Time-reversal asymmetries in ${\Lambda }_b\to \Lambda (\to p{\pi }^{-}){\ell }^{+}{\ell }^{-}$

Abstract

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 $15\le q^2\le 20{\text{GeV}}^2$. 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.

Comments: 21 pages, 1 figure