Angular analysis of the decay Λ b → Λ ( → Nπ ) ` + ` −

We study the differential decay rate for the rare Λb → Λ(→ Nπ)`+`− transition, including a determination of the complete angular distribution, assuming unpolarized Λb baryons. On the basis of a properly chosen parametrization of the various helicity amplitudes, we provide expressions for the angular observables within the Standard Model and a subset of new physics models with chirality-flipped operators. Hadronic effects at low recoil are estimated by combining information from lattice QCD with (improved) formfactor relations in Heavy Quark Effective Theory. Our estimates for large hadronic recoil — at this stage — are still rather uncertain because the baryonic input functions are not so well known, and non-factorizable spectator effects have not been worked out systematically so far. Still, our phenomenological analysis of decay asymmetries and angular observables for Λb → Λ(→ Nπ)`+`− reveals that this decay mode can provide new and complementary constraints on the Wilson coefficients in radiative and semileptonic b→ s transitions compared to the corresponding mesonic modes.