Lepton flavor nonuniversality in b → s ℓ + ℓ − processes

256

201

The LHCb measurement of the µ/e ratio R K * indicates a deficit with respect to the Standard Model prediction, supporting earlier hints of lepton universality violation observed in the R K ratio. We show that the R K and R K * ratios alone constrain the chiralities of the states contributing to these anomalies, and we find deviations from the Standard Model at the 4σ level. This conclusion is further corroborated by hints from the theoretically challenging b → sµ + µ − distributions. Theoretical interpretations in terms of Z , lepto-quarks, loop mediators, and composite dynamics are discussed. We highlight their distinctive features in terms of the chirality and flavour structures relevant to the observed anomalies.

Section: Jhep09(2017)010mentioning

confidence: 99%

Flavour anomalies after the R K ∗ measurement

D’Amico

Nardecchia

Panci

et al. 2017

256

201

“…Moreover, there are four types of lepto-quark (LQ) mediators giving rise to O (1) lq and O (3) lq at the tree-level [80,[94][95][96][97][98][99][100][101]. Two have spin one, U µ and U a µ , and two have spin zero, S and S a .…”

Section: Jhep09(2017)061mentioning

confidence: 99%

On the importance of electroweak corrections for B anomalies

Feruglio

Paradisi

Pattori

2017

137

180

Abstract:The growing experimental indication of Lepton Flavour Universality Violation (LFUV) both in charged-and neutral-current semileptonic B-decays, has triggered many theoretical interpretations of such non-standard phenomena. Focusing on popular scenarios where the explanation of these anomalies requires New Physics at the TeV scale, we emphasise the importance of including electroweak corrections to obtain trustable predictions for the models in question. We find that the most important quantum effects are the modifications of the leptonic couplings of the W and Z vector bosons and the generation of a purely leptonic effective Lagrangian. Although our results do not provide an inescapable no-go theorem for the explanation of the B anomalies, the tight experimental bounds on Z-pole observables and τ decays challenge an explanation of the current non-standard data. We illustrate how these effects arise, by providing a detailed discussion of the running and matching procedure which is necessary to derive the low-energy effective Lagrangian.

“…Concerning b → sµ + µ − , a solution is not particularly challenging, as one competes with a process which is in the SM loop and CKM suppressed, a rather small NP contribution, involving moderate couplings and not too light masses, is sufficient (like for example in Z models [24][25][26][27][28][29][30][31][32][33][34][35][36], models with loop effects of heavy scalars and fermions [37,38] and also by leptoquark models [9,10,12,[39][40][41][42] 2 ). Also for the a µ anomaly many possible solutions…”

Section: Introductionmentioning

confidence: 99%

Simultaneous explanation of R(D (∗)) and b→sμ + μ −: the last scalar leptoquarks standing

Crivellin

Müller

Ota

2017

322

254

Over the past years, experiments accumulated intriguing hints for new physics (NP) in flavor observables, namely in the anomalous magnetic moment of the muon (a µ ), in R(D ( * ) ) = Br(B → D ( * ) τ ν)/Br(B → D ( * ) ν) and in b → sµ + µ − transitions, which are all at the 3 − 4 σ level. In this article we point out that one can explain the R(D ( * ) ) anomaly using two scalar leptoquarks (LQs) with the same mass and coupling to fermions related via a discrete symmetry: an SU(2) L singlet and an SU(2) L triplet, both with hypercharge Y = −2/3. In this way, potentially dangerous contributions to b → sνν are avoided and non-CKM suppressed effects in R(D ( * ) ) can be generated. This allows for smaller overall couplings to fermions weakening the direct LHC bounds. In our model, R(D ( * ) ) is directly correlated to b → sτ + τ − transitions where an enhancement by orders of magnitude compared to the standard model (SM) is predicted, such that these decay modes are in the reach of LHCb and BELLE II. Furthermore, one can also naturally explain the b → sµ + µ − anomalies (including R(K)) by a C 9 = −C 10 like contribution without spoiling µ − e universality in charged current decays. In this case sizable effects in b → sτ µ transitions are predicted which are again well within the experimental reach. One can even address the longstanding anomaly in a µ , generating a sizable decay rate for τ → µγ. However, we find that out of the three anomalies R(D ( * ) ), b → sµ + µ − and a µ only two (but any two) can be explained simultaneously. We point out that a very similar phenomenology can be achieved using a vector leptoquark SU(2) L singlet with hypercharge 2/3. In this case, no tuning between couplings is necessary, but the model is non-renormalizable.