First Measurement of the Branching Fraction of the Decay psi(2S) --> tau tau

TL;DR

This paper reports the first measurement of the psi(2S) decay into tau pairs, confirming lepton universality and determining the resonance's total width with new experimental data from the BES detector.

Contribution

It provides the first experimental measurement of the psi(2S) to tau tau branching fraction and tests lepton universality in this decay mode.

Findings

Measured B_{tau tau} = (2.71 ± 0.43 ± 0.55) × 10^{-3}

Results support the sequential lepton hypothesis

Total width of psi(2S) is (252 ± 37) keV

Abstract

The branching fraction of the psi(2S) decay into tau pair has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is $B_{\tau\tau}=(2.71\pm 0.43 \pm 0.55) \times 10^{-3}$, where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into e+e- and mu+mu of this resonance, satisfy well the relation predicted by the sequential lepton hypothesis. Combining all these values with the leptonic width of the resonance the total width of the psi(2S) is determined to be $(252 \pm 37)$ keV.