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

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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.