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We study the differential decay rate for the rare $\Lambda_b \to \Lambda (\toN \pi)\ell^+\ell^-$ transition, including a determination of the completeangular distribution, assuming unpolarized $\Lambda_b$ baryons. On the basis ofa properly chosen parametrization of the various helicity amplitudes, weprovide expressions for the angular observables within the Standard Model and asubset of new physics models with chirality-flipped operators. Hadronic effectsat low recoil are estimated by combining information from lattice QCD with(improved) form-factor relations in Heavy Quark Effective Theory. Our estimatesfor large hadronic recoil -- at this stage -- are still rather uncertainbecause the baryonic input functions are not so well known, andnon-factorizable spectator effects have not been worked out systematically sofar. Still, our phenomenological analysis of decay asymmetries and angularobservables for $\Lambda_b \to \Lambda (\to N \pi)\ell^+\ell^-$ reveals thatthis decay mode can provide new and complementary constraints on the Wilsoncoefficients in radiative and semileptonic $b \to s$ transitions compared tothe corresponding mesonic modes.

Angular Analysis of the Decay $\Lambda_b \to \Lambda (\to N \pi) \ell^+\ell^-$