Violation of lepton flavour universality in composite Higgs models

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: Models With An Extra Zmentioning

confidence: 99%

Flavour anomalies after the R K ∗ measurement

D’Amico

Nardecchia

Panci

et al. 2017

256

201

“…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.

“…the experimentally observed hierarchical structure of Yukawa couplings. Some papers in the literature have attempted to obtain this connection in the context of partial compositeness [19,25,28] or leptoquark models [30]. Other works [18,22] considered gauged abelian symmetries that are able to reproduce some features of the CKM matrix (but not the SM quark mass hierarchies).…”

Section: Jhep11(2015)173mentioning

confidence: 99%

“…Nevertheless, it is intriguing that many of the discrepancies can be simultaneously explained by assuming new physics contributions to the Wilson coefficients of 4-fermion operators with a band s-quark and 2 leptons [4][5][6][7][8][9][10][11][12][13][14][15]. Such contributions can be easily generated in explicit models through the exchange of a new Z gauge boson [16][17][18][19][20][21][22][23][24][25][26] or leptoquarks [11,[27][28][29][30][31][32][33][34].…”

Section: Introductionmentioning

confidence: 99%

Lepton flavor non-universality in B-meson decays from a U(2) flavor model

Falkowski

Nardecchia

Ziegler

2015

125

Abstract:We address the recent anomalies in semi-leptonic B-meson decays using a model of fermion masses based on the U(2) flavor symmetry. The new contributions to b → s transitions arise due to a tree-level exchange of a Z vector boson gauging a U(1) subgroup of the flavor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the e-channel and destructive interference in the µ-channel. The current experimental data on semi-leptonic B-meson decays can be very well reproduced without violating existing constraints from flavor violation in the quark and lepton sectors. Our model will be tested by new measurements of b → s transitions and also by future electroweak precision tests, direct Z searches, and µ-e conversion in nuclei.