Diagnosing lepton-nonuniversality in b → sℓℓ

Abstract

Ratios of branching fractions of semileptonic B decays, (B → Hμμ) over (B → Hee) with H = K, K ∗, X s , K 0(1430), ϕ, . . . are sensitive probes of lepton universality. In the Standard Model, the underlying flavor changing neutral current process b → sℓℓ is lepton flavor universal. However models with new flavor violating physics above the weak scale can give substantial non-universal contributions. The leading contributions from such new physics can be parametrized by effective dimension six operators involving left- or right-handed quarks. We show that in the double ratios $$ {R_X}_{{}_s}/{R}_K $$ , R K ∗/R K and R ϕ /R K the dependence on new physics coupling to left-handed quarks cancels out. Thus a measurement of any of these double ratios is a clean probe of flavor nonuniversal physics coupling to right-handed quarks. We also point out that the observables $$ {R_X}_{{}_s} $$ , R K ∗, $$ {R}_K{{}_{{}_0}}_{(1430)} $$ and R ϕ depend on the same combination of Wilson coefficients and therefore satisfy simple consistency relations.