Search for the lepton-flavour violating decays B0 → K*0μ±e∓ and $$ {B}_s^0 $$ → ϕμ±e∓

A search for the lepton-flavour violating decays B0 → K∗0μ±e∓ and Bs0 → φμ±e∓ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of 9fb−1. No significant signals are observed and upper limits of B(B0 → K∗0μ+e−) < 5.7 × 10−9 (6.9 × 10−9), B(B0 → K∗0μ−e+) < 6.8 × 10−9 (7.9 × 10−9), B(B0 → K∗0μ±e∓) < 10.1 × 10−9 (11.7 × 10−9), B(Bs0 → φμ±e∓) < 16.0 × 10−9 (19.8 × 10−9) are set at 90% (95%) confidence level. These results constitute the world’s most stringent limits to date, with the limit on the decay Bs0 → φμ±e∓ the first being set. In addi- tion, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.