Search for the lepton-flavour violating decays $B^0 \to K^{*0} \mu^\pm e^\mp$ and $B_s^0 \to \phi \mu^\pm e^\mp$

TL;DR

This study searches for lepton-flavour violating decays of B mesons using LHCb data, finds no signals, and sets the most stringent upper limits to date, including the first limit on $B_s^0 o \,\phi \,\mu^\ ext{\pm} e^\mp$ decays.

Contribution

It provides the first limits on $B_s^0 \to \phi \mu^\pm e^\mp$ decays and improves constraints on lepton-flavour violation in B meson decays.

Findings

No significant signals observed.

Set new upper limits on branching fractions.

First limits on $B_s^0 \to \phi \mu^\pm e^\mp$ decays.

Abstract

A search for the lepton-flavour violating decays $B^0 \to K^{*0} \mu^\pm e^\mp$ and $B_s^0 \to \phi \mu^\pm e^\mp$ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of $9\,\text{fb}^{-1}$. No significant signals are observed and upper limits of \begin{align} {\cal B}( B^0 \to K^{*0} \mu^+ e^- ) &< \phantom{1}5.7\times 10^{-9}~(6.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} \mu^- e^+ ) &< \phantom{1}6.8\times 10^{-9}~(7.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} \mu^\pm e^\mp ) &< 10.1\times 10^{-9}~(11.7\times 10^{-9}),\newline {\cal B}( B_s^0 \to \phi \mu^\pm e^\mp ) &< 16.0\times 10^{-9}~(19.8\times 10^{-9}) \end{align} are set at $90\%~(95\%)$ confidence level. These results constitute the world's most stringent limits to date, with the limit on the decay $B_s^0 \to \phi \mu^\pm e^\mp$ the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.