Precision Measurement of the Mass of the $\ddot{I}„$ Lepton

Abstract

An energy scan near the $\ddot{I}„$ pair production threshold has been performed using the BESIII detector. About $24$ pb${}^{-1}$ of data, distributed over four scan points, was collected. This analysis is based on $\ddot{I}„$ pair decays to $ee$, $e\hat{I}¼$, $eh$, $\hat{I}¼\hat{I}¼$, $\hat{I}¼h$, $hh$, $e\ddot{I}$, $\hat{I}¼\ddot{I}$ and $\ddot{I}€\ddot{I}$ final states, where $h$ denotes a charged $\ddot{I}€$ or $K$. The mass of the $\ddot{I}„$ lepton is measured from a maximum likelihood fit to the $\ddot{I}„$ pair production cross section data to be $m_{\ddot{I}}„=(1776.91\pm 0.12_{-0.13}^{+0.10}$) MeV/$c^2$, which is currently the most precise value in a single measurement.

13 pages, 7 figures