Workshop on BSM Physics at Eta-Factories

13 May 2025, 08:00 → 16 May 2025, 17:00 Europe/Stockholm

Lund University Fysiska Institutionen, Sweden

Andrzej Kupsc, Bastian Kubis (University of Bonn (DE)), Corrado Gatto (INFN & NIU), Hao Qiu, Johan Bijnens (Lund University), Liping Gan, Marcin Zielinski (Jagiellonian University (PL)), Sean Tulin (York University), Valentina Santoro (ESS)

Workshop "Beyond Standard Model Physics at Eta-Factories".

Currently, there are three eta-factory facilities at various stages of development: JEF, REDTOP, and the HIAF Spectrometer. The MeV-GeV energy range is emerging as a compelling area for potential discoveries, offering exciting opportunities in high-energy physics. This workshop seeks to emphasize the discovery potential of eta-factories to the HEP community.

Tuesday 13 May

1 Welcome and introduction

Speakers: Andrzej Kupsc, Bastian Kubis (University of Bonn (DE)), Johan Bijnens (Lund University), Sean Tulin (York University)

2 New Physics near a GeV

Speaker: David McKeen (TRIUMF)

3 TBA

Speaker: Emilie Passemar

10:30 Coffee break (Fika)

4 Perspectives on C and CP violation in eta to 3pi decay from SMEFT

The decay $\eta\to\pi^+\pi^- \pi^0$ is an ideal channel in which to study flavor-conserving $C$ and $CP$ violation beyond the Standard Model. We use Standard Model Effective Field Theory (SMEFT) to deduce the mass-dimension-six operators that can contribute to its signature, i.e., an observed breaking of mirror symmetry in its Dalitz plot. Using chiral-effective theory, we present the leading hadron-level operators associated with the $I=0$ and $I=2$ transitions, and we use KLOE-2 and BES-III data to determine their strength. With this, we show how future $\eta\to \pi^+\pi^-\pi^0$ decay measurements at different experimental facilities can reveal the landscape of $C$ and $CP$ violation.

Speakers: Dr Jun Shi (South China Normal University), Susan Gardner (University of Kentucky)

5 CP Violation in η Decays to Leptons within the SMEFT framework

Currently and in the near future, a variety of $\eta$ factories are exploring the potential to search for new physics through different $\eta$ decays. One possibility being considered is to search for signs of CP-violation in eta decays. Since the Standard Model predicts an extremely suppressed rate for CP-violation in the $\eta$ sector, any positive signal would be a clear sign of new physics. In this talk, we will focus on C-even P-odd signals, which are stringently constrained by electric dipole moments. Employing the Standard Model effective field theory, we will discuss such bounds and the prospects for detecting CP-violation at different $\eta$ factories.

Speaker: Dr PABLO SÁNCHEZ PUERTAS (University of Granada)

12:00 Lunch

6 Status of eta, eta-prime lattice studies

Speaker: Konstantin Ottnad (University of Mainz)

7 JLab Eta Factory

The system of η and η′ offers a flavor-conserving laboratory to test the low-energy QCD and to search for new physics Beyond the Standard Model. The symmetry properties of QCD at low-energy, such as the chiral symmetry or the axial anomalies, are manifested in the decays of η and η′. Thus, a study of η/η′ will yield light on our understanding of the origin and the dynamics of QCD confinement. In addition, the η/η′ meson has quantum numbers of vacuum (except parity) with its strong and electromagnetic decays being either anomalous or forbidden to the lowest order due to symmetries or angular momentum conservation. This enhances the relative importance of higher order contributions, making rare η/η′ decays a sensitive hadronic probe for weakly-coupled new forces. Searching for sub-GeV dark gauge boson candidates and the C-violating, P-conserving interactions in various η/η′ decays will extend our knowledge of the dark sector and explore new sources of CP violation that are needed to explain the observed matter and anti-matter asymmetry in the universe. The JLab Eta Factory experiment is aimed at simultaneous measurements of η and η′ decays, with emphasis on rare neutral mode. This experiment will start in spring 2025 using the GlueX apparatus with a newly upgraded Forward calorimeter. The status and the new experimental opportunities for the η/η′ physics will be presented.

Speaker: Prof. Liping Gan (University of North Carolina Wilmington)

8 eta decays at KLOE/KLOE-2 and prospect at the future e+e- collider

The talk will include recent progress and insights on eta/eta' decays at KLOE/KLOE2 and the propest in this field that could be achieved with the future e+e- collider proposed like STCF.

Speaker: xiaolin Kang

15:00 Coffee break (Fika)

9 On general parameterization for the hadronic form factors

Speaker: Pere Masjuan

10 LFV eta/eta' decays and their relation to mu -> e conversion in nuclei

Speaker: Frederic Noël

End of the day discussion

Wednesday 14 May

11 TBA

Speaker: Andreas Crivellin (University Bern)

12 The REDTOP experiment: a η/η′ factory to explore dark matter and physics beyond the Standard Model

Speaker: Dr Marcin Zielinski (Jagiellonian University (PL))

13 TBA

Speaker: Frank Simon (KIT - Karlsruhe Institute of Technology (DE))

10:30 Coffee break (Fika)

14 Huizhou Hadron Spectrometer (HHaS)

Speaker: Hao Qiu

15 TBA

Speaker: Valentina Santoro (ESS)

12:00 Lunch

16 The Jefferson Lab Eta Factory Experiment and Applications of PbWO₄ Calorimeters in Future Experimental Facilities

The goal of the new JLab Eta Factory (JEF) experiment, conducted with the GlueX detector in Hall D at Jefferson Lab, is to perform measurements of various $\eta^{(\prime)}$ decays with a primary focus on rare neutral modes. The experiment's physics program ranges from precision tests of low-energy QCD to searches for gauge bosons with masses below 1 GeV that could couple the Standard Model (SM) sector to the dark sector. The experiment will collect a high-statistics data sample of $\eta^{(\prime)}$ mesons produced via a beam of tagged photons. The GlueX detector features a large, nearly uniform acceptance for both neutral and charged particles, enabling efficient identification of complex multi-particle final states. To meet the requirements of the JEF experiment, the inner section of the forward lead-glass calorimeter in the GlueX detector has been upgraded with lead tungstate (PbWO$_4$) scintillating crystals. PbWO$_4$ offers exceptional characteristics, such as a small radiation length and Molière radius, and large light yield, that make it ideal for constructing high-granularity, high-resolution, radiation-hard detectors. These properties enable excellent spatial separation and energy resolution of reconstructed electromagnetic showers, establishing PbWO$_4$ as the material of choice for many high-precision experiments. The JEF experiment began data collection in April 2025 and will operate concurrently with the GlueX experiment, whose primary objective is the search for gluonic excitations in the meson spectrum. I will give an overview of the JEF experiment, the GlueX detector, and the feasibility of further upgrades to support future $\eta$ physics studies. Special attention will be given to the newly constructed PbWO$_4$ scintillating calorimeter and recent advancements in calorimeter instrumentation.

Speaker: Dr Alexander Somov (Jefferson Lab)

17 TBA

Speaker: Jiancheng Yang

18 TBA

Speaker: Szymon Treliński

15:00 Coffee break

19 NA64 hadron mode: Search for η and η’ to invisible decays

Speaker: Marco Ayala (SAPHIR - UNAB)

20 Pseudoscalar mesons and emergent phenomena in QCD

The structural properties of pseudoscalar mesons are closely linked with QCD's emergent phenomena, confinement and emergent hadronic mass, as well as to fundamental symmetries of the Standard Model. Two-photon transition form factors (TFFs) offer an efficient way to explore these phenomena, while tracing the transition between soft and hard energy scales. Moreover, these form factors are related to precision observables. This talk presents a unified treatment of two-photon TFFs based on continuum Schwinger methods, a framework extensively used to investigate hadron-related quantities.

Speaker: Khépani Raya Montaño

End of the day discussion

Thursday 15 May

21 TBA

Speaker: Maxim Pospelov

22 Dark photon portal to dark matter, long-lived particles and gravitational waves

Dark photons arising at or below the GeV scale from a spontaneously broken gauge symmetry are one of the simplest and most exciting extensions of the Standard Model. They couple to known particles via kinetic mixing with the Standard Model gauge bosons and can act as mediators between visible matter and new hidden states, such as sub-GeV dark matter particles. I will show that these models can satisfy all current constraints from laboratory experiments, astrophysical observations and cosmological data and predict new signals that can be targeted with future accelerator experiments. Moreover, if the dark sector features a classically conformal symmetry, spontaneous symmetry breaking can happen via a strong first-order phase transition. For MeV-scale dark sectors, the resulting gravitational wave signals are predicted to lie in the nano-Hertz frequency range and can fit the stochastic background observed by pulsar timing arrays.

Speaker: Felix Kahlhoefer (Karlsruhe Institute of Technology)

23 Low-scale dark sectors motivated by the evidence of a stochastic gravitational waves background

The recently reported evidence of a Stochastic Gravitational Wave Background (SGWB) in the nHz frequency range is in tension with standard astrophysical expectations, motivating the study of new physics beyond the Standard Model. A preferred explanation relies on a supercooled first order phase transition at the 100 MeV - GeV scale. I will present a minimal model that can account for the SGWB, featuring a new U(1) gauge group and a dark scalar that dynamically breaks the symmetry, and discuss how the observed signal constrain the parameter space. Possible extensions of the model required to open new decay channels into the SM particles will also discussed.

Speaker: Dr Michele Lucente (University of Bologna)

10:30 Coffee break (Fika)

24 Pseudoscalar transition form factors and $P \to \gamma\gamma$ decays from Lattice QCD

Speaker: Antoine Gérardin (CPT Marseille)

25 eta(') transition form factors, the muon g-2, and beyond

The uncertainty of the Standard Model prediction of the anomalous magnetic moment of the muon $(g-2)_\mu$ is dominated by hadronic contributions. As part of these hadronic inputs, the pseudoscalar-pole contribution in hadronic-light-by-light scattering plays a vital role. The currently sought precision of the Standard Model prediction of $(g-2)_\mu$ requires a careful evaluation not only of the pion-pole contribution but also of the $\eta$ and $\eta'$.
We report on the first dispersive, data-driven determination of the $\eta$ and $ \eta'$ transition form factors in the space-like regime, which can be unambiguously related to their pole contributions. The analysis takes into account all of the lowest-lying singularities and encapsulates factorization-breaking effects by means of a left-hand-cut contribution due to the impact of the $a_2(1320)$ tensor meson.

Speaker: Simon Holz (Universität Bern)

12:00 Lunch

26 The process eta(') --> l+l- and pseudoscalar/axialvector mediators

Speaker: Bai-Long Hoid

27 Rare eta meson decays at the CMS experiment

Novel trigger strategies, combined with the large proton-proton collision datasets collected by the CMS experiment, have extended its sensitivity to rare flavor decay processes. Leveraging CMS’s ability to efficiently trigger on and reconstruct low-momentum muons, the experiment offers complementary sensitivity to rare meson decays.

This contribution presents recent results on η meson decays from the CMS experiment. We report the first observation of the rare double Dalitz decay η → μ⁺μ⁻μ⁺μ⁻, achieved through the use of dedicated low-pT dimuon triggers. We also discuss the innovative trigger developments introduced for Run 3 and outline the future prospects for probing rare η and η' decays at CMS.

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

28 eta/etaprime physics at BESIII

Speaker: Shuangshi Fang

15:00 Coffee break (Fika)

29 Theoretical calculation in the anomalous sector

This talk will provide an overview of the precise higher-order calculation in the anomalous sector. The focus will be on the two-photon decays for pion and eta and on the corresponding Dalitz decays.

Speaker: Karol Kampf (Charles University (CZ))

End of the day discussion

Friday 16 May

30 Leptophobic $B$-boson searches through $\eta^{(\prime)}\to\pi^0\gamma\gamma$ and $\eta^{\prime}\to\eta\gamma\gamma$ decays

We analyse vector, scalar and tensor meson-exchange contributions to the doubly radiative decays $\eta^{(\prime)}\to\pi^0\gamma\gamma$ and $\eta^{\prime}\to\eta\gamma\gamma$, and investigate the sensitivity of these processes to a leptophobic $B$ boson in the sub-GeV mass range. Our results are relevant for measurements of these decays at existing (A2, BESIII, KLOE-2) and forthcoming $\eta/\eta^\prime$-factories, such as the JEF and REDTOP experiments.

Speaker: Dr Emilio Royo (Universidad Cardenal Herrera CEU and IFAE)

31 Sub-GeV leptophobic B-boson exchange in $\phi\rightarrow\pi^0\eta\gamma$ decays

The sensitivity of the rare decay process $\phi\rightarrow\pi^0\eta\gamma$ to signatures of a leptophobic boson in the MeV-GeV mass range is analyzed in this talk. First, we improve the description of the dominant scalar contribution. Next, we incorporate an explicit B-boson resonance exchange alongside the Standard Model (SM) contributions from vector and scalar meson exchanges. By utilizing experimental data related to the associated branching ratios and invariant mass spectra, we are able to improve the current constraints on the B-boson mass, $m_B$, and its coupling to SM particles, $\alpha_B$. However, the decays $\phi\to\pi^0\eta\gamma$ provide weaker constraints on the B-boson parameters compared to $\eta\to\pi^0\gamma\gamma$ below the mass of the $\eta$ meson.

Speaker: Alejandro Miranda (IFAE)

32 Axio-hadronic $\eta/\eta^{\prime}$ decays

With upcoming $\eta/\eta^{\prime}$ factories in the horizon, dedicated searches for hadronic ALPs would open a new window into the exploration of strong CP solutions and low scale dark sectors. In this talk, we present branching ratio sensitivities of the axio-hadronic $\eta/\eta^{\prime}\to\pi\pi a$ decays, where $a$ is an axion or axion-like particle (ALP), using the leading order ALP-ChPT Lagrangian supplemented by dispersion relations to include the effects of pion-pion final state interactions. Decay channels with multiple emission of ALPs will be also briefly discussed.

Speaker: Sergi Gonzalez-Solis (University of Barcelona)

33 TBA

Speaker: xiaolin Kang

10:30 Coffee break (Fika)

34 TBA

Speaker: Susan Gardner (University of Kentucky)

35 Search for Axion-Like-Particles in the η->pi+pi-e+e- decay with the HADES Detector

Speaker: Krzysztof Prościński (Jagiellonian University)

Closing

12:15 Lunch