${\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau }$ Decay in the Standard Model and with New Physics

Abstract

Recently hints of lepton flavor non-universality emerged when the BaBar Collaboration observed deviations from the standard model predictions in $R(D^{(\ast )})\equiv \mathcal{B}(\bar{B}\to D^{(\ast )+}{\tau }^{-}{\bar{\nu }}_{\tau })/\mathcal{B}(\bar{B}\to D^{(\ast )+}{\ell }^{-}{\bar{\nu }}_{\ell })$ ($\ell =e,\mu$). Another test of this non-universality can be in the semi leptonic ${\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau }$ decay. In this work we present predictions for this decay in the standard model and in the presence of new-physics operators with different Lorentz structures. We present the most general four-fold angular distribution for this decay including new physics. For phenomenology, we focus on predictions for the decay rate and the differential distribution in the momentum transfer squared $q^2$. In particular, we calculate $R_{{\Lambda }_b}=\frac{BR[{\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau }]}{BR[{\Lambda }_b\to {\Lambda }_c\ell {\bar{\nu }}_{\ell }]}$ where $\ell$ represents $\mu$ or $e$, and find the standard model prediction to be around $0.3$ while the new physics operators can increase or slightly decrease this value.

Comments: 15 pages, 4 figures, 3 table. Accepted for publication in Physical review D