Search for the lepton-flavour violating decays $B^0_s \rightarrow e^{\pm}\mu^{\mp}$ and $B^0 \rightarrow e^{\pm} \mu^{\mp}$

TL;DR

This paper reports a search for lepton-flavour violating decays of B mesons, setting new upper limits on their branching fractions and deriving lower bounds on leptoquark masses, significantly improving previous constraints.

Contribution

The study provides the most stringent upper limits to date on the branching fractions of $B^0_s$ and $B^0$ decays to $e^{\pm}\mu^{\mp}$ and calculates higher lower bounds on Pati-Salam leptoquark masses.

Findings

Upper limits on branching fractions are $<1.1 imes 10^{-8}$ and $<2.8 imes 10^{-9}$ at 90% C.L.

New lower bounds on leptoquark masses are 107 TeV/c$^2$ and 126 TeV/c$^2$ at 95% C.L.

Limits are twenty times more restrictive than previous experiments.

Abstract

A search for the lepton-flavour violating decays $B^0_s \to e^{\pm} \mu^{\mp}$ and $B^0 \to e^{\pm} \mu^{\mp}$ is performed with a data sample, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7$ TeV, collected by the LHCb experiment. The observed number of $B^0_s \to e^{\pm} \mu^{\mp}$ and $B^0 \to e^{\pm} \mu^{\mp}$ candidates is consistent with background expectations. Upper limits on the branching fractions of both decays are determined to be $BR(B^0_s \to e^{\pm} \mu^{\mp}) < 1.1 \,(1.4) \times 10^{-8}$ and $BR (B^0 \to e^{\pm} \mu^{\mp}) < 2.8 \,(3.7) \times 10^{-9}$ at 90% (95%) confidence level (C.L.). These limits are a factor of twenty lower than those set by previous experiments. Lower bounds on the Pati-Salam leptoquark masses are also calculated, $M_{\rm LQ} (B^0_s \to e^{\pm} \mu^{\mp}) > 107$ TeV/c$^2$ and $M_{\rm LQ} (B^0 \to e^{\pm} \mu^{\mp}) > 126$ TeV/c$^2$ at 95% C.L., and are a factor of two higher than the previous bounds.