Search for the Lepton-Flavor Violating Decays B^{+}→π^{+}μ^{±}e^{∓}

The first search for the lepton-flavor violating decays B^{+}→π^{+}μ^{±}e^{∓} 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 B(B^{+}→π^{+}μ^{±}e^{∓})<1.8×10^{-9} at the 90% confidence level, 2 orders of magnitude more restrictive than the most stringent upper limit to date. This is the first constraint on lepton-flavor violating b→d quark transitions at the LHC and also sets the most stringent upper limits to date on b→dμ^{±}e^{∓} transitions. Limits on left-handed and scalar scenarios beyond the standard model are also reported.