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

Understand

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.