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.
Highlights
Both experiments find deviations from lepton universality with greater than 2σ significance, RXBeslle = 0.42 ± 0.25, RXBasBar = 0.58 ± 0.19
We show that in the double ratios RXs/RK, RK∗/RK and Rφ/RK the dependence on new physics coupling to left-handed quarks cancels out
We proposed a combined study of the ratios RH for final state hadrons H = K, K∗, Xs, φ, K0(1430), f0, η( ), K1, . . . to help decipher beyond the standard model (BSM) effects in b → s transitions
Summary
We give predictions for the ratios of branching fractions RH for the most promising exclusive final states which can be used to confirm and further explore potential lepton flavor nonuniversality in b → s decays. We show that p is close to 1 for the relevant q2-regions, and is rather precisely known This is good news because it implies that the vector meson channels XK∗ are almost maximally sensitive to right-handed currents and complementary to RK. Decays to the vector mesons behave to decays to scalars and are sensitive to right-handed currents in the combination C − C This is in contrast with decays to pseudoscalars K or η( ) where Wilson coefficients enter as C + C. The corresponding effects for Bs, Bs → φ(→ K K) are significantly smaller because of the φ’s narrow width, and because low-lying scalar ss mesons either have very little overlap with the φ or have small branching ratios to K K [6]
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