Revisit to the $b\to c\tau \nu$ transition: in and beyond the SM

Abstract

We perform an analysis of the $b\to c\tau \nu$ data, including $R(D^{(\ast )})$, $R(J/\psi )$, $P_{\tau }(D^{\ast })$ and $F_L^{D^{\ast }}$, within and beyond the Standard Model (SM). We fit the $B\to D^{(\ast )}$ hadronic form factors in the HQET parametrization to the lattice and the light-cone sum rule (LCSR) results, applying the general strong unitarity bounds corresponding to $J^P=1^{-}$, $1^{+}$, $0^{-}$ and $0^{+}$. Using the obtained HQET relations between helicity amplitudes, we give the strong unitarity bounds on individual helicity amplitudes, which can be used in the BGL fits. Using the fitted form factors and taking into account the most recent Belle measurement of $R(D^{(\ast )})$ we investigate the model-independent and the leptoquark model explanations of the $b\to c\tau \nu$ anomalies. Specifically, we consider the one-operator, the two-operator new physics (NP) scenarios and the NP models with a single $R_2$, $S_1$ or $U_1$ leptoquark which is supposed to be able to address the $b\to c\tau \nu$ anomalies, and our results show that the $R_2$ leptoquark model is in tension with the limit $\mathcal{B}(B_c\to \tau \nu )<10\%$. Furthermore, we give predictions for the various observables in the SM and the NP scenarios/leptoquark models based on the present form factor study and the analysis of NP.

arXiv comment35 pages, 5 figures and 14 tables; Matches the published version in NPB

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• Precise limits on the charge-$2/3$ $U_1$ vector leptoquark
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• A Complete Framework for Tau Polarimetry in $B\to D^{(\ast )}\tau \nu$ Decays
• Analysing the charged scalar boson contribution to the charged-current $B$ meson anomalies
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• Analysis of $b\to c\tau {\bar{\nu }}_{\tau }$ anomalies using weak effective Hamiltonian with complex couplings
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