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

Abstract

Measurements of the branching fractions of the semileptonic decays $B\to D^{(\ast )}\tau {\bar{\nu }}_{\tau }$ 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 $D^{\ast }$ 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.

Comments: 21 pages, 7 figures, 2019 Helmholtz International Summer School (HISS)-Dubna International Advanced School of Theoretical Physics (DIAS-TH)-Quantum Field Theory at the Limits: From Strong Fields to Heavy Quarks