A Challenge to Lepton Universality

A measurement of the ratio of the branching fractions of the B þ → K þ μ þ μ -and B þ → K þ e þ e -decays is presented using proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb -1 , recorded with the LHCb experiment at center-of-mass energies of 7 and 8 TeV.The value of the ratio of branching fractions for the dilepton invariant mass squared range 1 < q 2 < 6 GeV 2 =c 4 is measured to be 0.745 þ0.090 -0.074 ðstatÞ Æ 0.036ðsystÞ.This value is the most precise measurement of the ratio of branching fractions to date and is compatible with the standard model prediction within 2.6 standard deviations.