Flavour anomalies: a review

Abstract

The concept of lepton flavour universality (LFU), according to which the three lepton families are equivalent except for their masses, is a cornerstone prediction of the Standard Model (SM). LFU can be violated in models beyond the SM by new physics particles that couple preferentially to certain generations of leptons. In the last few years, hints of LFU violation have been observed in both tree-level b → c ν and loop-level b → s transitions. These measurements, combined with the tensions observed in angular observables and branching fractions of rare semileptonic b decays, point to a coherent pattern of anomalies that could soon turn into the first observation of physics beyond the SM. These proceedings review the anomalies seen by the LHC experiments and the B factories, and give an outlook for the near future.

The quest for new physics with the Physics Beyond Colliders programme

“…Decays of B-mesons show several deviations of ~2σ -3σ from the SM prediction 51 . Notably, in the SM model the gauge couplings of different leptons (e, μ or τ) are expected to be equal.…”

A search for charged lepton flavor violation in the Mu2e experiment

“…The observation of neutrino oscillations constitutes a de facto extension to the standard theory, with remaining questions regarding the nature of neutrinos; the muon anomalous magnetic moment measurement has yielded a > 3σ discrepancy from the SM prediction, 1 an important result that has guided experimental searches and BSM theory; and numerous signals in semi-leptonic B meson decays offer corroborating hints for violation of lepton flavor universality over a multitude of channels, [2][3][4][5][6][7] with combined statistical significance around the 4σ level. 8 This pattern of experimental deviations reveals precision searches in the flavor sector as a promising and well-motivated field.…”

Pursuing forbidden beauty: Search for the lepton-flavour violating decays B0 → e± μ∓ and Bs0 → e± μ∓ and study of electron-reconstruction performance at LHCb

“…in decays like J/ψ → + − [16]. In contrast, lepton-universality measurements of b-hadron decays in the last decade started to show discrepancies for b → s + − and b → c ν transitions, although none of the individual measurement shows the 5σ significance level conventionally required in the particle physics field to claim a new observation [3]. In the Standard Model, the b → s + − transitions (see e.g.…”

“…Typical distances are of the order of 1 cm. Subsequent decay products have a non-zero impact parameter (IP) 3 . With the high-precision detector modules of the VELO, the IPs of tracks and subsequent displacement of vertices formed by combinations of these tracks can be measured.…”

“…In the last decade, hints of deviations from the Standard Model have appeared in a class of b-hadron measurements testing the property of the Standard Model called lepton universality, i.e. the property that all three types of charged leptons (electrons, muons and taus) interact in the same way with force mediators [3]. A property of the Standard Model that is closely related to lepton universality is the conservation of lepton-family numbers, also known as lepton-flavour conservation.…”