Angular analysis of the rare decay ${\Lambda }_b\to \Lambda (1520)(\to NK){\ell }^{+}{\ell }^{-}$

Abstract

We study the differential decay rate for the rare decay ${\Lambda }_b\to \Lambda (1520)(\to NK){\ell }^{+}{\ell }^{-}$ where $\ell$ is a light lepton, as this decay mode can provide new and complementary constraints on the Wilson coefficients in $b\to s{\ell }^{+}{\ell }^{-}$ transitions compared to other modes. We provide a determination of the complete angular distribution, assuming unpolarised ${\Lambda }_b$ baryons and neglecting the lepton mass. The resulting angular observables are expressed in terms of helicity amplitudes involving hadronic form factors within the Standard Model and New physics models with chirality-flipped operators. We study these observables at low and large $\Lambda$ recoils, using effective theories to determine relations among the hadronic form factors involved. As there is currently no determination of the form factors available from lattice simulations or light-cone sum rules, we perform a first illustration of the sensitivity of some observables to New Physics contributions using hadronic inputs from quark models.

Comments: 37 pages, 5 figures. Corrected typos in Section 2 and Appendix A. Numerical Illustrations updated from arXiv:1108.6129v2 to arXiv:1108.6129v3