Lepton universality violation and lepton flavor conservation in $B$-meson decays

TLDR: In this paper, the authors explore symmetry arguments that lead to the hypothesis of minimal flavor violation in charge-current leptonic meson decays under the assumption of negligible neutrino mass effects in charged lepton processes, and the presence of lepton universality violation without lepton flavor violation naturally arises.

Abstract: Anomalies in (semi)leptonic $B$-meson decays present interesting patterns that might be revealing the shape of the new physics to come. In order to understand the experimental data, we explore symmetry arguments that lead to the hypothesis of minimal flavor violation. In particular, under the assumption of negligible neutrino mass effects in charged lepton processes, the presence of lepton universality violation without lepton flavor violation naturally arises. This can account for a deficit of $B^+\to K^+\mu\mu$ over $B^+\to K^+ee$ decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A prediction of this scenario is the modification of processes involving the third generation. In particular, accounting for the above ratio implies a large enhancement, by a factor $\sim10^3$ with respect to the standard model, of all the $b\to s\tau\tau$ decay rates. Although these are still below current experimental limits, they should be easily at reach in future experiments at $B$-factories. Another important consequence is the prediction of sizable effects in charge-current $B$ tauonic decays which could also explain the enhancements that have been observed in the $B\to D^{(*)}\tau\bar \nu$ and $B^-\to\tau^-\bar \nu$ decays. For the most part, the study is carried out in an effective field theory framework with an underlying $SU(2)_L\times U(1)_Y$ symmetry that emphasizes the model-independent correlations arising between low- and high-energy observables. For example, a connection between $B$-decays and top physics is pointed out. To complement the discussion, all possible (spin 0 and 1) leptoquark models are matched to the low-energy field theory so that the effective analysis can be used to survey these candidates for new physics. These models also serve as concrete examples where the hypotheses of this work can be implemented.