particle fluxes at the proposed detector location and of possibly detecting neutrino interactions for the first time at the LHC. We installed a 30-kg lead/tungsten emulsion detector and collected data of 12.2 fb$^{-1}$. The analysis of this data has yielded several neutrino interaction candidates, excluding the no-signal hypothesis with a statistical significance of 2.7$\sigma$. We have also studied the charged particle flux in regard to the characterization of the unprecedented collider neutrino beamline. During Run-3 of the LHC starting from 2022, we will deploy an emulsion detector with a target mass of 1.1 tons, coupled with the FASER magnetic spectrometer. This would yield roughly 2,000 $\nu_e$, 7,000 $\nu_{\mu}$, and 30 $\nu_{\tau}$ interacting in the detector. Here we present the status and plan of FASER$\nu$, as well as the neutrino detection in the 2018 data.

Speaker: Tomoko Ariga (Kyushu University (JP))

20210930_tariga_FASERnu_TAU2021.pdf

session4-nudm-04-ariga.mp4

09:25 The Scattering and Neutrino Detector at the LHC

SND@LHC is a compact and stand-alone experiment to perform measurements with neutrinos produced at the LHC in a hitherto unexplored pseudo-rapidity region of 7.2 < 𝜂 < 8.6, complementary to all the other experiments at the LHC. The experiment is to be located 480 m downstream of IP1 in the unused TI18 tunnel. The detector is composed of a hybrid system based on an 800 kg target mass of tungsten plates, interleaved with emulsion and electronic trackers, followed downstream by a calorimeter and a muon system. The configuration allows efficiently distinguishing between all three neutrino flavours, opening a unique opportunity to probe physics of heavy flavour production at the LHC in the region that is not accessible to ATLAS, CMS and LHCb. This region is of particular interest also for future circular colliders and for predictions of very high-energy atmospheric neutrinos. The detector concept is also well suited to searching for Feebly Interacting Particles via signatures of scattering in the detector target. The first phase aims at operating the detector throughout LHC Run 3 to collect a total of 150 fb−1. The experiment was recently approved by the Research Board at CERN. A new era of collider neutrino physics is just starting.

Speaker: Francesco Navarria (Universita e INFN, Bologna (IT))

Navarria_SND@LHC TAU2021.pdf

session4-nudm-05-navarria.mp4

08:00 → 09:40 Session 5b: Proton-proton and e+e- colliders

Convener: Michel Davier (IJCLab)

08:00 Current status and results of the experiments with CMD-3 detector at VEPP-2000

The VEPP-2000 ee collider has been operating at BINP (Novosibirsk) from 2010 in the center-of-mass energy range from 0.3 to 2 GeV. The project luminosity of this machine, exploiting the idea of the round beams, has to amount to 1032cm-2s-1. By now the luminosity up to 51031cm-2s-1 was achieved. Two detectors, CMD-3 and SND, are running at two interaction regions of the VEPP-2000. Each detector collected about 300 pb-1 in the new run. Considerable statistics was taken within an energy range around the nucleon-antinucleon pair production.
Precise study of the hadrons production in ee annihilation at low energies provides important information about interactions of light quarks and spectroscopy of their bound states.
Precise measurements of the total hadronic cross section, characterized by the ratio R, is needed for the calculation of the contribution of the hadronic vacuum polarization to the muon anomalous magnetic moment. It should be noted that at present the accuracy of the theoretical calculations of the muon (g-2) via the Standard Model is dominated by the precision of the hadronic contribution while the difference of theoretical and experimental values exceeds three standard deviations.
In this report we will discuss current status and results obtained by the of the CMD-3 experiment.

Speaker: Boris Shwartz (Budker Institute of Nuclear Physics, Novosibirsk)

talk-tau21-4.pdf

VideoTau2021_Shwartz.mp4

08:25 e+e- hadronic cross-sections with SND detector at VEPP 2000

Total amount of about 320 pb^{-1} of integrated luminocity has been collected with SND detector at VEPP-2000 collider. Here we present recent results on e+e- annihilation to hadrons below 2 GeV based on part of the data. In particular, we discuss measurements of the e^+e^-\to pi^+\pi^- and e^+e^- \to n\bar{n} cross sections, and study of the radiative processes e^+e^- \to \eta\gamma, e^+e^- \to \eta \pi^0 \gamma and e^+e^- \to \eta \eta \gamma.

Speaker: Dr Tatyana Dimova (Novosibirsk State University (RU))

Dimova-TAU21.pdf

VideoTau2021_Dimova.mp4

08:50 Study of e​+e​− ​annihilation into hadrons at low energies with ISR at BABAR

The measurement of exclusive e​+e​− to hadrons processes is a significant part of the physics program of BABAR experiment, aimed to improve the calculation of the hadronic contribution to the muon g−2 and to study the intermediate dynamics of the processes. We present the most recent studies performed on the full data set of about 470 fb^−1 collected at the PEP-II e+e− collider at a center-of-mass energy of about 10.6 GeV. In particular, we report the results on e+e​− annihilation into three pions and into states with six and seven pions or kaons, in an energy range from production threshold up to about 4 GeV.

Speaker: Peter Lukin

Lukin_BaBar_TAU2021.pdf

VideoTau2021_Lukin.mp4

09:15 Measuring the inclusive cross section of e+ e- annihilation into hadrons at the KEDR experiment

We present the results on the inclusive cross section of e+e- single
photon annihilation into hadrons in the energy range from 1.84 to 3.72 GeV obtained at the KEDR experiment, which include values of the R measurement in 22 points and of leptonic widths of J/psi and psi(2S) mesons. The results are important or determination of the muon anomalous magnetic moment, the fine structure constant at the Z^0 peak, the strong coupling constant as function of energy, and the heavy-quark masses.

Speaker: Korneliy Todyshev (BINP)

tau2021todyshev.pdf

tau2021todyshev.pdf

VideoTau2021_Todyshev.mp4

09:45 → 10:00 Conference Photo and Break

10:00 → 11:35 Session 6: g-2

Convener: Massimo Passera (INFN Padova)

10:00 First Result from the New Muon g-2 Experiment at Fermilab

Nearly twenty years ago a team from Brookhaven National Lab measured the anomalous part of the muon's dipole moment ($a_\mu$) finding a ~three-sigma discrepancy against contemporary Standard Model predictions. The mystery lingered for a time as there were no further measurements, but within a few years a small dedicated team set into motion a plan to revive the technique at Fermilab. This team grew into the new Muon g-2 Collaboration, whose result was published earlier this month. In our first run repeating the experiment, we confirmed the original measurement while utilizing major system upgrades and increased statistics to push the precision below one half ppm. The original incongruity persists and could potentially reach discovery-level significance within the planned run time of the experiment. I will provide a perspective on the result and our outlook for reaching a few times better resolution with further data.

Speaker: James Stapleton (Fermi National Lab)

tau2021_muong2.pdf

10:25 Theory overview

Speaker: Gilberto Colangelo (Universität Bern)

g-2_TAU2021.pdf

VideoTau2021_Colangelo.mp4

10:50 HVP contributions to the muon's anomalous magnetic moment from lattice QCD

With the Fermilab g-2 experiment’s first measurement of the muon anomalous magnetic moment ($a_\mu$), announced on 7 April 2021, the difference between the new experimental average and the Muon g-2 Theory Initiative’s Standard Model prediction now stands at $4.2\sigma$. Experimental measurements of $a_\mu$ are expected to improve drastically in the coming years. In order to maximize the impact of the experimental program, the uncertainties in the Standard Model prediction, which are dominated by hadronic effects and are currently commensurate with experiment, must be reduced concurrently. The largest of these is the hadronic vacuum polarization (HVP), which is also the dominant source of uncertainty. This talk will provide a review of the current status of lattice QCD calculations of this quantity, including future prospects for improvement.

Speaker: Aida El-Khadra (UIUC)

El-Khadra_TAU2021.pdf

VideoTau2021_El-Khadra.mp4

11:15 Measuring hadronic corrections to the muon g-2 at BESIII

The recent Fermilab result has confirmed the long standing discrepancy between the direct measurement and the Standard Model (SM) prediction of the anomalous magnetic moment of the muon aμ=(gµ-2)/2. The possibility to improve the SM prediction using data-driven approaches has motivated the BESIII collaboration to embark on a dedicated experimental program. The high statistics data samples collected with the BESIII experiment in e+e- collisions in the tau-charm region are analyzed exploiting the initial state radiation technique in order to measure hadronic cross sections needed in the dispersive analysis of the hadronic vacuum polarization contribution to aμ. The same data enable investigations of two-photon collisions. These allow the determination of the momentum dependence of transition form factors of light mesons in the relevant kinematic region, which dominate the hadronic Light-by-Light contribution to aµ. The current status and ongoing investigations will be discussed.

Speakers: Achim Denig (Univ. Mainz), Achim Denig (Universität Mainz)

Tau21_Denig_pdf.pdf

VideoTau2021_Denig.mp4

11:35 → 11:50 Break

11:50 → 12:50 Session 6: g-2

Convener: Rafel Escribano (UAB (Barcelona) and IFAE/BIST (Barcelona))

11:50 The pseudoscalar poles contributions to the muon g-2

The recent result from the Fermilab experiment on the anomalous magnetic moment of the muon ($g-2$) has revived the interest on this observable, that exhibits an interesting —and persistent— $4\sigma$ discrepancy with respect to its theoretical value.
Future runs at Fermilab will help deciphering the nature of such discrepancy, but that will require a commensurate improvement on the theory side, completely dominated by hadronic uncertainties.
In this talk, we revise the main piece of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon: the pseudoscalar poles. To that purpose, we use the framework of Padé and Canterbury approximants, that allows to analyze and to describe the relevant transition form factors entering the calculation in a model-independent and data-driven fashion.

Speaker: Pablo Sanchez Puertas (IFAE)

tau2021PabloSan.pdf

VideoTau2021_SanchezPuertas.mp4

12:10 A dispersive estimate of the f0(980) contribution to g-2

In my talk, I will present a dispersive estimate of the $f_0(980)$ contribution to $(g-2)_\mu$. The performed analysis is a coupled-channel extension of the previous work, where $f_0(500)$ has already been addressed. Important ingredients are $\gamma^*\gamma^*\to \pi \pi$ and $\gamma^*\gamma^*\to K\bar{K}$ S-wave helicity amplitudes which rely on the novel unitarizaiton scheme for studying hadronic interactions beyond the threshold region.

Speaker: Igor Danilkin (Johannes Gutenberg-Universitat Mainz)

Danilkin2021_tau.pdf

VideoTau2021_Danilkin.mp4

12:30 Two-photon physics at KLOE-2

KLOE-2, the continuation of the KLOE experiment at the Frascati $\phi-$factory, completed its data-taking by acquiring about 5 fb$^{-1}$ at the $\phi$ meson peak.
One of its distinctive feature is the possibility to study $e^+e^- \to \gamma^{\ast}\gamma^{\ast}e^+e^-\to \pi^0 e^+e^-$ processes by tagging final state leptons with two stations installed in both arms of the DA$\Phi$NE beam pipe.
The aim is to perform the high precision measurement of the $\pi^0$ width to test low-energy QCD dynamics.

The High Energy Tagger (HET) is a scintillator hodoscope whose counting rate is dominated by very low angle radiative Bhabha scattering events without any associated signal in the KLOE detector. The measurement of the effective low angle radiative Bhabha cross section per scintillator is used to monitor detector performance and infer acceptance$\times$efficiency of the HET.
The $\pi^0$ production from two-photon fusion is tagged by requiring the coincidence between the HET detector and the KLOE calorimeter when two clusters are reconstructed for one of the DAFNE bunch. The background is measured from events, continuously recorded in a time window where KOE and HET data acquisitions do not overlap.

The measurement of the low angle radiative Bhabha cross section and last results on the $\gamma^{\ast}\gamma^{\ast}\to \pi^0$ analysis will be reported.

Speaker: Dario Moricciani (INFN - National Institute for Nuclear Physics)

Moricciani_talk.pdf

VideoTau2021_Moricciani.mp4

12:50 → 13:20 Lunch

13:20 → 14:40 Session 6: g-2

Convener: Harvey Meyer (Johannes Gutenberg University Mainz)

13:20 Calculating HLbL on the Lattice

The hadronic light-by-light contribution to the muon g-2 can now
be calculated using first principles lattice QCD+QED methods. Independent results from two collaborations are available. I will review the status and recent developments.

Speaker: Christoph Lehner (University of Regensburg & BNL)

talk-nobackup.pdf

VideoTau2021_Lehner.mp4

13:40 HLbL in muon g-2 at large loop momenta

We study the HLbL contribution to g-2 in the kinematic region where the three loop momenta are large. We show how, even when the fourth photon is in the static limit, the massless quark loop gives the leading term of an operator product expansion. Power corrections are found to be small. Gluonic corrections are also included and the expansion is found to be well-behaved at relatively low-energies, which can be used to reduce uncertainties in the HLbL contribution to g-2.

Speaker: Antonio RODRIGUEZ SANCHEZ (CNRS)

talktaurodriguez.pdf

14:00 Two-pion contribution to hadronic vacuum polarization

The largest part of the hadronic-vacuum-polarization (HVP) contribution to the muon anomalous magnetic moment is due to the intermediate state of two pions. Analyticity and unitarity do not only allow us to write this contribution in terms of the pion vector form factor (VFF), but also constrain the VFF itself. I will discuss fits of a dispersive representation of the pion VFF to $e^+e^-$ cross-section data and I will comment on the implications of analyticity and unitarity in view of the tension between data-driven evaluations of HVP and recent lattice-QCD results.

Speaker: Peter Stoffer (University of Zurich)

Talk-Tau2021.pdf

14:20 New τ-based evaluation of the hadronic contribution to the vacuum polarization piece of the muon anomalous magnetic moment

We review the isospin-breaking and electromagnetic corrections to the $\tau^-\to\pi^-\pi^0\nu_\tau$ decays, which are used as an input to the two-pion contributions to the hadronic vacuum polarization (at LO) of the anomalous magnetic moment ($a_\mu$). We extend previous analyses by Cirigliano et al. working with ChPT with resonances. As an outcome, we improve the agreement between this determination and the other based on $e^+e^-$ data. The new results are in better agreement with an old estimation that uses Vector Dominance Model (VMD), and the discrepancy between the SM prediction and the combined results from BNL and FNAL is reduced to $2.1\sigma$ at $\mathcal{O}(p^4)$ and $2.3\sigma$ at $\mathcal{O}(p^6)$.

Speaker: Jesus Alejandro Miranda Hernandez (Cinvestav)

JM_TAU2021.pdf

14:40 → 14:55 Break

14:55 → 16:15 Session 6: g-2

Convener: Johan Bijnens

14:55 Perturbative heavy quark contributions to the anomalous magnetic moment of the muon

We discuss a method for calculating the heavy quark vacuum polarisation contribution to the muon anomalous magnetic moment, $a_\mu$, using perturbative QCD up to ${\cal O}\left(\alpha_s^3\right)$. This approach is independent of $e^+e^-$ cross-section data allowing a fully theoretical evaluation of these contributions. This method confirms an existing result at lower orders in $\alpha_s$ and we state a new explicit analytic formula which includes terms up to ${\cal O}\left(\alpha_s^3\right)$. Numerically the charm quark contribution to $a_\mu$ is found to be $a_\mu^c = (14.5 \pm 0.2)\times 10^{-10}$ and the bottom contribution is $a_\mu^b = (0.302 \pm 0.002) \times 10^{-10} $. Our uncertainty estimates include both parametric uncertainties, arising from $\hat{m}_q(\hat{m}_q)$ and $\alpha_s(\hat{m}_q)$, and theoretical uncertainties in the perturbative expansion. Comparison is then made between these results and those from alternative approaches such as, lattice QCD, or based on a dispersion relation and cross-section data.

Speaker: Philip David Kennedy (Johannes Gutenberg Universitaet Mainz (DE))

pkennedy_tau_presentation_amuq.pdf

15:15 A continuum determination of the strong isospin-breaking contribution to the muon anomalous magnetic moment

We present a continuum determination of a_mu^SIB, the strong isospin-breaking contribution to a_mu, the anomalous magnetic moment of the muon, using ChPT and the formulation of a_mu as a weighted integral of the electromagnetic current two-point function over Euclidean Q^2. Flavor-breaking hadronic tau decay sum rules are shown to provide a determination of a key higher-order chiral LEC encoding numerically important resonance region contributions. Implications of the structure of the result for the lattice determination of a_mu^SIB are also discussed.

Speaker: Kim Maltman (York University)

tau2021_amusib_sep30_21_maltman.pdf

15:35 Probing New Physics with the leptonic g-2

Speaker: Paride Paradisi (University of Padova and INFN)

paradisi_tau2021.pdf

15:55 Probing violation of CP & T invariance

The SM predicts zero values of CPV & TV in the decays of tau leptons except \tau^- \to \nu K^0_S [\pi,\pi\pi]. With neutrino oscillations I would give the golden medals for CPV & TV. I think that tau decays can get bronze medals in \tau^- \to \nu K [pi, \pi\pi, \eta]. Their hadronic final states are enough `complex'.

Speaker: Ikaros Bigi (Physics Department, University of Notre Dame)

MytalkTau2021CPVv2.pdf

Friday, 1 October

08:00 → 09:55 Session 7: Future directions

Convener: Changzheng Yuan (IHEP, Beijing)

08:00 Experimental Program for Super Tau-Charm Facility

The proposed STCF is a symmetric electron-positron beam collider designed to provide e+e− interactions at center of-mass energies from 2.0 to 7.0 GeV. The peaking luminosity is expected to be 0.5×10^35 cm−2s−1. The energy region of STCF covers the pair production thresholds for tau-leptons, charmed meson & baryons, and all of the strange hyperons. STCF is expected to deliver more than 1 ab−1 of integrated luminosity per year. Huge samples of XYZ, Jpsi , D+, D+s and Lambdac decays could be used to make precision measurements of the properties of XYZ particles, search for new ones, and study their rare decays; map out the spectroscopies of QCD hybrids; search for new sources of CP violation in the strange-hyperon and tau−lepton sectors with unprecedented sensitivity; make precise independent measurements of the unitarity of the CKM flavor-mixing matrix and address the Cabibbo Angle Anomaly; search for anomalous decays with sensitivities extending down to the level of SM-model expectations; qualify Lattice QCD calculations; and provide precise inputs that are essential for the interpretation of results from other experiments.

Speaker: Xiaorong Zhou (USTC)

STCF_tau2021.pdf

VideoTau2021_Zhou.mp4

08:25 Super Charm-Tau Factory in Russia

The Super Charm-Tau Factory (SCTF) project proposed at Budker Institute of Nuclear Physics is discussed. An electron-positron collider with the luminosity of about
10^35 1/cm^2/s operated at the center-of-mass energies from 3 to 7 GeV, and modern particle detector allow one to study on the new level of precision the physics of charmonium, exotic charmonium-like states, charmed mesons and baryons, and the tau lepton, as well as the production of light hadrons in e+ e--annihilation processes and in two-photon processes. A longitudinal polarization of the electron beam at the interaction point will provide a number of advantages of the SCTF over the existing B factories such as Belle II and LHCb. The tau physics program of the SCTF will be discussed in more detail.

Speaker: Denis Epifanov (BINP, Novosibirsk)

epifanov_sctf_tau2021.pdf

VideoTau2021_Epifanov.mp4

08:50 Muon g-2/EDM experiment at J-PARC

The muon g-2 experiment at J-PARC is under preparation and targeted to measure the muon anomalous magnetic moment with the precision of 450 ppb and muon electric dipole moment with 1.5e-21 e cm at its first stage,
thus contributing to investigation of discrepancy between Standard Model prediction and the current world average of g-2. The latter is dominated by two similar experiments E821 BNL and E989 FNAL, while we suggest a novel approach: pulsed primary proton beam provides surface muons, which are diffused through a silica aerogel target forming thermolised muonium atoms. They are laser ionised and re-accelerated by a multi-stage linac up to 300 MeV/c before spiral injection into the storage uniform 3 T MRI-like magnet volume at the stable orbit in the absence of E-field. The silicon strip detector placed inside the magnet measures decayed positron parameters used in data analysis.
We report the experimental approach, current status, and future prospects.

Speaker: Georgiy Razuvaev (BINP)

muon_g-2_edm_j-parc_tau2021_razuvaev_v2.pdf

VideoTau2021_Razuvaev.mp4

09:15 Feasibility of tau g-2 measurements in ultra-peripheral collisions of heavy ions

Anomalous magnetic moment of the tau lepton, $a_\tau = (g_\tau -2)/2$, is a sensitive probe of new physics but is extremely difficult to measure precisely in contrast to electron and muon moments. The best experimental limits were set by the DELPHI collaboration more than 15 years ago in the studies of the ditau production in the $e^+e^- \to e^+e^-\tau^+\tau^-$ process. Ultra-peripheral collisions (UPC) of heavy ions at the LHC may provide a unique opportunity to improve the $a_\tau$ constraints in the studies of ${\rm Pb+Pb} \to {\rm Pb+Pb} \tau^+\tau^-$ process. We review recent proposals to study ditau production via semi-leptonic tau decays in Pb-Pb UPC with the available ATLAS and CMS data and discuss feasibility to explore this process down to low transverse momenta of decay leptons with ALICE and LHCb detectors.

Speaker: Evgeny Kryshen (NRC Kurchatov Institute PNPI (RU))

2021-10-01-tau.pdf

VideoTau2021_Krishen.mp4

09:35 Physics measurements with Tau final state at the CEPC

Speaker: Manqi Ruan (IHEP)

Ruan_Tau at CEPC.pdf

VideoTau2021_Ruan.mp4

09:55 → 10:10 Break

10:10 → 10:40 Session 7: Future directions

Convener: Yury Kudenko (Russian Academy of Sciences (RU))

10:10 DUNE

For now two decades, an extensive neutrino oscillations experimental era dramatically improved our knowledge on the 3-neutrino oscillations paradigm. This program, though, almost only focused on the electron and muon neutrinos, and our knowledge on the tau neutrino mostly relies on leptonic universality.

DUNE (Deep Underground Neutrino Experiment) is a next generation of neutrino oscillations experiment tuned to probe electron neutrino appearance in an artificial beam of muon neutrinos between Fermilab and the Sanford Underground Research Facility (South Dakota). Its 1285 km baseline and the spatial/calorimetric precision of its gigantic far detectors makes it ideally suited to study tau neutrino appearance with an unprecedented sensitivity. This will allow, among others, performing a unique test of the consistency of the 3 flavour neutrino paradigm as well as help constraining the PMNS matrix unitarity.

In this talk I will discuss the main physics topics related to tau neutrino physics and present the current status of the non-trivial search for tau neutrino identification for the DUNE experiment at the simulation level.

Speaker: Thomas Kosc (CERN)

TauLeptonConference_ThomasKosc.pdf

VideoTau2021_Kosc.mp4

10:25 T2HK

Speaker: Sandro Bravar (Universite de Geneve (CH))

Bravar_Tau2021.pdf

VideoTau2021_Bravar.mp4

10:40 → 10:50 Break

10:50 → 12:50 Poster session: Breakout room 1

10:50 Tau lifetime measurement at Belle II

The tau-lepton lifetime represents a fundamental parameter within the Standard Model framework, contributing to the test of lepton flavor universality. Exploiting the vertex detector resolution and the tiny beam spot size at the interaction point, Belle II is expected to improve the present tau-lifetime value. The event topology where one tau decays to three charged pions (3-prong) and the other tau goes to a charged rho meson (1-prong), allows to have an higher event yield respect to 3-prong vs 3-prong topology studied by Belle. Therefore, a measurement with a statistical uncertainty competitive with the world average could already be performed with an early Belle II dataset.

Speaker: Stefano Moneta (INFN - National Institute for Nuclear Physics)

TAU2021_poster_taulifetime.pdf

10:50 The tau mass measurement at Belle II

Using the Belle II data from the early Phase III data taking, we reconstructed the tau
leptons using the 3-prong τ decays. This decay mode is used for the tau-lepton mass
measurement using the pseudomass technique developed by the ARGUS experiment. Though
this measurement is expected to be limited by statistics and imperfect knowledge of
the detector performance, we foresee that Belle II will provide the best tau mass measurement using the pseudomass technique once a larger data set with fully understood and operational detector
components is available.

Speaker: Navid Khandan Rad (Deutsches Elektronen-Synchrotron DESY)

PosterTauMass.pdf

10:50 → 12:50 Poster session: Breakout room 2

10:50 Lepton flavor violation in the Littlest Higgs Model with T parity including TeV Majorana neutrinos

LFV processes have been studied extensively in this model (JHEP 07 (2019) 154). We study systematically the phenomenological consequences of introducing inverse see-saw neutrino masses in the model (according to JHEP 12 (2019) 154) and obtain predictions on the $\tau \rightarrow \ell \gamma $ and $\tau \rightarrow \ell \ell' \ell'$ decays (other LFV decays are also analyzed) which are close to current experimental bounds. Besides, the introduction of these TeV Majorana neutrinos allows for wrong-sign $\tau \rightarrow \ell \ell \ell'$ decays, at a rate which can also be probed by Belle-II.

Speaker: Mr Iván Pacheco Zamudio (Centro de Investigación y de Estudios Avanzados )

Poster_IvanPacheco_TAU.pdf

10:50 Search for Tau -> 3mu decays with CMS experiment at LHC

New results are presented for a search for charged lepton flavor violating decays of tau leptons to three muons with the CMS detector. The search employs tau leptons produced in decays of heavy flavor B/D mesons and W bosons.

Speaker: Federica Maria Simone (Universita e INFN, Bari (IT))

TAU2021_tau3mu_poster_simone_v0.pdf

10:50 → 12:50 Poster session: Breakout room 3

10:50 Neutrino mixing and Leptogenesis with modular $S_3$ symmetry in the framework of type III seesaw

Discrete symmetries are being preferred to explain the neutrino phenomenology, we chose the simplest $S_3$ group and explore the implication of its modular form on neutrino masses and mixing. Non-trivial transformations of Yukawa couplings under this symmetry, make the model phenomenologically interesting by reducing the requirement of multiple scalar fields. This symmetry imposes a specific flavor structure to the neutrino mass matrix within the framework of less frequented type III seesaw mechanism and helps to explore the neutrino mixing consistent with the current observation. Apart, we also discuss the preferred scenario of leptogenesis to explain the baryon asymmetry of the universe by generating the lepton asymmetry from the decay of heavy fermion triplet at TeV scale.

Speaker: Subhasmita Mishra (Centurion University of Technology and Management, Odisha, India)

TAU2021.pdf

10:50 Non zero θ_13 and δ_CP in a realistic neutrino mass model with discrete A_4 family symmetry and perturbation to Tri-bimaximal mixing via z_2 ×z_2 invariant perturbation in the neutrino sector.

In this work, a flavour theory, of a neutrino mass model implementing an $A_4$ family symmetry is proposed. This scheme provides a simple way to derive tribimaximal mixing in the neutrino sector via spontaneous breaking of $A_4 $ symmetric model leading to acceptable values of $\theta_{13}$ and maximal CP violation. A $z_2 \times z_2$ invariant perturbations in this model is introduced in the neutrino sector which leads to testable predictions of $\theta_{13}$ and CP violation. By changing the magnitudes of perturbations in neutrino sector, one can generate viable values of $\theta_{13}$ and neutrino oscillation parameters.

Speaker: Gayatri Ghosh (Physics Department Gauhati University)

The 16th International Workshop on Tau Lepton Physics_GayatriGhosh.pdf

10:50 → 12:50 Poster session: Breakout room 4

10:50 Dark matter by sterile neutrino and the new Z burst road map

The presence of new relic sterile neutrinos at eV energy mass, their clustering as dark warm matter halos, their target for ZeV UHE neutrinos, might lead to UHECR uncorrelated signals with nearby sources, overcoming the GZK cut off. The ability of sterile neutrinos to solve at once different puzzle of theoretical physics and astrophysics will be shown in details.

Speaker: Daniele Fargion (Rome University)

Dark Matter and sterile neutrino2.pdf

Dark Matter and sterile neutrino2.pptx

10:50 Exploring Neutrino Masses and Mixing in the Seesaw Model with $L_e-L_\tau$ Gauged Symmetry

In the poster, we have taken $L_e-L_\tau$ gauge symmetry to study neutrino phenomenology in the framework of type-(I+II) seesaw mechanism. In the model, three heavy right-handed neutrinos, a scalar singlet, and one scalar triplet are added to the Standard Model. As a result, the active neutrino-mass matrix has a two-zero $A_1$ texture which helps explain neutrino oscillation parameters like $\theta_{13}, \theta_{23}. \theta_{12},$ the sum of active neutrino masses etc. The model also explains neutrinoless double $\beta $ decay and lepton flavor violation with reasonable accuracy. The branching ratio of $\tau \rightarrow e \gamma$ and $ \tau \rightarrow \mu \bar{\mu} \mu $ also stay well below the experimental upper bound.

Speaker: Papia Panda (University of Hyderabad)

main.pdf

10:50 → 12:50 Poster session: Breakout room 5

10:50 B boson search at KLOE/KLOE-2

Following the many contributions KLOE-2 has done to Dark Matter (DM) searches, an alternative model, where the Dark Force mediator is an hypothetical leptophobic B boson, in contra-position to the U boson or "dark photon", is investigated. The B boson couples mainly to quarks and it can be searched in the Phi decay to eta-B where B will decay in p0-gamma. So far, investigation of the pi0-gamma invariant mass shows no clear structure belonging to the signal of the DM mediator, hence, an upper limit in the number of events at 90\% with CLs the technique will be established for the decay.

Speaker: Elena Perez Del Rio (INFN - National Institute for Nuclear Physics)

poster_EPR_Tau2021.pdf

10:50 Search for upward-going air showers with the fluorescence detector of the Pierre Auger Observatory

The fluorescence detector (FD) of the Pierre Auger Observatory is sensitive to upward-going air showers for energies above 10^17 eV. Given its operation time and wide field of view, the FD has the potential to support or constrain the recent "anomalous" observations by the ANITA detector, interpreted as upward-going air showers of unexplained nature.

We have used 14 years of data collected by the FD to search for upward-going showers using a set of quality selection criteria defined using 10% of the full data sample.
To distinguish candidates from false positives, calculate the exposure and obtain the expected background, dedicated simulations for signal (upward-going events) and background (downward-going events) have been performed.
Results of the analysis after unblinding the data set are presented.

Finally, the exposure and sensitivity for the specific scenario of a signal being ascribed to tau lepton decay are calculated and the corresponding upper limits are shown as a function of primary energy and in different zenith angle ranges.

Speaker: Emanuele De Vito (INFN - National Institute for Nuclear Physics)

TAU_poster_EDV.pdf

10:50 → 12:50 Poster session: Breakout room 6

10:50 Search for lepton-flavor-violating decays of the Tau lepton at a future muon collider

Tau leptons can have lepton-flavor-violating (LFV) couplings to a muon or an electron and an Axion-Like Particle (ALP). ALPs are pseudo Nambu-Goldstone bosons associated with the spontaneously broken global $U(1)_{\mathrm{PQ}}$ symmetry. LFV ALPs have been of a great interest in the last several decades as they can solve some of the SM long-lasting problems. Assuming a future muon collider suggested by the Muon Accelerator Program (MAP), we search for LFV decays $\tau\rightarrow\ell a$ ($\ell=e,\mu$) of one of the tau leptons produced in the muon-anti muon annihilation. The ALP mass $m_a$ is assumed to be in the range 100 eV to 1 MeV and three different chiral structures are considered for the LFV coupling. Using a multivariate technique and performing a realistic detector simulation, we obtain expected 95$\%$ confidence level upper limits on the LFV couplings $c_{\tau e}$ and $c_{\tau \mu}$. Limits are computed assuming the center-of-mass energies of 126, 350 and 1500 GeV which the future muon collider is supposed to operate at. Furthermore, we study the two cases of unpolarized and polarized muon beams independently and show that taking advantage of tau polarization-induced effects in the polarized muon beams case can significantly suppress the main SM background $\tau\rightarrow e/\mu+\nu \bar{\nu}$ which overwhelms the signal. The obtained results indicate that current experimental limits on the $c_{\tau e}$ and $c_{\tau \mu}$ couplings can be improved by roughly one order of magnitude with the help of the present analysis.

Speaker: Gholamhossein Haghighat (Institute for Research in Fundamental Sciences (IR))

IndianaWS.pdf

10:50 Searching for lepton flavor violating interactions at future electron-positron colliders

Lepton flavor violating interactions are absent in the standard model but are expected in various beyond-standard models. In this work, the potential of the future circular electron-positron collider to probe the four-fermion lepton flavor couplings via the e-e+ → eτ process is revisited by means of an effective field theory approach. We provide constraints at 95% C.L. on the dimension-six Wilson coefficients including major sources of background processes and considering realistic detector effects at four expected operation energies, 157.5, 162.5, 240, and 365 GeV, according to their corresponding integrated luminosities. We demonstrate that the statistical combination of the results from four center-of-mass energies improves the sensitivity to the lepton flavor violation couplings significantly. We compare the results with the prospects from the Belle II Collaboration with 50 /ab and other studies at electron-positron colliders.

Speaker: Dr Reza Jafari (Institute for Research in Fundamental Sciences (IPM))

Tau2021Reza.pdf

10:50 → 12:50 Poster session: Breakout room 7

10:50 Neutrino phenomenology via Type-(I+II) seesaw in a $U(1)_{L_e-L_\mu}$ model

We realize neutrino phenomenology via Type-(I+II) seesaw
mechanism in a simple $U(1)_{L_e-L_\mu}$ gauge extension of standard model. With three additional right-handed neutrinos and a scalar triplet, we obtain two-zero $A_2$ texture in active neutrino mass matrix. We constrain the model parameters consistent with current neutrino oscillation data. Furthermore, we obtain new contributions to muon $g-2$ and also charged lepton flavor violating decays such as $\mu \to e\gamma$.

Speaker: Priya Mishra (University of Hyderabad)

taup_2021_poster-1.pdf

10:50 T violating effects in $\nu_{\tau} (\bar{\nu}_{\tau})-$nucleon quasielastic scattering

The future experiments like SHiP, DsTau, and DUNE are proposed to study the properties and the production cross sections of the $\tau$ lepton and its corresponding neutrino ($\nu_{\tau}$). Recently we have performed [1,2,3], a theoretical study of the production cross section as well as the polarization observables of the $\tau$ lepton and the final nucleon/hyperon produced in the quasielastic $\nu_{\tau} (\bar{\nu}_{\tau})-N$ scattering in the few GeV energy region relevant to the above experiments. The $\tau$ lepton produced in $\nu_{\tau}-N$ scattering decays to leptons and pions through the leptonic and hadronic decay modes. In this energy region, the production cross section of $\tau$, its decay and the characteristics of the decay products depend significantly on the $\tau$ polarization. The production cross section and polarization of $\tau$ lepton are calculated using weak nucleon form factors which are determined using various symmetry properties of the weak currents in the vector and axial vector sectors, assuming G and T invariances. We have studied the effect of G and T violating terms in the transition matrix element on the cross sections and the $\tau$ polarization in quasielastic $\nu_{\tau}(\bar{\nu}_{\tau})-N$ scattering induced by $\Delta S=0$ and $\Delta S=1$ weak currents. In the case of $\Delta S=1$ reactions, we have also studied the SU(3) symmetry breaking effects.

[1] A. Fatima, M. Sajjad Athar and S. K. Singh, Phys. Rev. D 102, 113009 (2020).
[2] A. Fatima, M. Sajjad Athar and S. K. Singh, [arXiv:2106.14590 [hep-ph]].
[3] A. Fatima, M. Sajjad Athar and S. K. Singh, Phys. Rev. D 98, 033005 (2018).

Speaker: Atika Fatima (Aligarh Muslim University)

Fatima_TAU2021.pdf

10:50 → 12:50 Poster session: Breakout room 8

10:50 Implications of new physics in $ \Lambda_b \to \Lambda_c \ell \nu_\ell $ decay processes.

Several indications of lepton non universality ratios, $R_{D^*}$, $R_{J/ \psi}$ and the measurements on hadronic and $\tau$ longitudinal polarizations in $b \to c \tau \bar{\nu_\tau}$ processes have attracted a lot of attentions. By considering the most general effective Lagrangian, we carry out a model independent analysis of the semileptonic $\Lambda_b$ decays, to inspect the nature of new physics. We constraint the new physics parameter space by using the measured branching ratios of $ {B_c}^+ \to \tau^+ \nu_\tau$ and the keep going experimental results on $R_{D^*}$, $R_{J/ \psi}$ through a chi square fitting. We study the implications of constrained new couplings on the observable such as branching fractions, forward-backward asymmetries, lepton non universality parameter and $ \Lambda_c$ and lepton longitudinal polarization fractions of the decay modes. Additionally, we also probe whether there could be any lepton universality violation in this decay processes.

Speaker: Ms Aishwarya Bhatta (University of Hyderabad)

Aish_tau_2021.pdf

10:50 Semi- and di-tauonic B decays and LFUV at future $e^+e^-$ colliders

$b\to s\tau^+\tau^-$ measurements are highly motivated for addressing lepton-flavor-universality (LFU)-violating puzzles such as $R_{K^{(\ast)}}$ anomalies. The anomalies of $R_{D^{(*)}}$ and $R_{J/\psi}$ further strengthen their necessity and importance, given that the LFU-violating hints from both involve the third-generation leptons directly. $Z$ factories at the future $e^-e^+$ colliders stand at a great position to conduct such measurements because of their relatively high production rates and reconstruction efficiencies for $B$ mesons at the $Z$ pole. To fully explore this potential, we pursue a dedicated sensitivity study in four $b\to s\tau^+\tau^-$ benchmark channels, namely $B^0\to K^{\ast 0} \tau^+ \tau^-$, $B_s\to\phi \tau^+ \tau^-$, $B^+ \to K^+ \tau^+ \tau^- $ and $B_s \to \tau^+ \tau^-$, at the future $Z$ factories. We develop a fully tracker-based scheme for reconstructing the signal $B$ mesons and introduce a semi-quantitative method for estimating their major backgrounds. The simulations indicate that branching ratios of the first three channels can be measured with a precision $\sim \mathcal O(10^{-7} - 10^{-6})$ and that of $B_s \to \tau^+ \tau^-$ with a precision $\sim \mathcal O(10^{-5})$ at Tera-$Z$. The impacts of luminosity and tracker resolution on the expected sensitivities are explored. The interpretations of these results in effective field theory are also presented.

Speaker: Lingfeng Li (Brown U.)

LFUV_ditau_tau_2021.pdf

10:50 → 12:50 Poster session: Breakout room 9

10:50 Dark matter and flavor anomalies with vector-like fermions and scalar leptoquark

We investigate vector-like fermionic dark matter and flavor anomalies in a simple extension of standard model, with doublet vector-like fermions of quark and lepton type and also a $S_1(\bar{\textbf{3}},\textbf{1},1/3)$ scalar leptoquark. An additional vector-like lepton singlet is included, whose admixture with vector-like lepton doublet plays the role of dark matter and is examined in relic density and direct detection perspective. We utilize the bounds from electroweak precision observables and also constrain the new couplings from the branching ratio and angular observables associated with $b \to sll (\nu_l \bar \nu_l)$, $b \to s \gamma$ decays. We estimate the branching ratios of the rare lepton flavor violating $\tau$ decays such as $\tau \to \mu (\gamma, \phi, \eta, \eta^\prime)$.

Speaker: Shivaramakrishna Singirala (School of physics, University of Hyderabad, Hyderabad 500046)

Poster_Shivaramakrishna.pdf

10:50 Impact of (axial)vector coefficients on $B\to K_1 \ell \ell$ decay modes

We analyse the rare semileptonic decays of $B$ meson to axial vector mesons $K_1(1270)$ and $K_1(1400)$ mediated by the flavor changing neutral current $b \to s ll$ quark level transition, in an effective field theory approach. We perform a global fit to all the relevant and up-to-date $b \to sl^+ l^-$ data for various sets of (axial)vector couplings. We then look over the implications of the allowed parameter space on the branching ratios and several phyical observables such as forward-backward asymmetry, lepton polarization asymmetry and lepton flavor universality violating parameters of $B \to K_1 l^+ l^-$ processes.

Speaker: Suchismita Sahoo (Central University of Karnataka)

Tau21-Poster_Suchismita.pdf

12:50 → 13:20 Lunch

13:20 → 14:35 Session 7: Future directions

Convener: Thomas Browder (University of Hawaii)

13:20 Breaking the Myth of the "Non-Traditional" Physicist

Speaker: Crystal Bailey (Americal Physical Society)

Bailey_TAU2021 Career Talk.pdf

VideoTau2021_Bailey.mp4

13:50 Future directions on tau physics with Belle II

Speaker: Ami Rostomyan (DESY)

Rostomyan_tau2021.pdf

VideoTau2021_Rostomyan.mp4

14:15 FCC-ee - also a tau factory

The proposed FCC-ee collider provides optimal conditions for ultimate statstics studies of the
four heaviest particles of the Standard Model, the Z, W and Higgs bosons and the top quark.
With the phenomenal sample of 5x10^12 Z decays in the very clean e+e- environment it also
provides optimal conditions for precision studies of heavy flavour, among that studies of the
tau lepton. Possibilities are opened of much improved determinations of τ-lepton properties and,
via the measurement of the τ polarisation, of the neutral-current couplings of electrons and τs.
Improved measurements of τ-lepton properties – lifetime, leptonic branching fractions, and mass –
allow important tests of lepton universality. The experimental challenge is to match as far as
possible statistical uncertainties at the 10−5 level. This applies in particular to the lifetime
measurement, and to the branching fraction and polarisation measurements, where the cross-channel
contamination is of particular concern. These issues raise strict requirements, in particular, on
the accuracy of the construction and alignment of the vertex detector and of the precise calorimetric
separation and measurement of photons and π0s in the collimated τ decay topologies.

The talk will review the status of the FCC-ee project and summarise the opportunities for ultimate
precision tau-lepton measurements.

Speaker: Mogens Dam (University of Copenhagen (DK))

tau21_011021.pdf

VideoTau2021_Dam.mp4

14:35 → 14:50 Break

14:50 → 16:30 Session 7: Future directions

Convener: Zhiqing Philippe Zhang (IJCLab, Orsay (FR))

14:50 Status of the MUonE experiment

The latest measurement of the muon g-2, recently announced at Fermilab, exhibits a 4.2$\sigma$ discrepancy from the currently accepted Standard Model prediction. The leading hadronic contribution $a_{\mu}^{HLO}$ represents the main source of uncertainty on the theoretical value, and is traditionally determined by a data-driven dispersive approach. In contrast, a recent evaluation of $a_{\mu}^{HLO}$ based on lattice QCD weakens the discrepancy between theory and experiment to 1.5$\sigma$. Therefore, an independent crosscheck of $a_{\mu}^{HLO}$ is required to solve this tension and consolidate the theoretical prediction.
The MUonE experiment proposes a novel approach to determine $a_{\mu}^{HLO}$ by measuring the running of the electromagnetic coupling constant in the space-like region, via $\mu-e$ elastic scattering. The measurement will be performed by scattering a 160 GeV muon beam, currently available at CERN's North Area, on the atomic electrons of a low-Z target. A Test Run on a reduced detector is planned in 2021-2022, to validate this proposal. The status of the experiment in view of the Test Run will be presented.

Speaker: Riccardo Nunzio Pilato (Universita & INFN Pisa (IT))

TAU2021_MUonE_Pilato.pdf

VideoTau2021_Pilato.mp4

15:10 Status of Mu2e II

Speaker: Gianantonio Pezzullo (Yale University)

Pezzullo-Mu2e-II-talk-20211001-TAU2021.pdf

VideoTau2021_Pezzullo.mp4

15:35 Physics Prospects of Beam Polarization at Belle II

Having the SuperKEKB e+e- collider upgraded with a polarized e- beam is under consideration, providing a unique program of precision electroweak and other physics at 10.6 GeV, thereby opening exciting new windows in search of new physics. Measurements of left-right asymmetries ($A_{LR}$) of e+e- transitions to pairs of taus, muons, electrons, c- and b-quarks would yield improvements to the determination of $\sin^2\theta_W$ compared to those made at the Z-pole precision but at much lower energy. These will probe the running and universality of neutral current couplings with unprecedented precision, opening new ways to search for dark sector effects. Other tau and QCD physics is also enhanced. This paper will focus on the physics prospects with a special emphasis on tau physics.

Speaker: Michael Roney (University of Victoria)

ChiralBelle-Tau2021-1Oct2021.pdf

VideoTau2021_Roney.mp4

16:00 Outlook

Speaker: Kiyoshi Hayasaka (Niigata University)

Hayasaka_Outlook_tau2021.pdf

VideoTau2021_Hayasaka.mp4