Diagnosing lepton-nonuniversality in $b\to s\ell \ell$

Abstract

Ratios of branching fractions of semileptonic B decays, $(B\to H\mu \mu )$ over $(B\to Hee)$ with $H=K,K^{\ast },X_s,K_0(1430),\varphi ,\dots$ are sensitive probes of lepton universality. In the Standard Model, the underlying flavor changing neutral current process $b\to s\ell \ell$ 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 $R_{X_s}/R_K$, $R_{K^{\ast }}/R_K$ and $R_{\varphi }/R_K$ 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 $R_{X_s}$, $R_{K^{\ast }}$, $R_{K_0(1430)}$ and $R_{\varphi }$ depend on the same combination of Wilson coefficients and therefore satisfy simple consistency relations.

Comments: 20 pages, 4 plots; axialvector K1 included