# Precise predictions for $\Lambda_b \to \Lambda_c$ semileptonic decays

## TL;DR

This paper provides precise, model-independent predictions for $ ext{Lambda}_b o ext{Lambda}_c$ semileptonic decays, including potential new physics effects, by calculating form factors and decay rates with higher-order corrections and minimal unknown parameters.

## Contribution

It introduces a systematic approach to predict $ ext{Lambda}_b o ext{Lambda}_c$ decay observables with reduced theoretical uncertainties, incorporating new physics and higher-order effects in heavy quark effective theory.

## Key findings

- Predictions for decay rates and form factors with higher-order corrections.
- Model-independent estimates for $R( ext{Lambda}_c)$ including new physics.
- Emphasis on measuring differential decay rates to test factorization.

## Abstract

We calculate the $\Lambda_b \to \Lambda_c \ell \nu$ form factors and decay rates for all possible $b\to c \ell\bar\nu$ four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order $\alpha_s\, \Lambda_\text{QCD}/m_{c,b}$ and $\Lambda_\text{QCD}^2/m_c^2$ in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic $\Lambda_b \to \Lambda_c$ decays in terms of only two unknown sub-subleading Isgur-Wise functions, which can be determined from fitting LHCb and lattice QCD data. We thus obtain model independent results for $\Lambda_b\to \Lambda_c\ell\bar\nu$ decays, including predictions for the ratio $R(\Lambda_c) = {\cal B}(\Lambda_b\to \Lambda_c \tau\bar\nu) / {\cal B}(\Lambda_b\to \Lambda_c \mu\bar\nu)$ in the presence of new physics, that are more precise than prior results in the literature, and systematically improvable with better data on the decays with $\mu$ (or $e$) in the final state. We also explore tests of factorization in $\Lambda_b \to \Lambda_c\pi$ decays, and emphasize the importance of measuring at LHCb the double differential rate $d^2\Gamma(\Lambda_b\to\Lambda_c\ell\bar\nu) / (d q^2\, d\cos\theta)$, in addition to the $q^2$ spectrum.