Measurements of the ratio of B → K * µµ to B → K * ee branching fractions, RK * , by the LHCb collaboration strengthen the hints from previous studies with pseudoscalar kaons, RK , for the breakdown of lepton universality, and therefore the Standard Model (SM), to ∼ 3.5σ. Complementarity between RK and RK * allows to pin down the Dirac structure of the new contributions to be predominantly SM-like chiral, with possible admixture of chirality-flipped contributions of up to O(few10%). Scalar and vector leptoquark representations (S3, V1, V3) plus possible (S2, V2) admixture can explain RK,K * via tree level exchange. Flavor models naturally predict leptoquark masses not exceeding a few TeV, with couplings to third generation quarks at O(0.1), implying that this scenario can be directly tested at the LHC.
Introduction.Gauge interactions of the leptons within the Standard Model (SM) exhibit exact universality. The only known source of lepton non-universality (LNU) are the Yukawa couplings of the leptons to the Higgs. Tests of lepton universality are provided by rare (semi)leptonic |∆B| = |∆S| = 1 transitions, which are induced in the SM at one loop and additionally suppressed by the Glashow-Iliopoulos-Maiani mechanism, therefore allowing to probe physics from scales significantly higher than the weak scale. Useful observables are the ratios of branching fractions of B meson decays into strange hadrons H and muon pairs over electron pairs [1]