New predictions for $Λ_b\toΛ_c$ semileptonic decays and tests of heavy quark symmetry

Abstract

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