B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B ! " decay, where the large mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B ! D ðÃÞ " rates. Because of the rich spin structure of the final state, the decay mode B ! D à " offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b ! c (semi)leptonic transitions. We explore such contributions to the B ! D à " decay amplitudes by determining the differential decay rate, longitudinal D à polarization fraction, D à À opening angle asymmetry and the helicity asymmetry. We identify the size of possible new physics contributions to these observables constrained by the present B ! D ðÃÞ " rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the helicity asymmetry can still deviate by as much as 80%.

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]

Present measurements of b→cτν and b→uτν transitions differ from the standard model predictions of lepton flavor universality by almost 4σ. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super-)B factories are presented.

The combined analysis of the BABAR, Belle, and LHCb data on B → Dτν, B → D à τν and B c → J=Ψτν decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one-and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of F L ðD Ã Þ and of the τ polarizations, and especially on the constraint from the branching fraction BRðB c → τνÞ. We critically review this constraint and do not support the aggressive limit of BRðB c → τνÞ < 10% used in some analyses. While the impact of F L ðD Ã Þ is currently still limited, the BRðB c → τνÞ constraint has a significant impact: depending on whether one uses a limit of 60%, 30% or 10%, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative 60% limit a scenario with scalar operators gives the best fit to data, while for an aggressive 10% limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of B → Dτν, B → D à τν and Λ b → Λ c τν which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of BRðΛ b → Λ c τνÞ over its SM prediction by ð24 AE 6Þ% for the current RðD ðÃÞ Þ data.