New predictions for ${\Lambda }_b\to {\Lambda }_c$ semileptonic decays and tests of heavy quark symmetry

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 ${\Lambda }_{\mathrm{QCD}}/m_{c,b}$, and only two sub-subleading functions enter at order ${\Lambda }_{\mathrm{QCD}}^2/m_c^2$. These features allow us to fit the form factors and decay rates calculated up to order ${\Lambda }_{\mathrm{QCD}}^2/m_c^2$ to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio $\mathcal{B}({\Lambda }_b\to {\Lambda }_c\tau \bar{\nu })/\mathcal{B}({\Lambda }_b\to {\Lambda }_c\mu \bar{\nu })$ than prior results, and find the expansion in ${\Lambda }_{\mathrm{QCD}}/m_c$ 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.

5 pages, 7 figures