# $\tau$ lepton mass measurement at BESIII

## TL;DR

This paper reports on the precise measurement of the $ au$ lepton mass at BESIII using a dedicated beam energy system, with data collected to achieve an uncertainty below 100 keV.

## Contribution

The paper introduces a new high-precision method for measuring the $ au$ lepton mass at BESIII, including the development of a beam energy measurement system and a detailed scan plan.

## Key findings

- Collected over 130 pb^{-1} of $ au$ mass scan data.
- Achieved an expected uncertainty of less than 100 keV in $m_{\tau}$.
- Established a scenario for high-precision $ au$ mass measurement.

## Abstract

In order to obtain the mass of $\tau$ lepton at BESIII precisely, a beam energy measurement system was built at BEPCII. A scenario for high precision $\tau$ mass measurement was put forth before data taken. More than 130 $pb^{-1}$ $\tau$ mass scan data were collected in April 2018, and the uncertainty of $m_{\tau}$ is expected to be less than 100 keV.