Revisiting the vector leptoquark explanation of the B-physics anomalies

Abstract

We present a thorough investigation of the vector leptoquark hypothesis for a combined explanation of the B-physics anomalies. We analyze this hypothesis from a twofold perspective, taking into account recent results from B-physics observables and high-p T searches. First, using a simplified model, we determine the general conditions for a successful lowenergy fit in presence of right-handed leptoquark couplings (neglected in previous analyses). Second, we show how these conditions, in particular a sizable 2-3 family mixing, can be achieved in a motivated ultraviolet completion. Our analysis reinforces the phenomenological success of the vector leptoquark hypothesis in addressing the anomalies, and its compatibility with motivated extensions of the Standard Model based on the idea of flavor non-universal gauge interactions. The implications of right-handed leptoquark couplings for a series of key low-energy observables, namely B s → τ τ and τ → µ lepton flavor violating processes, both in τ and in B decays, are discussed in detail. The role of the ultraviolet completion in precisely estimating other low-energy observables, most notably ∆F = 2 amplitudes, is also addressed. also [32,37]).We provide a detailed implementation of the U 1 leptoquark in a renormalizable model based on the (flavor non-universal) gauge group SU (4) 3 × SU (3) 1+2 × SU (2) L × U (1) , which in turn can be embedded in PS 3 [31]. In this context, we complement the simplified-model analysis by including oneloop contributions to low-energy observables (most notably ∆F = 2 amplitudes and dipole operators) which can be reliably computed only within a UV-complete framework.The paper is organized as follows. In Section 2 we present the simplified-model analysis: we introduce the Lagrangian describing the U 1 couplings to SM fermions, and analyze its low-energy limit. We discuss all the observables insensitive to the UV completion (Section 2.2), which are later used to fit low-energy data (Section 2.3). We finally comment on the high-p T constraints (Section 2.4).The UV-complete model is presented and discussed in Section 3: on the model-building side we pay particular attention to the flavor structure of the model (Section 3.2); on the phenomenological side we present complete expressions for the UV-dependent (loop-induced) observables, which were omitted in the low-energy fit (Section 3.3). The results are summarized in Section 4.

Effective theory approach to new physics with flavour: general framework and a leptoquark example

“…As an illustration of the method, we consider in detail the minimally extended MFV scenario that accounts for the present flavour anomalies. Since one of the most promising setups to fit the anomalies is a U 1 leptoquark model [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40], one simply needs to consider the spurions which are associated with the fermion currents that the U 1 leptoquark is coupling to (see below). These spurions will be referred to as ∆ QL and ∆ DE in the following.…”

Testing lepton flavour universality with (semi)-leptonic $$\varvec{D_{(s)}}$$ decays

“…physics lying beyond the SM, have been proposed that could explain the B-decay anomalies, allowing in particular also for violations of LFU. Important specific scenarios are given by leptoquark models [7][8][9][10][11][12], Z models [13][14][15] or Two-Higgs-Doublet models [16][17][18], implying usually a rich phenomenology of patterns and correlations among various observables.…”

With or without U(2)? Probing non-standard flavor and helicity structures in semileptonic B decays

“…In the limit of vanishing neutrino masses, the size of |V | cannot be unambiguously determined. As discussed below (see also [20]), a good fit of the anomalies in semileptonic B decays is obtained for…”

“…The EFT in (12), with factorized flavor couplings as in (13) and (18), nicely matches the structure generated by integrating out a U 1 vector leptoquark, transforming as (3, 1) 2/3 under the SM gauge group. As noted first in [16], this field provides indeed an excellent mediator to build in a natural, and sufficiently general way, an EFT for semileptonic B decays built on the U (2) 5 flavor symmetry broken only by the leading V q and V spurions (see [20][21][22][23][25][26][27][28][29][30] for other phenomenological analysis of the U 1 leptoquark in B physics).…”

“…This numerical value provides an important contraint on the size of the leptonic spurion (λ µ ): since ∆ sµ q = O(λ s q λ µ ), setting λ s q = O(10 −1 ) and C V = O(10 −2 ), as suggested by the R D ( * ) fit, the value of R K ( * ) implies λ µ = O(10 −1 ). Within the UV leptoquark completion, the fact that ∆R K ( * ) ≡ R K ( * ) − 1 < 0 allows us also to determine a non-trival relative sign among the U (2) 5 breking terms: according to (20) one has C V > 0, which implies ∆ sµ q λ µ * < 0. Other data involving b → sµμ transitions, such as the measurements of P 5 [63][64][65][66] and other differential distributions, also deviate from the SM predictions consistently with R K ( * ) , further supporting the hypothesis of…”