LF(U)V Workshop

BIN-0-K.02 (University of Zurich)


University of Zurich

Binzmühlestrasse 14 8050 Zürich
Claudio Andrea Manzari (University of Zurich (CH)), Felix Wilsch, Fiona Kirk, Julie Pages, Nudzeim Selimovic

LF(U)V Workshop: July 04-06, 2022

This workshop focuses on lepton flavor violation (LFV) and lepton flavor universality violation (LFUV) in particle physics. Searches for these effects are particularly promising tests of the Standard Model (SM) as they involve very clean observables within the reach of current and near-future experiments. Hints for LFUV have shown up in different semileptonic B decays at LHCb and in other experiments. If confirmed, this would be a clear sign for physics beyond the SM, requiring the development of new fundamental theories. 

This workshop is geared towards young researchers (of both theoretical and experimental particle physics) and aims at giving them the opportunity to gain a better overview of the theory and current efforts in the experimental searches for lepton flavour (universality) violation. Theoretical review talks and more in-depth presentations on recent theoretical and experimental developments will be given by leading experts in the field. During and after all presentations there will be ample time for exchange and discussions. Furthermore, a platform will be offered to the young researchers to present their current work.

Confirmed Speakers:

  • Wolfgang Altmannshofer (UC Santa Cruz)

  • Marzia Bordone (CERN)

  • Sacha Davidson (U. Montpellier)

  • Javier Fuentes-Martín (Granada U.)

  • Martin Hoferichter (Bern U.)

  • Tobias Hurth (Mainz U.)

  • Patrick Owen (Zurich U.)

  • Angela Papa (PSI)

  • Peter Stangl (Bern U.)


Due to an improved funding situation, we are able to wave the participation fee for the workshop dinner.

Registrations are now closed.

The workshop is held at the University of Zurich in Switzerland in a completely on-site format.

Funding by the UZH Graduate Campus, the Pauli Center for Theoretical Studies, and the UZH Alumni is gratefully acknowledged.


Fiona Kirk, Claudio Andrea Manzari, Julie Pagès, Nudžeim Selimović, and Felix Wilsch

  • Aleks Smolkovic
  • Amir Khan
  • Anders Eller Thomsen
  • Andrea Gurgone
  • Andreas Crivellin
  • Annarita Buonaura
  • Antonio Coutinho
  • Arman Korajac
  • Ben Stefanek
  • Caterina Aruta
  • Clara Fernandez Castañer
  • Claudio Andrea Manzari
  • Cristina Martin Perez
  • Daniel Unverzagt
  • Emily Hampshire
  • Felix Wilsch
  • Fiona Kirk
  • Gino Isidori
  • Giovanni Dal Maso
  • Hector Gisbert Mullor
  • Hicham Benmansour
  • Iaroslava Bezshyiko
  • Jason Aebischer
  • Javier Fuentes-Martin
  • Javier M. Lizana
  • Jean-Loup Tastet
  • Joseph Davighi
  • Julie Pagès
  • Julien Laux
  • Katharina Schleicher
  • Kuldeep Deka
  • Luc Schnell
  • Ludovico VITTORIO
  • Lukas Allwicher
  • Majid Ekhterachian
  • Marco Ardu
  • Marco Milesi
  • Maria Carolina Feliciano Faria
  • Marko Pesut
  • Martina Ferrillo
  • Marvin Schnubel
  • Marzia Bordone
  • Mehmet Asim Gumus
  • Miaoyuan Liu
  • Michele Atzeni
  • Naman Kumar Bhalla
  • Nathaniel Sherrill
  • Nudzeim Selimovic
  • Patrick Owen
  • Peter Stangl
  • Peter Stoffer
  • Sacha Davidson
  • Sandro Mächler
  • Tobias Hurth
  • Tom Steudtner
  • Wolfgang Altmannshofer
  • Yannick Demets
  • Yi Chung
  • Òscar Lara Crosas
    • 8:30 AM
      Registration BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
    • 1
      LFUV review BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Wolfgang Altmannshofer (UC Santa Cruz)
    • 10:30 AM
      Coffee break
    • 2
      Overview of experimental LFU tests BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Patrick Haworth Owen (University of Zurich (CH))
    • 3
      LFV experiments BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Angela Papa
    • 12:45 PM
      Lunch break
    • Young researcher session BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      • 4
        Explaining B-Anomalies with safe Z' extensions

        $U(1)'$ extensions of the SM with generation-dependent couplings to quarks and leptons are a popular explanation of anomalies in rare B-decays, such as the $R_K$ and $R_K^*$ observables. However, a critical issue with such models is often ignored -- most configurations feature a Landau pole in the new gauge coupling that is well below the Planck scale. We present an ansatz that avoids such Landau poles, stabilises the Higgs potential and remains well-behaved and predictive up to the Planck scale. To these ends, we introduce three generations of vector-like standard model singlet fermions, an enlarged, flavourful scalar sector, and, possibly, right-handed neutrinos, all suitably charged under the $U(1)'$ gauge interaction. We identify several benchmark models consistent with theory as well as phenomenological constraints and their respective predictions of Wilson coefficients for a global $b \to s$ fit. We further investigate the complete two-loop running of gauge, Yukawa and quartic couplings up to the Planck scale to constrain low-energy parameters and map out the BSM critical surface. Moreover, some key phenomenological aspects are highlighted.

        Speaker: Tom Steudtner
      • 5
        A novel theoretical approach to $R(D^{(*)})$ through the Dispersive Matrix method

        The ratios $R(D^{(*)})$ are defined as the $\tau/\mu$ ratio of the branching fractions of exclusive semileptonic $B \to D^{(*)}$ decays. They are a powerful test of Lepton Flavour Universality, one of the pillars of the SM, and are determined by the hadronic Form Factors (FFs) describing the $B \to D^{(*)}$ decays. Through the novel Dispersive Matrix (DM) approach, we describe these FFs without assumptions on their functional dependence on the momentum transfer. The DM method is based on the non-perturbative determination of the dispersive bounds due to unitarity and analyticity, and allows to determine in a model-independent way the FFs in the full kinematical range, starting from existing Lattice QCD data which are available only at large momentum transfer. Using the unitarity bands of the FFs, which are thus independent of the experimental determinations of the differential decay widths, we compute new fully-theoretical expectations of the anomalies. Our results read $R(D) = 0.296(8)$ and $R(D^*) = 0.275(8)$, each of which is compatible with the corresponding world average of the measurements at the $\sim 1.3 \sigma$ level. The origin of the discrepancy between the DM estimates and the HFLAV SM predictions will be also discussed.

        Speaker: Ludovico VITTORIO (SNS, Pisa)
      • 7
        Connecting the B anomalies with the hierarchy problem

        Intriguing hints for lepton flavor universality violation have emerged from the measurement of B meson decays by LHCb. These B anomalies point to the new physics at the low TeV scale, exactly where we expect a solution to the hierarchy problem! In this talk, I will present an economical model that can address the two problems at once. The model is based on a SU(4)/SP(4) fundamental composite Higgs model. The hierarchy problem can be solved by the compositeness of the pNGB Higgs, and the observed neutral current B anomalies can be explained by the additional $Z’$ boson in the model. The model connects the energy scales of the EWSB and the B anomalies. I will discuss the experimental constraints from Higgs physics, flavor physics, and LHC direct searches.

        Speaker: Yi Chung
      • 8
        New Physics in missing transverse energy tails with $b$-tagged jets

        Dilepton tail distributions have proven to be a very good ground for limiting different New Physics (NP) scenarios. Assuming NP in left-handed particles,the operators $c^{(1)}_{ijk}(\bar{Q_i} \gamma^{\mu} Q_{j})(\bar{L_k}\gamma_\mu L_k)$ and $c^{(3)}_{ijk} (\bar{Q_i} \gamma^{\mu} \sigma_{a} Q_{j})(\bar{L_k}\gamma_\mu \sigma^{a} L_k) $ also induce a neutral-current interaction with neutrinos, which would eventually leave imprints in high-$p_T$ measurements. We, therefore, take a look into the analysis of missing transverse energy tails with accompanying $b$-jets, experimentally done in ATLAS searches \cite{ATLAS:2017hoo, ATLAS:2014dbf}, and derive current bounds on the linear combination $(c^{(1)} - c^{(3)})$. Furthermore, we investigate in detail different cuts and their efficiencies and see how these limits, together with the limits from the dilepton signatures, can give us an insight about the preferred region in the parameter space of these Wilson Coefficients. Finally, we also take into account constraints from low-energy experiments, like $\Upsilon(1S) \to \bar{\nu} \nu$ and $B \to K \bar{\nu} \nu$.

        Speaker: Mr Arman Korajac (Jožef Stefan Institute)
      • 9
        On Drell-Yan production of leptoquarks coupling to heavy quark flavours

        Given the hints of lepton-flavour non-universality in semi-leptonic~$B$ decays, leptoquark (LQ) models with sizeable couplings to heavy quark flavours are enjoying a renaissance. While such models are subject to stringent constraints from low-energy experiments also bounds from non-resonant dilepton searches at the Large Hadron Collider~(LHC) turn out to be phenomenologically relevant. Based on the latest LHC dilepton analyses corresponding to an integrated luminosity of around~$140 \, {\rm fb}^{-1}$ of proton-proton collisions at~$\sqrt{s} = 13 \, {\rm TeV}$, we present improved limits on the scalar~LQ couplings that involve heavy quark flavours and light or heavy~dileptons. In particular, we show that effects beyond the leading order that are related to real QCD emissions are relevant in this context, since the inclusion of additional heavy-flavoured jets notably improves the exclusion limits that derive from the high-mass dilepton tails. A dedicated Monte Carlo code within the {\tt POWHEG-BOX}~framework that allows for an on-the-fly signal event generation including all LQ corrections we considered is introduced.

        Speaker: Luc Schnell (Max Planck Institute for Physics)
    • 3:45 PM
      Coffee break
    • 10
      From the global b→sll fit to models of new physics BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Peter Stangl (University of Bern)
    • 11
      Non perturbative physics in rare and semileptonic decays BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Marzia Bordone (CERN)
    • 12
      Searching for physics beyond the Standard Model with lepton dipole moments BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Martin Hoferichter
    • 10:30 AM
      Coffee break
    • 13
      New Physics Models for the muon (g-2) BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Wolfgang Altmannshofer (UC Santa Cruz)
    • 12:45 PM
      Lunch break
    • Young researcher session BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      • 14
        Measurement of the X17 anomaly with the MEG II detector

        In 2016 the ATOMKI collaboration measured an anomaly in the angular distribution of the pair produced by the M1 transition of the isoscalar 1+ state on 8Be, which might be explained by creation and decay of a boson, the X17, with mass 16.7 MeV/c2.

        The MEGII detector is thought to measure the cLFV decay mu to e and gamma, but it could measure the X17 anomaly with a different detection technique and refuse or validate the results from ATOMKI.

        We plan on using the Cylindrical Drift Chamber and the Constant Bending RAdius magnet of the MEG II experiment to measure the momentum of the electron-positron pair, with timing information coming from the pixelated Timing Counter.

        The reaction will be produced impinging a 1050 keV proton beam on a LiPON target, and a set of scintillating detectors will be used to detect the gammas coming from concurrent nuclear processes to monitor the status of the target as time elapses.

        We present the current status of the set-up, the first data taking period in February 2022 and the plan for the coming beam time.

        Speaker: Giovanni Dal Maso
      • 15
        Gearing up for the next generation of LFV experiments, via on-shell methods

        Lepton Flavor Violating (LFV) observables such as $\mu\to e\gamma$, $\mu\to 3e$ and $\mu N \to eN$ are among the best probes for new physics at the TeV scale. In the near future the bounds on these observables will improve by many orders of magnitude. In this work we use the SM EFT to understand the impact of these measurements. The precision reach on the measurement of these processes is such that the interpretation of the bounds requires an analysis of the dimension-six operator mixing up to the two-loop level. Using on-shell amplitude techniques, which make transparent many selection rules, we classify and calculate the different operator mixing chains. At the leading order, on-shell techniques allow to calculate anomalous dimensions of SM EFT operators from the product of tree-level amplitudes, even for two-loop renormalization group mixings. We illustrate the importance of our EFT approach in models with extra vector-like fermions.

        Speaker: Clara Fernandez Castañer (IFAE)
      • 16
        Axion-like Particles and Lepton-Flavor Violation

        Axion-like Particles (ALPs) appear in many well-motivated extensions of the Standard Model. We present a detailed study of the phenomenology of ALPs with lepton flavor-violating couplings. Studied observables include rare LFV decays of muons and taus, non-decay experiments such as the electron and muon electric dipole moment (EDM) and muonium oscillations, and as well the effect flavor-violating ALP-couplings could have on the long-lasting discrepancy between Standard Model expectation value and measurement in $(g-2)_\mu$. We highlight that for ALPs that can be resonantly produced, the sensitivity of three-body decays such as $\mu\to3e$ and $\tau\to3\mu$ exceeds by many orders of magnitude that of radiative decays like $\mu\to e\gamma$ and $\tau\to\mu\gamma$. Searches for these two types of processes are therefore highly complementary.

        We derive bounds on LFV ALP couplings in dependence of the ALP mass and consistently take lifetime-effects and experimental features like detector geometry and experimental timing cuts into account. Our results are presented in benchmark-scenarios where only one LFV coupling is present at a time.

        Keywords: Axion-like Particles, ALP, Lepton-Flavor Violation

        Speaker: Marvin Schnubel
      • 17
        ALP-EFTs for additionally gauged U(1) symmetries

        We construct an effective theory (EFT) of an axion-like particle (ALP) for an additionally gauged $U(1)$ symmetry. The Z' is associated to a global current of the Standard Model (SM), such as baryon ($B$) or lepton ($L_i$) numbers. In order to fulfill anomaly conditions as well as a trace condition, new fermions have to be introduced. Integrating out these fermions induces a kinetic mixing between the $\gamma$, Z and Z' bosons as well as the operators in the ALP-EFT. The matching conditions for the corresponding Wilson coefficients are determined by general calculations in which we allow all couplings to be flavor violating. We find new operators which are not covered by standard ALP-EFTs.

        Speaker: Julien Laux
    • 3:15 PM
      Coffee break
    • 18
      Interactive model building session BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Javier Fuentes-Martin (Universidad de Granada)
    • 7:00 PM
      Workshop dinner eCHo Restaurant, Zürich Marriott Hotel

      eCHo Restaurant, Zürich Marriott Hotel

      Neumühlequai 42, CH-8001 Zürich, https://goo.gl/maps/TgGFYdHCKs4ouZh5A
    • 19
      LFV, Mostly EFT BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Sacha Davidson (Centre National de la Recherche Scientifique (FR))
    • 10:30 AM
      Coffee break
    • 20
      Leptogenesis BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Sacha Davidson (Centre National de la Recherche Scientifique (FR))
    • 21
      Semileptonic Inclusive Penguins BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      Speaker: Tobias Hurth (Johannes Gutenberg Universitaet Mainz (DE))
    • 1:00 PM
      Lunch break
    • Young researcher session BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      • 22
        A viable $L_e−L_\mu$ model with $\mu\to e$ violation

        We extend the Standard Model gauge group by $U(1)_{L_e-L_\mu}$ and introduce two scalars, a doublet and a singlet, that are charged under this new group and have lepton flavour violating couplings. Since in this model $\mu \to e$ processes can only be mediated by $\mu\to \tau\times \tau\to e$ interactions, bounds from $\mu\to e$ transitions can be avoided while allowing for accessible new physics. We consider a $Z'$ boson with a mass of $M_{Z'}\simeq10~$GeV and a gauge coupling $g'\simeq 10^{-4}$, which is in reach of Belle-II. Neutrino masses and mixing angles can also be accounted for if sterile neutrinos are added to the spectrum.

        Speaker: Marco Ardu
      • 23
        Search for cLFV mediated by a new light particle at PSI

        The search for charged Lepton Flavour Violation (cLFV) in muon decays is a sensitive tool to probe the Standard Model (SM). In this context, the MEG II and Mu3e experiments at the Paul Scherrer Institut (PSI) search for $\mu^+ \to e^+\gamma$ and $\mu^+ \to e^+e^-e^+$, respectively. In addition to their main channels, both experiments appear to be competitive in searching for more exotic processes, in which the flavour violation occurs in presence of an invisible axion-like particle (ALP), denoted with $X$. A suitable candidate is the two-body decay $\mu^+ \to e^+ X$, whose only signature is a monochromatic positron close to kinematic endpoint of the $\mu^+ \to e^+ \nu_e \bar\nu_\mu$ background.

        Since the higher-order radiative corrections in this region are greatly enhanced by the emission of soft photons, the experimental hunt for such an elusive signal requires extremely accurate theoretical predictions. This is one of the problems that led to the development of McMule, acronym of Monte Carlo for MUons and other LEptons. McMule is a generic framework for the numerical computation of fully-differential QED corrections for low-energy processes involving leptons, based on the FKS$^2$ subtraction scheme.

        In this presentation, we focus on the implementation of $\mu \to e X$ and $\mu \to e \nu \bar\nu$ in the McMule framework. In both cases the muons are assumed to be polarised and the electron mass is kept at its physical value. The signal $\mu \to e X$ is computed using an effective field approach, including the QED corrections at next-to-leading order (NLO). The background $\mu \to e \nu \bar\nu$ includes weak corrections at NLO, hadronic contributions at LO, exact QED corrections at NNLO and logarithmically approximate at N$^3$LO and N$^4$LO. Going beyond fixed-order calculations, an analytical resummation of soft emissions is also included. This results in a theoretical precision of $10^{-5}$ on the positron energy spectrum, the highest achieved for polarised muon decay.

        As a preliminary study, we assume the nominal performances of the MEG II and Mu3e experiments to estimate their sensitivity on $\mu \to e X$ for different masses and couplings of the ALP. The branching ratio of this hypothetical process has been limited to $5.8\cdot10^{-5}$ by the TWIST experiment. We show that MEG II can provide an independent measurement close to this limit, while Mu3e can improve it up to three orders of magnitude. In this regard, the McMule predictions are currently being implemented in the experimental analysis codes for more detailed studies.

        Speaker: Andrea Gurgone (Pavia U. & INFN)
      • 24
        Search for lepton flavour violation in charm decays at LHCb

        LHCb is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which might reveal effects beyond the Standard Model. This talk aims in particular to motivate the search for lepton flavour violation (LFV) in charm decays. A short overview of recent LFV results in the charm sector at LHCb is presented, as well as the prospects for future measurements exploiting new topologies, such as four-body decays of the form $D^0\to hh \mu e$ with $h = \pi^\pm,K^\pm$.

        Speaker: Daniel Unverzagt (Ruprecht Karls Universitaet Heidelberg (DE))
      • 25
        Dineutrino modes probing LFV

        $SU(2)_L$-invariance links charged dilepton $\bar{q}\,q^\prime\,\bar{\ell}\,\ell$ and dineutrino $\bar{q}\,q^\prime\,\bar{\nu}\,\nu$ couplings. This connection can be established using SMEFT and holds if only SM-like left-handed light neutrinos are present. It allows to perform complementary experimental tests of lepton universality and charged lepton flavor conservation with flavor-summed dineutrino observables. Phenomenological implications are worked out for flavor changing neutral currents involving strange, charm and beauty quark transitions in a model-independent way.

        Speaker: Hector Gisbert
    • 3:30 PM
      Coffee break
    • Young researcher session BIN-0-K.02


      University of Zurich

      Binzmühlestrasse 14 8050 Zürich
      • 26
        Mu3e looking for rare muon decays

        Mu3e is a dedicated experiment to search the rare charged lepton flavor violating (cLFV) decay $\mu^{+} \rightarrow{} e^{+}e^{-}e^{+}$ with a sensitivity down to $10^{-16}$ under construction at PSI. In the Standard Model, this decay is heavily suppressed with a branching fraction of $10^{-54}$. The Mu3e experiment will be able to reconstruct low momentum electrons and positrons from rare $\mu$ decays. Mu3e apparatus consists of a tracking detector based on monolithic active pixel sensors for very precise momentum and vertex reconstruction, combined with scintillating fibers and tiles for very high timing measurements. The motivation for the $\mu^{+} \rightarrow{} e^{+}e^{-}e^{+}$ search will be presented, along with experimental design and subsequent expected sensitivity.

        Speaker: Yannick Demets (University of Geneva)
      • 27
        Precision timing in the Mu3e experiment towards the search for the muon decay to three electrons

        Mu3e is an experiment under construction at the Paul Serrer Institute to search for the charged lepton flavor violating $\mu^+ \rightarrow e^+e^-e^+$ decay at branching fractions above $10^{-16}$. As this decay is heavily suppressed in the Standard Model, its observation would unambiguously indicate the existence of new physics. Achieving such sensitivity requires a high rate of muons and a detector with large kinematic acceptance; hence, excellent time resolution is essential to suppress the accidental background and to facilitate the global event reconstruction. In particular, the scintillating fiber (SciFi) sub-detector is mounted in the central Mu3e region and is designed to achieve a very precise time measurement with a very high efficiency and rate capability. In this talk, the SciFi design and performance is presented in the context of the demanding Mu3e detector requirements.

        Speaker: Cristina Martin Perez (ETH Zurich (CH))
      • 28
        Lepton number & flavor violation in realistic HNL models

        Heavy neutral leptons (HNLs) are hypothetical heavy states mixing with the Standard Model neutrinos. They were proposed to explain neutrino masses, dark matter and the baryon asymmetry of the Universe. HNLs have been searched for in numerous experiments, however most of them report their limits under the assumption of one HNL mixing with a single neutrino flavor. Although convenient, we will see that this assumption is inconsistent with neutrino oscillation data. This has important consequences for searches. First, an approximate lepton number symmetry between two HNLs (required in order to reproduce light neutrino masses with large mixing angles) may suppress lepton-number-violating signatures. Second, lepton-flavor-violating decays are a solid prediction of most realistic HNL models, but they are absent if HNLs are assumed to mix with only one flavor. In this talk I will present a reinterpretation of an existing ATLAS search for HNLs decaying promptly to three leptons, as well as the results from a recent ATLAS search for displaced HNLs, in which limits were interpreted from the start within realistic models. I will discuss the roadblocks encountered along the way, and propose a method that enables theorists to easily and accurately interpret the limits for any mixing pattern and any number of HNLs.

        Speaker: Jean-Loup Tastet (UAM-IFT)
      • 29
        Constraints on Lorentz and CPT violation from lepton-flavor-violating decays

        Lorentz and CPT invariance are among the most fundamental physical symmetries. Progress in recent years has led to stringent constraints on several coefficients parametrizing Lorentz and CPT violation coupled to the fields of the standard model and general relativity. However, nonrenormalizable operators inducing tree-level lepton-flavor-violating decays remain completely unexamined. In this talk, I discuss the dominant dimension-five operators of this type initiating flavor-changing muon and tau decays and their resulting bounds using existing MEG and BaBar measurements. The outlook for improved constraints in future experiments will also be discussed.

        Speaker: Nathaniel Sherrill (University of Sussex)