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 → cℓν and loop-level b → sℓℓ 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 collider experiments, and give an outlook for the near future.
Highlights
Quarks and leptons, the fundamental fermions of the Standard Model (SM), exist in three generations, each comprised of two members
lepton flavour universality (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
Summary
The fundamental fermions of the SM, exist in three generations, each comprised of two members. LFU is an accidental symmetry, broken only by Yukawa interactions, and states that the electroweak gauge bosons couple with equal strength to the three families of leptons. This property is well established in decays of light mesons, e.g. K → ν decays [1]. A violation of LFU would clearly indicate that new particles participate in quark flavour changing processes, modifying their dynamics. Flavour-changing neutral current (FCNC) transitions between same-charge quarks are not directly mediated by the neutral weak boson Z0, but rather occur through much rarer loop processes involving virtual W ± and additional virtual quarks, in penguin- and box-like Feynman diagrams. BaBar, PRL109,101802(2012) Belle, PRD92,072014(2015) LHCb, PRL115,111803(2015) Belle, PRD94,072007(2016) Belle, PRL118,211801(2017) LHCb, PRL120,171802(2018) Average
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