Test of lepton universality with $B^0\to K^{\ast 0}{\ell }^{+}{\ell }^{-}$ decays

Abstract

A test of lepton universality, performed by measuring the ratio of the branching fractions of the $B^0\to K^{\ast 0}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $B^0\to K^{\ast 0}{e}^{+}{e}^{-}$ decays, $R_{K^{\ast 0}}$, is presented. The $K^{\ast 0}$ meson is reconstructed in the final state $K^{+}\ddot{I}{€}^{-}$, which is required to have an invariant mass within 100$\mathrm{MeV}{c}^2$ of the known $K^{\ast }(892{)}^0$ mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3${\mathrm{fb}}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8$\mathrm{TeV}$. The ratio is measured in two regions of the dilepton invariant mass squared, $q^2$, to be \begin{eqnarray*} R_{K^{*0}} = \begin{cases} 0.66~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.03\mathrm{\,(syst)} & \textrm{for } 0.045 < q^{2} < 1.1~\mathrm{\,GeV^2}c^4 \, , \\ 0.69~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.05\mathrm{\,(syst)} & \textrm{for } 1.1\phantom{00} < q^{2} < 6.0~\mathrm{\,GeV^2}c^4 \, . \end{cases} \end{eqnarray*} The corresponding 95.4\% confidence level intervals are $[0.52,0.89]$ and $[0.53,0.94]$. The results, which represent the most precise measurements of $R_{K^{\ast 0}}$ to date, are compatible with the Standard Model expectations at the level of 2.1--2.3 and 2.4--2.5 standard deviations in the two $q^2$ regions, respectively.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2017-013.html