Search for Lepton Flavor Violation in $\Upsilon (3S)\rightarrow e^{\pm}\mu^{\mp}$

TL;DR

This paper reports the first search for lepton flavor violation in the decay of the $S$ meson to an electron and muon, setting a new upper limit on the branching fraction and constraining new physics models.

Contribution

It presents the first experimental search for LFV in $S$ decays and establishes a new upper limit on the branching fraction, constraining the energy scale of potential new physics.

Findings

No evidence for LFV decay observed.

Set an upper limit on the branching fraction: < 3.6×10^{-7}.

Limits on new physics energy scale: > 80 TeV.

Abstract

We report on the first search for electron-muon lepton flavor violation (LFV) in the decay of a $b$~quark and $b$~antiquark bound state. We look for the LFV decay $\Upsilon (3S)\rightarrow e^{\pm}\mu^{\mp}$ in a sample of 118~million $\Upsilon (3S)$ mesons from 27~fb$^{-1}$ of data collected with the \babar\ detector at the SLAC PEP-II $e^+e^-$ collider operating with a 10.36~GeV center-of-mass energy. No evidence for a signal is found and we set a limit on the branching fraction $\mathcal{B}(\Upsilon(3S)\rightarrow e^{\pm}\mu^{\mp})<3.6\times10^{-7}\mathrm{at~ 90\%~ CL}$. This result can be interpreted as a limit $\Lambda_{NP}/g^2_{NP} > 80~$TeV on the energy scale $\Lambda_{NP}$ divided by the coupling-squared $g^2_{NP}$ of relevant new physics.