SciPost: SciPost Physics Proceedings: issue detail

SciPost Physics Proceedings issue 17

17th International Workshop on Tau Lepton Physics (TAU2023)

Event dates: from Dec. 4, 2023 to Dec. 8, 2023.

The goal of this workshop is to bring theorists and experimentalists together to discuss recent progress in the field. Recent results on tau lepton production and decays at lepton and hadron colliders will be presented, as well as recent results from g-2 and neutrino oscillation experiments. The program will also include properties of tau leptons, lepton universality and flavor violation, precision electroweak physics, QCD, CP violation, and electromagnetic dipole moments. Review of current results and future opportunities will be discussed. For more information, visit https://indico.cern.ch/event/1303630/.

(Guest) Fellows responsible for this Issue

Prof. Swagato Banerjee
Dr David Brown
Prof. Christopher Davis

Tau lepton averages by the HFLAV group

Alberto Lusiani

SciPost Phys. Proc. 17, 001 (2025) · published 23 July 2025 | · pdf

We document some features of the preliminary draft of the tau lepton averages for the incoming HFLAV report. We include the tau mass and lifetime averages, and a fit of measurements of tau branching fraction measurements. Elaborations of these results provide lepton universality tests and determination of $|V_{us}|$ with tau measurements.
Monte Carlos for tau lepton - Standard Model and new physics signatures

Zbigniew Was

SciPost Phys. Proc. 17, 002 (2025) · published 23 July 2025 | · pdf

One of the purposes of High Energy accelerator experiments is confrontation of theory and measurements in ever new realms. Any new agreement extends theory applicability domain, any discrepancy hints to unexplained, calling for better calculations or new, deeper theory. Often one has to search for small contributions over large Standard Model background. Multidimensional signatures and complex background subtraction cuts imply that Monte Carlo techniques are indispensable. My talk included description of: KKMC Monte Carlo for $e^+e^- \to \tau^+\tau^- (n\gamma)$ (with $\tau$ decays), generation of additional lepton pairs of SM and New Physics, and contributions from anomalous magnetic and electric dipole moments.
Electromagnetic corrections in hadronic tau decays

Jesús Alejandro Miranda Hernández

SciPost Phys. Proc. 17, 003 (2025) · published 23 July 2025 | · pdf

We briefly review electromagnetic radiative corrections in semileptonic tau decays and their main applications.
Electric and magnetic tau dipole moments revisited

G. A. González-Sprinberg

SciPost Phys. Proc. 17, 004 (2025) · published 23 July 2025 | · pdf

Precise measurements of magnetic and electric dipole moments are important tests of the Standard Model and beyond Standard Model physics, particularly for the electron and the muon. However, the situation presents distinctive challenges when dealing with the tau lepton due to its very short lifetime and relatively high mass. Here, we review the theoretical predictions and experimental measurements of both the anomalous magnetic and electric dipole moments of the tau lepton.
Lepton flavor violation with tau leptons

Julian Heeck

SciPost Phys. Proc. 17, 005 (2025) · published 23 July 2025 | · pdf

We review the status and importance of lepton flavor violation with tauons, focusing on overlooked flavor-breaking patterns as well as tau-flavor violation in nucleon decays.
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.
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.
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 $\tau$.
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.
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.
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.
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.
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.
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.
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.
Search for lepton flavour violating decays involving tau leptons in the final state at CMS

Luca Guzzi

SciPost Phys. Proc. 17, 016 (2025) · published 23 July 2025 | · pdf

In the Standard Model (SM) lepton flavour numbers are approximately conserved at energies accessible to current experiments, where lepton flavour violation is extremely suppressed. The observation of neutrino oscillations, however, proves that neutrinos are massive particles and allows for additional Lepton Flavour Violating (LFV) processes also at low energies. Nevertheless, these processes are predicted with very low branching ratios and are sensitive to new physics effects, which could manifest as an enhancement in the decay probability. Similarly, Lepton Flavour Universality Violating (LFUV) observables allows for the test of the SM and the study of beyond the SM theories. The latest CMS results from the Higgs, B-physics and exotica groups are presented on the search for LFV and LFUV with tau leptons in the final state. The results are based on data collected in proton-proton collisions at the centre-of-mass energy of 13 TeV.
Matter effect in $P_{\mu \tau}$ at long baselines

Srubabati Goswami, Supriya Pan, Animesh Chatterjee

SciPost Phys. Proc. 17, 017 (2025) · published 23 July 2025 | · pdf

In the simple two-generation case, the probability $P_{\mu \tau}$ is not affected by interactions of neutrinos in matter. But for three-generation case, at baselines of the order of 9000 km, matter effects become important for this channel. This is a genuine three-flavour effect. We study how the presence of non-standard interactions (NSI) alters the $P_{\mu \tau}$ at these baselines. We observe large deviations from the standard matter effect. In particular, we find energies and baselines for which the phases governing the NSIs do not play any role. This may facilitate a better determination of NSI parameters if tau neutrinos can be detected.
FASER experiment and first results from LHC run 3

Umut Kose, on behalf of FASER collaboration

SciPost Phys. Proc. 17, 018 (2025) · published 23 July 2025 | · pdf

FASER is designed to search for light, extremely weakly interacting and long-lived beyond standard model particles at the CERN Large Hadron Collider. Such particles, e.g., dark photons, may be produced in the high-energy proton-proton collisions at the ATLAS interaction point and then decay to visible particles in FASER, which is placed 480 m downstream and aligned with the collision axis line-of-sight. The detector covers a previously unexplored range of pseudorapidity larger than 8.8, which allows it to have sensitivity to new physics in the far-forward region. FASER also has a sub-detector called FASER$\nu$, which is specifically designed to detect and investigate high-energy collider neutrino interactions in the TeV regime, extending current cross-section measurements. In this proceeding, the FASER detector and present recent results obtained during LHC Run 3 will be introduced.
The Hyper-Kamiokande experiment: Status and prospect

Umut Kose, on behalf of Hyper-Kamiokande collaboration

SciPost Phys. Proc. 17, 019 (2025) · published 23 July 2025 | · pdf

Hyper-Kamiokande is the next-generation neutrino observatory, aiming to tackle a broad spectrum of physics programs. These include probing leptonic CP violation through long baseline accelerator neutrino oscillations, determining neutrino mass ordering, potentially discovering the proton decay, and capturing neutrinos from supernova relics, other astrophysical sources, and the sun. As the world's largest underground Cherenkov detector, Hyper-Kamiokande boasts a fiducial volume eight times that of its predecessor, Super-Kamiokande. With 258 kton of ultrapure water as its medium, Hyper-Kamiokande is scheduled to begin its groundbreaking operations in 2027, located in Kamioka, Japan. This proceeding outlines the ambitious physics program of Hyper-Kamiokande, details of the design of the detector, and provides an update on its current status.
Status and prospects of the JUNO experiment

Matthias Raphael Stock, on behalf of the JUNO collaboration

SciPost Phys. Proc. 17, 020 (2025) · published 23 July 2025 | · pdf

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment currently under construction in China. It is located 52.5 km away from two nuclear power plants in a newly constructed 700-m-deep underground laboratory. JUNO will be the largest liquid scintillator (LS) detector in the world comprising 20 kt of ultra-pure LS filled in an acrylic sphere. Its main goal is to determine the neutrino mass ordering by measuring the energy spectrum of reactor neutrinos with highest accuracy. In addition, JUNO will cover precision measurements of oscillation parameters and several aspects in the field of astroparticle physics. Data taking will start in late 2024.
TAMBO: Searching for tau neutrinos in the Peruvian Andes

William G. Thompson, on behalf of the TAMBO collaboration.

SciPost Phys. Proc. 17, 021 (2025) · published 23 July 2025 | · pdf

TAMBO is a planned next-generation neutrino observatory sensitive to 1-100 PeV Earth-skimming tau neutrinos. This tau neutrino specificity will provide a high-purity sample of astrophysical neutrinos whose locations on the sky can then be used to improve the sensitivities of all-flavor neutrino observatories. The observatory will be located in a deep canyon and will comprise an array of water-Cherenkov and plastic scintillator air-shower detectors. In this proceeding, I will summarize the current status of the updated TAMBO simulation package, which is currently being used for detector optimization and reconstruction studies.
Measurement of the muon magnetic anomaly to 0.20 ppm by the Muon g-2 experiment at Fermilab

Lorenzo Cotrozzi, on behalf of the Muon g-2 collaboration

SciPost Phys. Proc. 17, 022 (2025) · published 23 July 2025 | · pdf

The Muon $g-2$ experiment at Fermilab aims to measure the muon magnetic moment anomaly, $a_{\mu}=(g-2)/2$, with a final accuracy of 0.14 parts per million (ppm). The experiment’s first result was published in 2021, based on data collected in 2018, and in 2023 a new result was published based on two more years of data taking, 2019 and 2020. The combination of the two results from Fermilab and the previous one from Brookhaven National Laboratory brought the uncertainty on the experimental measurement of $a_{\mu}$ to the unprecedented value of 0.19 ppm. This paper will present details about the improvements of statistical and systematic uncertainties on $a_{\mu}$ since the 2021 result.
Tau data-based evaluations of the hadronic vacuum polarization contribution to the muon g-2

Pere Masjuan, Alejandro Miranda, Pablo Roig

SciPost Phys. Proc. 17, 023 (2025) · published 23 July 2025 | · pdf

We review the $\tau$ data-driven computation of Euclidean windows for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment ($a_\mu$), which agree with the lattice results, making the difference of the $e^+e^-$ data-driven methods with them more intriguing. This conundrum needs to be solved by next year, when the final FNAL $a_\mu$ measurement will be published, in order to extract firm conclusions on possible new physics effects.
An alternative evaluation of the leading-order hadronic contribution to the muon $g-2$ with MUonE

Riccardo N. Pilato

SciPost Phys. Proc. 17, 024 (2025) · published 23 July 2025 | · pdf

The MUonE experiment proposes a novel approach to determine the hadronic contribution to the muon $g-2$, $a_\mu^\text{HLO}$, based on the measurement of the hadronic running of the QED coupling through the analysis of $\mu-e$ elastic scattering events. This could clarify the tensions in the current evaluations of $a_\mu^\text{HLO}$, which are limiting the comparison between theory and experiment for the muon $g-2$. The measurement will be performed at CERN’s North Area by scattering a 160 GeV muon beam on the atomic electrons of a low-Z target. The status and future plans of the experiment will be presented. Furthermore, an alternative method to extract $a_\mu^\text{HLO}$ from MUonE data will be discussed.
A data-driven method to estimate the antiproton background in Mu2e

Namitha Chithirasreemadam, Simone Donati, Pavel Murat, on behalf of the Mu2e collaboration

SciPost Phys. Proc. 17, 025 (2025) · published 23 July 2025 | · pdf

The Mu2e experiment will search for the CLFV process of neutrinoless, coherent conversion of muon to electron in the field of an Al nucleus. One of the expected backgrounds is antiprotons produced by the proton beam at the Production Target and annihilating in the Stopping Target to produce signal-like electrons. Although not a dominant background, it has a large uncertainty and cannot be suppressed by the timing cuts used to reduce the prompt background. However, at Mu2e energies, $p\bar{p}$ annihilation is the only source of events with multiple, simultaneous particles coming from the Stopping Target. We utilized this unique feature and developed a novel approach to reconstruct multi-track events and estimate the antiproton background.