Impact of $Λ_b\to Λ_cτν$ measurement on New Physics in $b\to c \, l ν$ transitions

Abstract

Measurements of the branching ratios of $B \to D^{(*)}τ\barν/B \to D^{(*)}\ell\barν$ and $B_c\to J/ψ\, τ\barν/B_c\to J/ψ\, \ell\barν$ by the BaBar, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio $Λ_b \toΛ_c τ\barν/Λ_b \toΛ_c \ell\barν$ 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 $2σ$ or $1.5σ$, depending on the ${\cal R}(Λ_c)$ 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.