Interpreting LHCb's Λb → Λc τ(ν)over-bar measurement and puzzles in semileptonic Λb decays

Normalizing the recent LHCb measurement of Ab-+ Ac tau nu over bar to the standard model (SM) prediction for the Ab-+ Ac mu nu over bar rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio R(Ac). This modestly increases R(Ac) 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 Ab decay may be significantly greater than the corresponding fraction in B decays. Possible implications are speculated upon.

Automated summary

Normalizing the recent LHCb measurement of Ab-+ Ac tau nu over bar to the standard model (SM) prediction for the Ab-+ Ac mu nu over bar rate can provide a more consistent comparison with the SM prediction for the lepton flavor universality ratio R(Ac). This eliminates the previous hint of a suppression compared to the SM, which would be difficult to explain in new physics scenarios. We also speculate about the implication of a higher fraction of excited states in inclusive semileptonic Ab decay compared to B decays.