${\Lambda }_b\to \Lambda {\ell }^{+}{\ell }^{-}$ and ${\Lambda }_b\to {\Lambda }^{\ast }{\ell }^{+}{\ell }^{-}$ decays with polarized leptons

Abstract

We present an analysis of lepton polarization observables in the rare flavor-changing neutral current decays ${\Lambda }_b\to \Lambda {\ell }^{+}{\ell }^{-}$ and ${\Lambda }_b\to {\Lambda }^{\ast }{\ell }^{+}{\ell }^{-}$. These decays are driven by the quark-level transition $b\to s{\ell }^{+}{\ell }^{-}$ and are sensitive to short-distance physics within and beyond the Standard Model. We work within an effective Hamiltonian framework and derive single lepton polarization asymmetries, double lepton polarization asymmetries, and polarized forward-backward asymmetries using the helicity method. The observables are expressed in terms of transversity amplitudes and evaluated using state-of-the-art lattice QCD form factors. We analyze the $q^2$ dependence of several observables for both the ground-state $\Lambda$ and the excited ${\Lambda }^{\ast }(1520)$ final states and discuss sensitivities of the observables to chiral structure of the effective operators and complex Wilson coefficients.