$Λ_b \to (Λ_c,\,p)\,τ\,ν$ decays within standard model and beyond (1512.04034v1)

Abstract: Deviations from the standard model prediction have been observed in several leptonic and semileptonic $B$ meson decays to $\tau\nu$ final states mediated via $b \to u$ and $b \to c$ charged current interactions. The measured value of ratio of branching ratios $R_{\pi}^l$ of $B^- \to \tau^-\,\nu_{\tau}$ to $B^0 \to \pi^+\,\l^-\,\nu$ decays, where $l = (e,\,\mu)$, is larger than the standard model prediction by more than a factor of two. Similarly, a combined excess of $3.9\sigma$ from the standard model expectation has been reported by HFAG for the values of $R_D$ and $R_{D^{\ast}}$, where $R_{D,\,D^{\ast}}$ represents the ratio of branching ratios of $B \to (D,\,D^{\ast})\,\tau\nu$ to corresponding $B \to (D,\,D^{\ast})\,l\nu$ decays, respectively. Very recently, hint of lepton flavor violation has been observed in the ratio of branching fractions of $B \to K\,e^+\,e^-$ to $B \to K\,\mu^+\,\mu^-$ decays as well. In this context, we employ an effective Lagrangian approach to study the decay branching fractions and the ratio of branching fractions of $\Lambda_b \to \Lambda_c\,l\,\nu$ and $\Lambda_b \to p\,l\,\nu$ decays within the standard model and beyond. We constrain the new physics parameter space using the existing experimental data on $R_D$, $R_{D^{\ast}}$, and $R_{\pi}^l$. We give predictions for various observables in the context of various new physics scenarios.