Measurement of the tau anomalous magnetic moment using Ultra-peripheral collisions with the ALICE detector in Run 3 Pb-Pb data

TL;DR

The paper assesses the feasibility of measuring the tau anomalous magnetic moment $a_ au$ using ultra-peripheral Pb--Pb collisions at the LHC with the ALICE detector in Run 3. It proposes a strategy based on $ extgamma extgamma o au^+ au^-$ production in mixed tau-decay channels, leveraging ALICE's PID capabilities to suppress backgrounds in the intermediate $W_{ extgamma extgamma}$ region. Simulation studies predict on the order of $3.6 imes10^4$ $ au^+ au^-$ events with favorable channel fractions and a sensitivity approach that could tighten $a_ au$ bounds beyond current limits; first Run 3 data show results consistent with STARlight and validate the analysis framework. With Run 3+4 luminosities, ALICE is poised to set world-leading constraints in the intermediate $W_{ extgamma extgamma}$ range, offering a complementary probe to CMS/ATLAS in $pp$ collisions and contributing to tests of SM robustness and potential new physics.

Abstract

The anomalous magnetic moment of the tau lepton ($a_τ$) is a sensitive probe for the search for deviations from the Standard Model predictions and thus for new physics. This study investigates the feasibility of measuring $a_τ$ using ultra-peripheral collisions (UPCs) at the LHC, where photon-photon interactions ($γγ\to τ^+ τ^-$) produce tau lepton pairs. We focus on events recorded by the ALICE detector during Run 3 Pb-Pb data-taking. Events are selected in the decay channel where one tau decays into an electron and neutrinos, and the other decays into a charged pion, or three charged pions, and neutrinos. These samples are enhanced with decays into muons, which are inseparable in the ALICE detector. The clean environment of UPCs minimizes hadronic background, while the advanced particle identification capabilities of the ALICE Time Projection Chamber (TPC) and Time-of-Flight (TOF) systems allow for efficient separation of electrons, pions, and background particles. In this talk, prospects for measuring this process by ALICE in Run 3, which benefits from high statistics and improved systematics uncertainties, will be discussed. Results will provide tighter constraints on $a_τ$, contributing to the broader effort to test the Standard Model's robustness and explore physics beyond it.

Figures (4)

• Figure 1: TPC $\mathrm{d}E/\mathrm{d}x$ vs. track momentum distributions showing electron--muon/pion separation and the absence of kaon/proton contamination in the selected samples. Black curves indicate Bethe-Bloch expectations for various particles.
• Figure 2: Acoplanarity distributions in data vs. STARlight MC for the $e^+e^-$ and $e+(\pi/\mu)$ channels after requiring system $p_{\mathrm{T}}\xspace>0.2GeV/c\xspace$.'
• Figure 3: Acoplanarity (left) and invariant mass (right) distributions in data vs. STARlight MC for the $e^+e^-$ and $e+3\pi$ channel after requiring system $p_{\mathrm{T}}\xspace>0.2GeV/c\xspace$.'
• Figure 4: The electron $p_{\mathrm{T}}$ spectra in the $e^+e^-$ and $e+(\pi/\mu)$ selection compared to STARlight.