Angular Analysis of \boldmath $Λ_b\to Λ_c (\to Λπ)\ell\barν$

Abstract

We revisit the decay $Λ_b^0\to Λ_c^+ \ell^-\barν$ ($\ell = e,μ,τ$) with a subsequent two-body decay $Λ_c^+ \to Λ^0 π^+$ in the Standard Model and in generic New Physics models. The decay's joint four-differential angular distribution can be expressed in terms of ten angular observables, assuming negligible polarization of the initial $Λ_b$ state. We present compact analytical results for all angular observables, which enables us to discuss their possible New Physics reach. We find that the decay at hand probes more and complementary independent combinations of Wilson coefficients compared to its mesonic counter parts $\bar{B}\to D^{(*)}\ell^-\barν$. Our result for the angular distribution is at variance with some of the results on scalar-vector interference terms in the literature. We provide numerical estimates for all angular observables based on lattice-QCD results for the $Λ_b \to Λ_c$ form factors and account for a recent measurement of the parity-violating parameter in $Λ_c^+\to Λ^0π^+$ decays by BESIII. A numerical implementation of our results is made publicly available as part of the EOS software.