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Register a new userReading the footprints of the B-meson flavor anomalies
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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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We assume that the quark-flavor coefficients matrix of the semileptonic operators addressing the neutral-current B-meson anomalies has rank-one, i.e. it can be described by a single vector in quark-flavor space. By correlating the observed anomalies to other flavor and high-$p_T$ observables, we constrain its possible directions and we show that a large region of the parameter space of this framework will be explored by flavor data from the NA62, KOTO, LHCb and Belle II experiments.
We explore the connection of the leptoquark solution to the recently reported $B$-meson anomalies with a mechanism of neutrino mass generation and a viable dark matter candidate. We consider a model consisting of two scalar leptoquarks and three generations of triplet fermions: neutrino masses are radiatively generated at the 3-loop level and, by imposing a discrete $Z_2$ symmetry, one can obtain a viable dark matter candidate. We discuss the constraints on the flavour structure of this model arising from numerous flavour observables. The rare decay $Kto pi^+ ubar u$ and charged lepton flavour violating $mu-e$ conversion in nuclei are found to provide the most stringent constraint on this class of models.
Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by $K_Ltomu e$ and $Ktopimu e$, the scale can be lowered to a few TeV by adding vector-like fermions. Furthermore, in this case, the intriguing hints for lepton flavour universality violation in $bto smu^+mu^-$ and $bto ctau u$ processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension and the resulting massive vector leptoquark automatically has the same mass scale as the vector-like fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavour universality violation in semileptonic $B$ decays. Assuming flavour alignment in the down sector we find that in $bto sell^+ell^-$ transitions the observed deviations from the SM predictions (including $R(K)$ and $R(K^*)$) can be explained with natural values for the free parameters of the model. Even though we find sizable effects in $R(D)$, $R(D^*)$ and $R(J/Psi)$ one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from $D-bar D$ mixing and $tauto 3mu$.
In this work we study possible connections between $B$-meson anomalies and Kaon physics observables in the context of combined solutions with the singlet and triplet scalar leptoquarks $S_1$ and $S_3$. By assuming a flavor structure for the leptoquark couplings dictated by a minimally broken $U(2)^5$ flavor symmetry we can make a sharp connection between these two classes of observables. We find that bound on $Br(K^+ to pi^+ u u)$ from NA62 puts already some tension in the model, while the present limits on $Br(K_L to mu^+ mu^-)$ and $mu to e$ conversion in nuclei can be saturated. Relaxing instead the flavor assumption we study what values for $Br(K^+ to pi^+ u u)$, as well as for $Br(K_L to pi^0 u u)$ and $Br(K_{L,S} to mu^+ mu^-)$, are viable compatibly with all other phenomenological constraints.
Recently reported anomalies in various $B$ meson decays and also in the anomalous magnetic moment of muon $(g-2)$ motivate us to consider a particular extension of the standard model incorporating new interactions in lepton and quark sectors simultaneously. Our minimal choice would be leptoquark. In particular, we take vector leptoquark ($U_1$) and comprehensively study all related observables including ${(g-2)_{mu}}, R_{K^{(*)}}, R_{D^{(*)}}$, $B to (K) ell ell $ where $ellell$ are various combinations of $mu$ and $tau$, and also lepton flavor violation in the $tau$ decays.
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