$D^\ast$ Polarization as an Additional Constraint on New Physics in the $b\to c\tau\bar\nu_\tau$ Transition

TL;DR

This paper investigates how the $D^*$ polarization observable can serve as an additional constraint on new physics models explaining anomalies in $b o c auar u_ au$ transitions, using updated experimental data and a model-independent analysis.

Contribution

It introduces the use of $D^*$ polarization as a new constraint in analyzing new physics effects in semileptonic B decays, with an updated fit of Wilson coefficients based on covariant quark model form factors.

Findings

Updated Wilson coefficient constraints incorporating polarization data.

Identification of viable new physics scenarios consistent with all current data.

Predictions for polarization observables to guide future experiments.

Abstract

Measurements of the branching fractions of the semileptonic decays $B\to D^{(*)}\tau\bar\nu_\tau$ and $B_c\to J/\psi\tau\bar\nu_\tau$ systematically exceed the Standard Model (SM) predictions, pointing to possible signals of new physics that can violate lepton flavor universality. The unknown origin of new physics realized in these channels can be probed using a general effective Hamiltonian constructed from four-fermion operators and the corresponding Wilson coefficients. Previously, constraints on these Wilson coefficients were obtained mainly from the experimental data for the branching fractions. Meanwhile, polarization observables were only theoretically studied. The situation has changed with more experimental data having become available, particularly those regarding the polarization of the tau and the $D^*$ meson. In this study, we discuss the implications of the new data on the overall picture. We then include them in an updated fit of the Wilson coefficients using all hadronic form factors from our covariant constituent quark model. The use of our form factors provides an analysis independent of those in the literature. Several new-physics scenarios are studied with the corresponding theoretical predictions provided, which are useful for future experimental studies.