Phenomenology of an SU(2) × SU(2) × U(1) model with lepton-flavour non-universality

323

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.

Section: Jhep09(2017)040mentioning

confidence: 99%

“…Therefore, it is well motivated to search for a simultaneous explanation of these two anomalies [8][9][10][11][12][13][14][15][16][17][18][19][20]. Furthermore, since a e agrees with the SM prediction, also a µ can be considered as a LFUV quantity and one can address it together with R(D ( * ) ) and/or b → sµ + µ − [13,[21][22][23].…”

Section: Introductionmentioning

confidence: 99%

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

Crivellin

Müller

Ota

2017

323

254

“…Models featuring extra Z to explain the anomalies are very popular, see the partial list of references [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61]. Typically these models contain a Z with mass M Z savagely coupled to …”

Section: Models With An Extra Zmentioning

confidence: 99%

Flavour anomalies after the R K ∗ measurement

D’Amico

Nardecchia

Panci

et al. 2017

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.

“…At the level of concrete NP models, most analyses focus on a generation of the required NP effects at tree level, either by the exchange of Z vector bosons [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] or through leptoquarks [31][32][33][34][35][36][37][38]. An explanation of the anomalies via loop effects, on the other hand, typically leads to correlated imprints on other observables like the anomalous magnetic moment of the muon (a µ ).…”

Section: Jhep04(2017)043mentioning

confidence: 99%

Loop effects of heavy new scalars and fermions in b → sμ + μ −

Arnan

Crivellin

Hofer

et al. 2017

127

190

Recent measurements of b → sµ + µ − processes at LHCb and BELLE have revealed tensions at the 2 − 3 σ level between the Standard Model (SM) prediction and the experimental results in the channels B → K * µ + µ − and B s → φµ + µ − , as well as in the lepton-flavor universality violating observable R K = Br(B → Kµ + µ − )/Br(B → Ke + e − ). Combined global fits to the available b → sµ + µ − data suggest that these tensions might have their common origin in New Physics (NP) beyond the SM because some NP scenarios turn out to be preferred over the SM by 4 − 5 σ. The fact that all these anomalies are related to muons further suggests a connection (and a common NP explanation) with the long-standing anomaly in the anomalous magnetic moment of the muon, a µ . In this article, we study the impact of a generic class of NP models featuring new heavy scalars and fermions that couple to the SM fermions via Yukawa-like interactions. We consider two different scenarios, introducing either one additional fermion and two scalars or two additional fermions and one scalar, and examine all possible representations of the new particles under the SM gauge group with dimension up to the adjoint one. The models induce one-loop contributions to b → sµ + µ − and a µ which are capable of solving the respective anomalies at the 2σ level, albeit a relatively large coupling of the new particles to muons is required. In the case of b → sµ + µ − , stringent constraints from B s − B s mixing arise which can be relaxed if the new fermion is a Majorana particle.