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 → s 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 Xs /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 Xs , R K * , R K 0 (1430) and R φ depend on the same combination of Wilson coefficients and therefore satisfy simple consistency relations.

Lepton flavor nonuniversality inb→sℓ+ℓ−processes

“…From the current data on B(Bs → µµ) and B(B → Kµµ) [15,22]GeV 2 , we obtain constraints on ReC 10 and ImC 10 which we then assume to be lepton specific, and find RK = B(B → Kµµ)/B(B → Kee) [1,6]GeV 2 = 0.88(8), consistent with recent value measured at LHCb. A specific realization of this scenario is the one with a scalar leptoquark state ∆, in which C 10 is related to the mass of ∆ and its Yukawa couplings.…”

Flavour anomalies after the R K ∗ measurement

“…Future data will be able to further reduce the statistical uncertainty on these quantities. In addition, measurements of other ratios R H analogous to R K , with H = X s , φ, K 0 (1430), f 0 will constitute relevant independent tests [2,24].…”

“…The same remark remains valid for the simplified expression proposed in ref. [24], expanded up to quadratic terms in new physics coefficients. The reason is that the expansion is controlled by the parameter…”

Large hadronic power corrections or new physics in the rare decay B → K ∗ μ + μ −?

“…Thus, a confirmation of the deviation in R K would also indirectly confirm the NP interpretation of the anomalies in the angular observables. Updated measurement of R K with a larger dataset as well as other theoretically clean ratios of b → s + − observables which test lepton universality [15,26,[29][30][31][32] would be illuminating in this regard.…”