We study the decays of $\Lambda_b \to \Lambda(\to p \pi^-) \ell ^+ \ell^-$ with $\ell = (e, \mu, \tau)$. In particular, we examine the full angular distributions with polarized $\Lambda_b$ and identify the time-reversal asymmetries or T-odd observables. By using the homogeneous bag model, we find that the decay branching fractions of $\Lambda_b \to \Lambda \ell^+\ell^-$ are $(9.1\pm 2.5,7.9\pm 1.8, 2.1\pm 0.2)\times 10^{ -7}$ for $\ell =(e, \mu ,\tau)$, respectively. In addition, we obtain that $A_{ FB}^{ \ell} = -0.369\pm 0.007$ and $A_{ FB}^{ h}=-0.333\pm 0.004$, averaged in the range of $15 \leq q^2 \leq 20~ \text{ GeV}^2$. These results are well consistent with the current experimental data. We also explore the T-odd observables in $\Lambda_b \to \Lambda(\to p \pi^-) \mu^+ \mu^-$, which are sensitive to new physics~(NP). Explicitly, we illustrate that the current experimental measurement from one of the T-odd observables favors the existence of NP, such as the extra $Z$-boson model.
Categories: hep-ph hep-ex