Search

Measurements of the branching ratios of B→D(*)τν¯/B→D(*)ℓν¯ and Bc→J/ψτν¯/Bc→J/ψℓν¯ by the BABAR, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio Λb→Λcτν¯/Λb→Λcℓν¯ shows a relative deficit in taus. In this paper, we critically address whether data still points towards a coherent pattern of deviations, in particular in light of the sum rule relating these decays in a model-independent way. We find that no common new physics explanation of all ratios is possible (within 2σ or 1.5σ, depending on the R(Λc) normalization to light lepton channels). While this inconsistency could be a statistical fluctuation, further measurements are required in order to converge to a coherent pattern of experimental results.

The rare flavor-changing top quark decay t→cZ is a clear sign of new physics and experimentally very interesting due to the huge number of top quarks produced at the LHC. However, there are few (viable) models which can generate a sizable branching ratio for t→cZ—in fact vectorlike quarks seem to be the only realistic option. In this paper, we investigate all three representations (under the Standard Model gauge group) of vectorlike quarks (U, Q1 and Q7) that can generate a sizable branching ratio for t→cZ without violating bounds from B physics. Importantly, these are exactly the three vectorlike quarks which can lead to a sizable positive shift in the prediction for W mass, via the couplings to the top quark also needed for a sizable Br(t→cZ). Calculating and using the one-loop matching of vectorlike quarks on the Standard Model effective field theory, we find that Br(t→cZ) can be of the order of 10−6, 10−5 and 10−4 for U, Q1 and Q7, respectively, and that in all three cases the large W mass measurement can be accommodated.

Recently, the standard model predictions for the B-meson hadronic decays, B¯0→D(*)+K− and B¯s0→Ds(*)+π−, have been updated based on the QCD factorization approach. This improvement sheds light on a novel puzzle in the B-meson hadronic decays: there are mild but universal tensions between data and the predicted branching ratios. Assuming the higher-order QCD corrections are not huge enough to solve the tension, we examine several new physics interpretations of this puzzle. We find that the tension can be partially explained by a left-handed W′ model, which can be compatible with other flavor observables and collider bounds.

The KOTO experiment recently reported four candidate events in the signal region of KL→π0νν¯ search, where the standard model only expects 0.10±0.02 events. If confirmed, this requires physics beyond the standard model to enhance the signal. We examine various new physics interpretations of the result including these: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of “νν¯” as a new light long-lived particle, or (3) reinterpretation of the whole signal as the production of a new light long-lived particle at the fixed target. We study the above explanations in the context of a generalized new physics Grossman-Nir bound coming from the K+→π+νν¯ decay, bounded by data from the E949 and the NA62 experiments.

In this addendum to Ref. [1], we update our results to include the recent measurement of R(D) and R(D*) by the Belle Collaboration [2]: R(D)Belle=0.307±0.037±0.016 and R(D*)Belle=0.283±0.018±0.014, resulting in the new HFLAV fit result R(D)=0.340±0.027±0.013, R(D*)=0.295±0.011±0.008, exhibiting a 3.1σ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for R(Λc) from our sum rule reads R(Λc)=RSM(Λc)(1.15±0.04)=0.38±0.01±0.01. We also comment on theoretical predictions for the fragmentation function fc of b→Bc and their implication on the constraint from Bu/c→τν data.

The combined analysis of the BABAR, Belle, and LHCb data on B→Dτν, B→D*τν and Bc→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 FL(D*) and of the τ polarizations, and especially on the constraint from the branching fraction BR(Bc→τν). We critically review this constraint and do not support the aggressive limit of BR(Bc→τν)<10% used in some analyses. While the impact of FL(D*) is currently still limited, the BR(Bc→τν) 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*τν and Λb→Λcτν which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of BR(Λb→Λcτν) over its SM prediction by (24±6)% for the current R(D(*)) data.