New predictions for $\Lambda_b\to\Lambda_c$ semileptonic decays and tests of heavy quark symmetry

TL;DR

This paper uses heavy quark effective theory to make precise, model-independent predictions for $ ext{Lambda}_b o ext{Lambda}_c$ semileptonic decays, fitting form factors to data and lattice calculations, and improving the prediction of decay rate ratios.

Contribution

It derives a more precise standard model prediction for the ratio of tau to muon decay modes in $ ext{Lambda}_b$ decays using heavy quark symmetry and fits to data, addressing previous uncertainties.

Findings

More accurate prediction of $ ext{Lambda}_b o ext{Lambda}_c au ar{ u}$ to $ ext{Lambda}_b o ext{Lambda}_c \mu ar{ u}$ ratio.

Validation of the $ ext{Lambda}_b o ext{Lambda}_c$ form factor expansion's convergence.

Systematic framework for improving decay rate calculations with better data.

Abstract

The heavy quark effective theory makes model independent predictions for semileptonic $\Lambda_b \to \Lambda_c$ decays in terms of a small set of parameters. No subleading Isgur-Wise function occurs at order $\Lambda_{\rm QCD}/m_{c,b}$, and only two sub-subleading functions enter at order $\Lambda_{\rm QCD}^2/m_c^2$. These features allow us to fit the form factors and decay rates calculated up to order $\Lambda_{\rm QCD}^2/m_c^2$ to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio ${\cal B}(\Lambda_b\to \Lambda_c \tau\bar\nu) / {\cal B}(\Lambda_b\to \Lambda_c \mu\bar\nu)$ than prior results, and find the expansion in $\Lambda_{\rm QCD}/m_c$ well-behaved, addressing a long-standing question. Our results allow more precise and reliable calculations of $\Lambda_b\to \Lambda_c\ell\bar\nu$ rates, and are systematically improvable with better data on the $\mu$ (or $e$) modes.