Measurement of the mass of the $\tau$-lepton and an upper limit on the mass difference between $\tau^+$ and $\tau^-$

TL;DR

This paper reports a precise measurement of the tau lepton mass using decay data from the Belle detector and establishes an upper limit on the mass difference between tau positive and negative leptons, testing CPT symmetry.

Contribution

The study provides the most accurate tau mass measurement to date and sets a new upper limit on the mass difference between tau charge states.

Findings

Tau mass measured as 1776.61 ± 0.13 (stat) ± 0.35 (sys) MeV/c^2.

Upper limit on mass difference ratio is 2.8 × 10^{-4} at 90% CL.

Results improve constraints on CPT violation in the lepton sector.

Abstract

The mass of the $\tau$ lepton has been measured in the decay mode $\tau \to 3\pi \nu_\tau$ using a pseudomass technique. The result obtained from $414 \mathrm{fb}^{-1}$ of data collected with the Belle detector is $M_\tau = (1776.61\pm 0.13 {(stat.)} \pm 0.35 {(sys.)})$ MeV/$c^2$. The upper limit on the relative mass difference between positive and negative $\tau$ leptons is $|M_{\tau^+}-M_{\tau^-}|/M_{\tau} < 2.8 \times 10^{-4}$ at 90% confidence level.