Interpreting LHCb's $Λ_b\to Λ_cτ\barν$ measurement and puzzles in semileptonic $Λ_b$ decays

Abstract

Normalizing the recent LHCb measurement of $Λ_b \to Λ_c τ\barν$ to the standard model (SM) prediction for the $Λ_b \to Λ_c μ\barν$ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio $R(Λ_c)$. This modestly increases $R(Λ_c)$ 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 $R(D^{(*)})$. We point out that the fraction of excited states in inclusive semileptonic $Λ_b$ decay may be significantly greater than the corresponding fraction in $B$ decays. Possible implications are speculated upon.