Investigation of Effects of New Physics in $\Lambda_b\to\Lambda_c \tau\bar\nu_\tau$ Decay

Abstract

Recent experimental results of ${\cal R}(D^{(*)})$ deviate from the standard model (SM) by $3.1\sigma$, suggesting a new physics (NP) that affects the $b\to c \tau \bar\nu_\tau$ transition. Motivated by this, we investigate the possible NP effects in the $\Lambda_b\to\Lambda_c \tau\bar\nu_\tau$ decay. For this purpose, assuming the neutrinos are left-handed, we calculate in detail the helicity amplitudes of $\Lambda_b\to\Lambda_c \ell\bar\nu_\ell$ decays with all possible four-fermion operators. Within the latest results of $\Lambda_b\to\Lambda_c$ form factors from lattice QCD calculations, we study these decays in a model-independent manner. The differential and total branching fractions and other observables are calculated. In SM, we obtain the ratio ${\cal R}(\Lambda_c)=0.33\pm0.01$. Supposing that NP only affects the third generation fermions, we present the correlations among ${\cal R}(D)$, ${\cal R}(D^*)$ and ${\cal R}(\Lambda_c)$. We perform a minimum $\chi^2$ fit of the wilson coefficient of each operator to the latest experimental data of different observables. It is found that the left-handed scalar operator ${\cal O}_{SL}$ affects the branching fraction remarkably, and the ratio ${\cal R}(\Lambda_c)$ can be enhanced by $30\%$. For other operators, the ratio amounts to $0.38\pm0.02$, which is larger than prediction of SM by $20\%$. Using the fitted values of the wilson coefficients of the single NP operators, we also give a prognosis for the physical observables of $\Lambda_b\to\Lambda_c \tau\bar\nu_\tau$. Furthermore, we also study the effects of three typical NP models on $\Lambda_b\to\Lambda_c \tau\bar\nu_\tau$. We hope our results can be tested in the current LHCb experiment and the future high energy experiments.