Reading the footprints of the B-meson flavor anomalies

Abstract

Motivated by the recent LHCb announcement of a $3.1\sigma$ violation of lepton-flavor universality in the ratio $R_K=\Gamma (B\to K{\mu }^{+}{\mu }^{-})/\Gamma (B\to K{e}^{+}{e}^{-})$, we present an updated, comprehensive analysis of the flavor anomalies seen in both neutral-current ($b\to s{\ell }^{+}{\ell }^{-}$) and charged-current ($b\to c\tau \bar{\nu }$) decays of $B$ mesons. Our study starts from a model-independent effective field-theory approach and then considers both a simplified model and a UV-complete extension of the Standard Model featuring a vector leptoquark $U_1$ as the main mediator of the anomalies. We show that the new LHCb data corroborate the emerging pattern of a new, predominantly left-handed, semileptonic current-current interaction with a flavor structure respecting a (minimally) broken $U(2{)}^5$ flavor symmetry. New aspects of our analysis include a combined analysis of the semileptonic operators involving tau leptons, including in particular the important constraint from $B_s$--${\bar{B}}_s$ mixing, a systematic study of the effects of right-handed leptoquark couplings and of deviations from minimal flavor-symmetry breaking, a detailed analysis of various rare $B$-decay modes which would provide smoking-gun signatures of this non-standard framework (LFV decays, di-tau modes, and $B\to K^{(\ast )}\nu \bar{\nu }$), and finally an updated analysis of collider bounds on the leptoquark mass and couplings.

Comments: 43 pages, 14 figures, 4 tables. v2: new appendix, matching the journal version