Time-reversal asymmetries and angular distributions in $\Lambda_b \to \Lambda V$

TL;DR

This paper investigates T-odd spin correlations and angular distributions in Lambda_b decays to probe time-reversal asymmetries, providing a framework to test the Standard Model and explore new physics effects.

Contribution

It introduces a detailed analysis of T-odd spin correlations and angular distributions in Lambda_b decays, including the effects of nonfactorizable contributions and potential new physics.

Findings

Polarization asymmetries of Lambda are close to -1.

Standard Model predicts suppressed T and CP asymmetries in Lambda_b decays.

Nonfactorizable effects or new physics may significantly influence decay rates and asymmetries.

Abstract

We study the spin correlations to probe time-reversal~(T) asymmetries in the decays of $\Lambda_b \to \Lambda V ~(V=\phi, \rho^0, \omega, K^{*0})$. The eigenstates of the T-odd operators are obtained along with definite angular momenta. We obtain the T-odd spin correlations from the complex phases among the helicity amplitudes. We give the angular distributions of $\Lambda_b \to \Lambda(\to p \pi^-)V(\to PP')$ and show the corresponding spin correlations, where $P^{(\prime)}$ are the pseudoscalar mesons. Due to the helicity conservation of the $s$ quark in $\Lambda$, we deduce that the polarization asymmetries of $\Lambda$ are close to $-1$. Since the decay of $\Lambda_b \to \Lambda \phi$ in the standard model~(SM) is dictated by the single weak phase from the product of CKM elements, $V_{tb}V_{ts}^*$, the true T and CP asymmetries are suppressed, providing a clean background to test the SM and search for new physics. In the factorization approach, as the helicity amplitudes in the SM share the same complex phase, T-violating effects are absent. Nonetheless, the experimental branching ratio of $Br(\Lambda_b \to \Lambda \phi) = (5.18\pm 1.29 )\times 10^{-6}$ suggests that the nonfactorizable effects or some new physics play an important role. By parametrizing the nonfactorizable contributions with the effective color number, we calculate the branching ratios and direct CP asymmetries. We also explore the possible T-violating effects from new physics.