Search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^\pm e^\mp$

TL;DR

This paper reports the first search for lepton-flavour violating decays of B+ mesons into pi+ mu e, setting the most stringent upper limits to date and constraining new physics scenarios.

Contribution

It presents the first experimental search for lepton-flavour violation in $b o d$ transitions at the LHC, providing new upper limits on these rare decays.

Findings

No significant signal observed for $B^+ o ext{pi}^+ ext{mu}^ ext{±} ext{e}^ ext{∓}$ decays.

Upper limit on branching fraction set at $1.8 imes 10^{-9}$ at 90% CL.

Most restrictive limits on $b o d ext{mu}^ ext{±} ext{e}^ ext{∓}$ transitions to date.

Abstract

The first search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^{\pm} e^{\mp}$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of 9 fb$^{-1}$. No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^+ \to \pi^+ \mu^{\pm} e^{\mp}) < 1.8 \times 10^{-9}$ at the $90\%$ confidence level, two orders of magnitude more restrictive than the current world average. This is the first constraint on lepton-flavour violating $b \to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b \to d \mu^{\pm} e^{\mp}$ transitions. Limits on left-handed and scalar scenarios beyond the Standard Model are also reported.