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

TL;DR

This paper analyzes the angular distribution of the rare decay b  b (
a N
e
d)    , providing a framework to explore potential new physics effects in b  s transitions.

Contribution

It offers a comprehensive angular analysis including Standard Model and new physics scenarios, with estimates based on lattice QCD and heavy quark effective theory.

Findings

Decay asymmetries can constrain Wilson coefficients in b  s transitions.

The decay mode provides complementary information to mesonic decays.

Uncertainties remain large at high recoil due to hadronic input limitations.

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.