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

Abstract

We study the differential decay rate for the rare ${\Lambda }_b\to \Lambda (\to N\pi ){\ell }^{+}{\ell }^{-}$ transition, including a determination of the complete angular distribution, assuming unpolarized ${\Lambda }_b$ 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) form-factor 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 ${\Lambda }_b\to \Lambda (\to N\pi ){\ell }^{+}{\ell }^{-}$ reveals that this decay mode can provide new and complementary constraints on the Wilson coefficients in radiative and semileptonic $b\to s$ transitions compared to the corresponding mesonic modes.

Comments: 31 pages, 1 figure; v2 as published: add numerical study of optimized observables in the SM and two benchmark scenarios, conclusions unchanged; v3: fixed typo in eq. (3.28), conclusions unchanged