Motivated by the recent LHCb announcement of a $3.1sigma$ violation of lepton-flavor universality in the ratio $R_K=Gamma(Bto Kmu^+mu^-)/Gamma(Bto K e^+ e^-)$, we present an updated, comprehensive analysis of the flavor anomalies seen in both neutral-current ($bto sell^+ell^-$) and charged-current ($bto ctaubar u$) 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 $Bto K^{(*)} ubar u$), and finally an updated analysis of collider bounds on the leptoquark mass and couplings.
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