Impact of ${\Lambda }_b\to {\Lambda }_c\tau \nu$ measurement on New Physics in $b\to cl\nu$ transitions

Abstract

Measurements of the branching ratios of $B\to D^{(\ast )}\tau \bar{\nu }/B\to D^{(\ast )}\ell \bar{\nu }$ and $B_c\to J/\psi \tau \bar{\nu }/B_c\to J/\psi \ell \bar{\nu }$ by the BaBar, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio ${\Lambda }_b\to {\Lambda }_c\tau \bar{\nu }/{\Lambda }_b\to {\Lambda }_c\ell \bar{\nu }$ shows a relative deficit in taus. In this paper, we critically address whether data still points towards a coherent pattern of deviations, in particular in light of the sum rule relating these decays in a model-independent way. We find that no common new physics explanation of all ratios is possible (within $2\sigma$ or $1.5\sigma$, depending on the $\mathcal{R}({\Lambda }_c)$ normalization to light lepton channels). While this inconsistency could be a statistical fluctuation, further measurements are required in order to converge to a coherent pattern of experimental results.

Comments: 12 pages, 3 tables. Minor corrections, conclusions unchanged, matches journal version