Publications

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Searches for new physics that couple with third generation fermions

Valeria D'Amante, on behalf of the CMS collaboration

SciPost Phys. Proc. 17, 015 (2025) · published 23 July 2025 | · pdf

The $\tau$ lepton, with a mass of $1776.86 ± 0.12$ MeV, is unique in its ability to decay into hadrons and a neutrino. Approximately one-third of $\tau$ decays produce an electron or a muon and two neutrinos, denoted as $\tau_e$ and $\tau_\mu$. The remaining decays, mainly involving hadrons and a tau neutrino, are denoted as $\tau_h$. At the CERN LHC, searches involving $\tau$ leptons are crucial for studying the decay of Higgs bosons to $\tau$ pairs, probing Yukawa couplings, and CP properties of the Higgs. These measurements support Standard Model (SM) tests and searches for Beyond Standard Model (BSM) physics, including new or heavy Higgs bosons, leptoquarks, supersymmetric particles, or gauge bosons. The $\tau$ lepton polarization in Z boson decays is also significant for probing the SM. Despite its potential as a portal to new physics, the $\tau$ lepton's decay products, especially neutrinos and hadrons, make its reconstruction and identification challenging at the LHC. At the CMS experiment, neutrinos contribute to Missing Transverse Energy (MET), and hadronically decaying $\tau$ leptons are often misidentified as jets, complicating the separation of $\tau$-involved processes from background processes.
Searches for new phenomena in final states with taus using the ATLAS detector

Giuseppe Carratta

SciPost Phys. Proc. 17, 014 (2025) · published 23 July 2025 | · pdf

Many theories beyond the Standard Model predict new phenomena, such as leptoquarks, vector like leptons, $Z'$ and $W'$ bosons, supersymmetry, new scalars or heavy leptons, in final states with isolated, high-$p_{T}$ taus. Searches for new physics with such signatures, produced either resonantly or non-resonantly, are performed using the ATLAS experiment at the LHC. The most recent results at $\sqrt{s} =$ 13 TeV in proton-proton collision, are presented in this contribution.
Measurements of Higgs boson properties in decays to two Tau leptons and search for lepton-flavor-violating Higgs boson decays into Tau leptons using the ATLAS detector

Ömer Oğul Öncel

SciPost Phys. Proc. 17, 013 (2025) · published 23 July 2025 | · pdf

Three analyses exploiting the $H→\tau\tau$ process using 139 fb$^{-1}$ of proton--proton collision data collected at a center-of-mass energy of 13 TeV by the ATLAS detector at the LHC are presented. A measurement of $VH$ production, with $V(=W, Z)→ e\nu/\mu\nu,ee/\mu\mu$ decays, has established the first evidence for this process with an observed (expected) significance of 4.2 (3.6) standard deviations. The second analysis, first of its kind by the ATLAS collaboration, probes CP-invariance if the decay products of the $\tau$ leptons using an angular observable $\phi_\tau$, and measured an observed (expected) value of $\phi_\tau=9 ± 16^\circ (0 ± 28^\circ)$ at 68% confidence level. The last analysis is a search for lepton-flavour-violation in $H→ e\tau/\mu\tau$ decays, where most stringent exclusion limits at 95% confidence level on $H→ e\tau$ branching ratio of 0.2%, is obtained.
Chiral Belle: e-Beam polarization upgrade of SuperKEKB

J. Michael Roney

SciPost Phys. Proc. 17, 012 (2025) · published 23 July 2025 | · pdf

Recent R&D work associated with upgrading the SuperKEKB $e^+e^-$ collider with polarized electron beams is presented. The Chiral Belle physics program enables a set of unique precision measurements using the Belle II detector. It includes a set of measurements of $\sin^2\theta_W$ via separate left-right asymmetry ($A_{LR}$) measurements in annihilations to pairs of electrons, muons, taus, charm and b-quarks that yield precisions matching or exceeding those of the current world averages from measurements at the $Z^0$-pole, but at 10GeV, thereby also probing the running of the couplings. Chiral Belle will also probe for new physics via the highest precision measurements of neutral current universality ratios and precision measurements of tau lepton properties, including the tau magnetic moment. After summarizing the physics goals, developments related to provision of the polarized source, the new components of the accelerator lattice that rotate the electron spin from transverse to longitudinal at the interaction point, and polarimetry of the electron beam will be reported.
Precision $e^-$ beam polarimetry at an $e^+e^-$ B factory using tau-pair events

Caleb Miller

SciPost Phys. Proc. 17, 011 (2025) · published 23 July 2025 | · pdf

A novel technique for measuring the average beam polarization in $e^+e^-$ collisions, known as Tau Polarimetry, has been developed by the BABAR collaboration. This technique makes use of the relationship between the polarization of the beams, the resulting polarization of the $\tau^+\tau^-$ produced, and the kinematics of the decay products. Using this technique BABAR has measured the average $e^-$ beam polarization of PEP-II to be 0.0034 $± 0.0024_{stat} ± 0.0029_{sys}$. This technique is expected to be used at Belle II after a proposed polarization upgrade as the most precise method with which beam polarization can be determined.
Search for charged lepton flavor violation in $J/\psi$ decays at BESIII

Xudong Yu

SciPost Phys. Proc. 17, 010 (2025) · published 23 July 2025 | · pdf

In the Standard Model, charged lepton flavor violation (CLFV) is heavily suppressed by tiny neutrino mass, while many theoretical models can enhance CLFV effects up to a detectable level. The observation of any CLFV process would be a clear signal of new physics beyond SM. BESIII experiment collected 10 billion $J/\psi$ data and searched for CLFV processes $J/\psi\to e\tau$ and $e\mu$. The upper limits at the 90\% confidence level are determined to be $\mathcal{B}(J/\psi\to e\tau)<7.5× 10^{-8}$ and $\mathcal{B}(J/\psi\to e\mu)<4.5× 10^{-9}$, respectively, which are the most stringent constraints in the heavy quarkonium system.
Searches for lepton flavor violation in meson decays at Belle

Sourav Patra

SciPost Phys. Proc. 17, 009 (2025) · published 23 July 2025 | · pdf

We report the results of recent searches on charged lepton flavor violation using the $e^+e^-$ collision data collected by the Belle detector. In the first section, we present a search for the lepton-flavor-violating decays $B{}^0_s → \ell^{\mp}\tau^{±}$, where $\ell = e, \mu$, using $121\ \mathrm{fb}^{-1}$ data collected at the $\Upsilon(5S)$ resonance. We also search for the lepton-flavour-violating decays $B^+ \to K^+ \tau^± \ell^\mp$, with $\ell = (e, \mu)$, using $711\ \mathrm{fb}^{-1}$ $\Upsilon(4S)$ data sample. Finally, we present a search for the charged lepton-flavor-violating decays $\Upsilon(1S)→\ell^±\ell^{\prime\mp}$ and radiative charged lepton-flavour-violating decays $\Upsilon(1S)→\gamma\ell^±\ell^{\prime\mp}$ [$\ell,\ell^\prime = e, \mu, \tau$] using $25\ \mathrm{fb}^{-1}$ data recorded at $\Upsilon(2S)$ resonance. This search uses $\Upsilon(1S)$ mesons produced in $\Upsilon(2S)→\pi^+\pi^-\Upsilon(1S)$ transitions.
Measurement of the presence of $a_1(1420)$ and $\omega(782)$ in $\tau^{-}\to \pi^{-}\pi^{+}\pi^{-}\nu_{\tau}$ at Belle

Andrei Rabusov, Daniel Greenwald, Stephan Paul

SciPost Phys. Proc. 17, 008 (2025) · published 23 July 2025 | · pdf

We present preliminary results of a partial-wave analysis of $\tau^-\to\pi^-\pi^-\pi^+\nu_\tau$ using data from the Belle experiment at the KEKB $\mathrm{e}^+\mathrm{e}^-$ collider. We validate our model with a model-independent analysis. We see the $\mathrm{a}_1(1420)$ and a G-parity-violating $1^-[\omega(782)\pi]_\mathrm{P}$ wave in tauon decays. Our results will improve models used in simulation studies necessary for measuring the electric and magnetic dipole moments and Michel parameters of the $\tau$.
Leptophilic portals to new physics at colliders

P. S. Bhupal Dev

SciPost Phys. Proc. 17, 007 (2025) · published 23 July 2025 | · pdf

Observed neutrino oscillations imply that the global lepton flavor symmetry of the Standard Model must be broken. Therefore, searches for lepton flavor violation (LFV) are promising probes of new physics beyond the Standard Model. High-energy colliders provide a powerful tool to study LFV effects, which are complementary to the low-energy charged LFV searches. Here we discuss the possibility of LFV signals at colliders arising from exotic Higgs decays, and from leptophilic scalar and vector portal scenarios.
Lepton flavor violation at FCC-ee and CEPC

Pankaj Munbodh

SciPost Phys. Proc. 17, 006 (2025) · published 23 July 2025 | · pdf

Lepton flavor violating processes are highly suppressed in the Standard Model. Therefore, if observed, lepton flavor violation would be a clear indication of new physics beyond the Standard Model. We study the process $e^+e^-\to\tau\mu$ at tree-level in the Standard Model Effective Field Theory both on the $Z$-pole and at higher center-of-mass energies. We show that the constraints derived from the future circular $e^+e^-$ colliders FCC-ee and CEPC, are complementary to those obtained from low-energy tau decays at BaBar and Belle, as well as projections from Belle-II.

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Publications

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