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

TL;DR

This study explores the potential of a future muon collider to detect lepton-flavor-violating decays of tau leptons involving Axion-Like Particles, aiming to improve existing limits on LFV couplings through simulation and analysis.

Contribution

It presents a novel simulation-based analysis of LFV tau decays at a future muon collider, considering different energies, polarizations, and chiral structures to set expected upper limits.

Findings

Expected 95% CL upper limits on LFV couplings are established.

Tau polarization can significantly reduce background noise.

Current limits on LFV couplings could be improved by about an order of magnitude.

Abstract

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.