Explaining the RK and RK∗ anomalies

Abstract

Recent LHCb results on RK∗, the ratio of the branching fractions of B→ K∗μ+μ- to that of B→ K∗e+e-, for the dilepton invariant mass bins q2≡mℓℓ2= [0.045–1.1] GeV2 and [1.1–6] GeV2 show approximately 2.5 σ deviations from the corresponding Standard Model prediction in each of the bins. This, when combined with the measurement of RK(q2=[1-6]GeV2), a similar ratio for the decay to a pseudo-scalar meson, highly suggests lepton non-universal new physics in semi-leptonic B meson decays. In this work, we perform a model independent analysis of these potential new physics signals and identify the operators that do the best job in satisfying all these measurements. We show that heavy new physics, giving rise to q2 independent local 4-Fermi operators of scalar, pseudo-scalar, vector or axial-vector type, is unable to explain all the three measurements simultaneously, in particular RK∗ in the bin [0.045–1.1], within their experimental 1 σ regions. We point out the possibility to explain RK∗ in the low bin by an additional light (≲20MeV) vector boson with appropriate coupling strengths to (b¯s) and (e¯e).