Interpreting LHCb's ${\Lambda }_b\to {\Lambda }_c\tau \bar{\nu }$ measurement and puzzles in semileptonic ${\Lambda }_b$ decays

Abstract

Normalizing the recent LHCb measurement of ${\Lambda }_b\to {\Lambda }_c\tau \bar{\nu }$ to the standard model (SM) prediction for the ${\Lambda }_b\to {\Lambda }_c\mu \bar{\nu }$ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio $R({\Lambda }_c)$. This modestly increases $R({\Lambda }_c)$ compared to the quoted LHCb result, such that it no longer hints at a suppression compared to the SM, which would be hard to accommodate in new physics scenarios that enhance $R(D^{(\ast )})$. We point out that the fraction of excited states in inclusive semileptonic ${\Lambda }_b$ decay may be significantly greater than the corresponding fraction in $B$ decays. Possible implications are speculated upon.

Comments: 3 pages