Interpreting LHCb's $\Lambda_b\to \Lambda_c\tau\bar\nu$ measurement and puzzles in semileptonic $\Lambda_b$ decays

TL;DR

This paper re-evaluates LHCb's measurement of Lambda_b decays, showing that normalization choices affect the interpretation of lepton flavor universality tests and highlighting potential puzzles in semileptonic Lambda_b decay fractions.

Contribution

It demonstrates that normalizing to the SM prediction rather than LEP data alters the significance of deviations in R(Lambda_c), and discusses the possible larger excited state contributions in Lambda_b decays.

Findings

Re-normalization reduces tension with the SM in R(Lambda_c)

Excited state fractions in Lambda_b decays may be larger than in B decays

Implications for new physics scenarios are discussed

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 $R(D^{(*)})$. 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.