Reading the footprints of the B-meson flavor anomalies

TL;DR

This paper analyzes recent B-meson flavor anomalies, combining model-independent and specific leptoquark models, revealing a pattern of new left-handed interactions with potential experimental signatures and collider constraints.

Contribution

It provides a comprehensive analysis of B-meson flavor anomalies using effective field theory and leptoquark models, including new insights on flavor structure and experimental signatures.

Findings

Confirmation of a left-handed semileptonic interaction pattern

Constraints from B_s mixing and rare decays

Updated collider bounds on leptoquark parameters

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 $B\to K^{(*)}\nu\bar\nu$), and finally an updated analysis of collider bounds on the leptoquark mass and couplings.