Publications

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• Probing $\delta_{CP}$ phase and charged lepton flavour violation with $A_4$ flavor symmetry and deviations to tri-bi-maximal mixing via $z_2 \times z_2$ invariant perturbation in the neutrino sector

  Gayatri Ghosh

  SciPost Phys. Proc. 16, 025 (2025) · published 15 July 2025 | Toggle abstract · pdf

  In this work, a flavour theory of a neutrino mass model based on $A_{4}$ symmetry is considered to explain the phenomenology of CP violation phase and charged lepton flavour violation. The spontaneous symmetry breaking of $A_{4}$ symmetry in this model leads to tribimaximal mixing in the neutrino sector at a leading order. $\mathrm{A} z_{2} × z_{2}$ invariant perturbations in this model is introduced in the neutrino sector which leads to testable predictions of $\theta_{13}$ and $\mathbf{C P}$ violation. We consider an effective theory with an $A_{4} × z_{2} × z_{2}$ symmetry, which after spontaneous symmetry breaking at high scale which is much higher than the electroweak scale leads to charged lepton flavour violation processes once the heavy Majorana neutrino mass degeneracy is lifted either by renormalization group effects or by a soft breaking of the $A_{4}$ symmetry.

• Exploring neutrino masses and mixing in the seesaw model with $L_e-L_\tau$ gauged symmetry

  Papia Panda, Mitesh Kumar Behera, Priya Mishra, Shivaramakrishna Singirala, Rukmani Mohanta

  SciPost Phys. Proc. 16, 024 (2025) · published 15 July 2025 | Toggle abstract · pdf

  We investigate neutrino phenomenology in the $U(1)_{L_e-L_\tau}$ gauge extension of the Standard Model with symmetry following the framework of type-(I+II) seesaw mechanism. For this, we have added three right handed neutrinos, one singlet and one scalar triplet in our model. We show that the model can successfully accommodate the observed neutrino oscillation data and consistent with the results from lepton flavor violation and neutrinoless double beta decay process.

• Neutrino phenomenology via Type-(I+II) seesaw in the $U(1)_{L_e-L_\mu}$ model

  Priya Mishra, Mitesh Kumar Behera, Papia Panda, Shivaramakrishna Singirala, Rukmani Mohanta

  SciPost Phys. Proc. 16, 023 (2025) · published 15 July 2025 | Toggle abstract · pdf

  We show how, $L_e-L_\mu$ gauged symmetry is used as the Standard Model (SM) extension with the type (I+II) seesaw mechanism for neutrino mass generation. The particle content of this model involves three extra right handed neutrinos, one scalar triplet and one singlet with the SM spectrum. We discuss about lepton flavour violating decays $\mu → e\gamma$, $\tau \to \mu \bar \mu\mu$ and anomalous magnetic moment of muon.

  1 citation

• 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.