A measurement of the ratio of the branching fractions of the $B^{+}\rightarrow K^{+}\mu^{+}\mu^{}$ and $B^{+}\rightarrow K^{+}e^{+}e^{}$ decays is presented using protonproton collision data, corresponding to an integrated luminosity of 3.0 fb$^{1}$, recorded with the LHCb experiment at centerofmass energies of 7 and 8 $\mathrm{ Te V}$. The value of the ratio of branching fractions for the dilepton invariant mass squared range $1<q^{2}<6\mathrm{ Ge V^2 /}c^4$ is measured to be $0.745^{+0.090}_{0.074}\mathrm{ (stat)} \pm0.036\mathrm{ (syst)}$. This value is the most precise measurement of the ratio of branching fractions to date and is compatible with the SM prediction within $2.6$ standard deviations.
Dilepton invariant mass squared, $ q^2$ , as a function of the $ K ^+ \ell^+ \ell^ $ invariant mass, $m( K ^+ \ell^+ \ell^ )$, for selected (a) $ B ^+ \rightarrow K ^+ \mu ^+\mu ^ $ and (b) $ B ^+ \rightarrow K ^+ e ^+ e ^ $ candidates. The radiative tail of the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ mesons is most pronounced in the electron mode due to the larger bremsstrahlung and because the energy resolution of the ECAL is lower compared to the momentum resolution of the tracking system. 
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Mass distributions with fit projections overlaid of selected $ B ^+ \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} (\rightarrow e ^+ e ^ ) K ^+ $ candidates triggered in the hardware trigger by (a) one of the two electrons, (b) by the $ K ^+$ and (c) by other particles in the event. Mass distributions with fit projections overlaid of selected $ B ^+ \rightarrow K ^+ e ^+ e ^ $ candidates in the same categories, triggered by (d) one of the two electrons, (e) the $ K ^+$ and (f) by other particles in the event. The total fit model is shown in black, the combinatorial background component is indicated by the dark shaded region and the background from partially reconstructed $ b $ hadron decays by the light shaded region. 
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Created on 20 February 2021.