We calculate the ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\ell \nu$ form factors and decay rates for all possible $b\to c\ell \overline{\nu }$ four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order ${\alpha }_s{\mathrm{\Lambda }}_{\mathrm{QCD}}/m_{c,b}$ and ${\mathrm{\Lambda }}_{\mathrm{QCD}}^2/m_c^2$ in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c$ 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 ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\ell \overline{\nu }$ decays, including predictions for the ratio $R({\mathrm{\Lambda }}_c)=\mathcal{B}({\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\tau \overline{\nu })/\mathcal{B}({\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\mu \overline{\nu })$ 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 ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\pi$ decays, and we emphasize the importance of measuring at LHCb the double differential rate $d^2\mathrm{\Gamma }({\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\ell \overline{\nu })/(d{q}^{2}d\cos \theta )$, in addition to the $q^2$ spectrum.

• Received 10 January 2019

DOI:https://doi.org/10.1103/PhysRevD.99.055008

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society