Publications

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• Feasibility of tau g-2 measurements in ultra-peripheral collisions of heavy ions

  Nazar Burmasov, Evgeny Kryshen, Paul Buehler, Roman Lavicka

  SciPost Phys. Proc. 16, 022 (2025) · published 15 July 2025 | Toggle abstract · pdf

  The 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 studies of the ditau production in the $ee\to ee\tau\tau$ process. Ultra-peripheral collisions (UPCs) 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 the feasibility to explore this process down to low transverse momenta of decay leptons with the ALICE and LHCb experiments.

• Lepton flavour universality tests and lepton flavour violation searches at LHCb

  Sara Celani

  SciPost Phys. Proc. 16, 021 (2025) · published 15 July 2025 | Toggle abstract · pdf

  The Standard Model (SM) of particle physics represents our most fundamental knowledge of elementary particles and their interactions. One of the cardinal properties of the SM which has been thoroughly studied during the past years is the so-called Lepton Flavour Universality (LFU): W and Z bosons are predicted to be equally coupled to the three lepton generations. Hints for deviations from LFU have been found by the LHCb collaboration in $b\to s\ell\ell$ and $b\to c\ell\nu$ decays, sparking great interest and further motivating the searches for lepton flavour violating processes. A general review of these results is presented, including prospects in view of the LHC and LHCb upgrade.

• LFV searches by ATLAS and CMS experiments

  Luca Fiorini

  SciPost Phys. Proc. 16, 020 (2025) · published 15 July 2025 | Toggle abstract · pdf

  The appearance of lepton-flavour-violating processes in LHC proton-proton collisions is one of the possible ways that new physics beyond the Standard Model could manifest itself. This proceeding summarizes the most recent searches for lepton-flavour-violating processes and tests of lepton flavour universality with the ATLAS and CMS detectors, using proton-proton collisions at a centre-of-mass energy of 13 TeV.

• Tau leptons as a tool to investigate the CP properties of the Higgs boson at CMS

  Andrea Cardini

  SciPost Phys. Proc. 16, 019 (2025) · published 15 July 2025 | Toggle abstract · pdf

  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 tau Yukawa coupling 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% confidence level allowing to constrain the allowed phase space for possible BSM scenarios.

• Tau identification in CMS during LHC run 2

  Mohammad Hassan Hassanshahi

  SciPost Phys. Proc. 16, 018 (2025) · published 15 July 2025 | Toggle abstract · pdf

  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.

• Search for lepton-flavor-violating decays of the tau lepton at a future muon collider

  Gholamhossein Haghighat, Mojtaba Mohammadi Najafabadi

  SciPost Phys. Proc. 16, 017 (2025) · published 14 July 2025 | Toggle abstract · pdf

  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 spontaneously broken global U(1) symmetries. LFV ALPs have been of a great interest in the last several decades as they can address some of the SM long-lasting problems. Assuming a future muon collider proposed 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 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 parameterized simulation based on the ideal target performance, we obtain expected 95$\%$ confidence level upper limits on the LFV couplings tau-electron-ALP and tau-muon-ALP. 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. We study the two cases of unpolarized and polarized muon beams and show that taking advantage of tau polarization-induced effects, the main background $\tau\rightarrow e/\mu + \nu\bar{\nu}$ can be significantly reduced. Results indicate that current limits on the LFV couplings can be improved by roughly one order of magnitude using the present analysis.

• Probing lepton flavor violation via $e\tau$ production in an effective framework at a future electron-positron circular coll

  Seyed Mohsen Etesami, Reza Jafari, Mojtaba Mohammadi Najafabadi, Seddigheh Tizchang

  SciPost Phys. Proc. 16, 016 (2025) · published 14 July 2025 | Toggle abstract · pdf

  In the Standard Model framework, lepton flavor is conserved in interactions. However, it can be violated in certain scenarios beyond the Standard Model. In this study, we consider $(e^{±}\tau^{\mp})$ production through a four-Fermi lepton flavor violation interaction at the future electron-positron circular collider (FCC-ee). To constrain the Wilson coefficients at four center-of-mass energies $157.5, 162.5, 240$, and $365$ GeV that have been proposed for the FCC-ee, a Standard Model Effective Field Theory (SMEFT) is used. The main sources of background are accounted for, and the study incorporates realistic detector effects. To enhance the results, we combine the individual constraints obtained at each center-of-mass energy. We show that the combined limits improve the sensitivity to lepton flavor violation couplings by a factor of approximately three compared to the individual results and are competitive with the Belle II prospects.

• Status and prospects for $\tau$ property measurements at Belle II

  Michel Hernández Villanueva

  SciPost Phys. Proc. 16, 015 (2025) · published 14 July 2025 | Toggle abstract · pdf

  Belle II is a major upgrade of the Belle detector, operating at the SuperKEKB asymmetric-energy $e^+e^-$ collider with a design luminosity of 8×$10^{35}$ cm$^{-2}$s$^{-1}$. To date, the detector has recorded over 200 fb$^{-1}$ of data and aims to collect 50 ab$^{-1}$, a factor of 50 more than its predecessor. Belle II has a broad $\tau$ physics program, from high-precision measurements of Standard Model parameters to searches of new non-Standard-Model interactions. In this work, we review the status of the Belle II experiment and the prospects for the measurement of the $\tau$ lepton mass and lifetime, which are fundamental inputs in tests of LFU violation.

• $e^+e^-$ hadronic cross-sections with SND detector at VEPP 2000

  T. V. Dimova, M. N. Achasov, A. Yu. Barnyakov, K. I. Beloborodov, A. V. Berdyugin, A. G. Bogdanchikov, A. A. Botov, V. P. Druzhinin, L. V. Kardapoltsev, A. G. Kharlamov, A. A. Korol, D. P. Kovrizhin, A. S. Kupich, N. A. Melnikova, A. E. Obrazovsky, E. V. Pakhtusova, K.V. Pugachev, S. I. Serednyakov, D. A. Shtol, Z. K. Silagadze, I. K. Surin, Yu. V. Usov, V. N. Zhabin

  SciPost Phys. Proc. 16, 014 (2025) · published 14 July 2025 | Toggle abstract · pdf

  Data with an integrated luminosity of about 370 pb$^{-1}$ have been collected with the SND detector at the VEPP-2000 $e^+e^-$ collider. Here we present recent results on $e^+e^-$ annihilation into hadrons below 2 GeV based on a part of these data. In particular, we discuss measurements of the $e^+e^-\to \pi^+\pi^-$, $e^+e^- \to n\bar{n}$, $e^+e^- \to p\bar{p}$, and $e^+e^-\to \pi^+\pi^-\pi^0\pi^0$ cross sections, and study of the processes $e^+e^-\to \pi^+\pi^-\pi^0$, $e^+e^-\to K^+K^-\pi^0$, $e^+e^- \to \eta\pi^0 \gamma$, and $e^+e^- \to \eta \eta \gamma$.

• B boson search at KLOE/KLOE-2

  Elena Perez del Rio

  SciPost Phys. Proc. 16, 013 (2025) · published 14 July 2025 | Toggle abstract · pdf

  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 $\pi^{0}$-$\gamma$. So far, investigation of the $\pi^{0}$-$\gamma$ 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.

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