$CP$ violation in two-body hadronic $\Lambda_b$ decays in the PQCD approach

TL;DR

This paper calculates CP-violating effects in two-body hadronic Lambda_b decays using perturbative QCD, predicting small overall CPV but larger effects in specific channels, with implications for experimental tests of baryon CP violation.

Contribution

It provides the first systematic PQCD-based predictions of CP violation in Lambda_b decays, including detailed partial-wave analyses and angular asymmetries, highlighting channels with sizable CPV effects.

Findings

Predicted direct CPV in Lambda_b→pπ− and pK− is small, consistent with current data.

Partial-wave CPVs can exceed 10%, indicating potential observability.

Large CPV asymmetries (>20%) are predicted in certain angular distributions of Lambda_b→pK1 decays.

Abstract

We systematically investigate the $CP$-averaged branching ratios and $CP$ violations (CPVs) for the two-body hadronic decays $\Lambda_b\to ph$, where $h$ runs through the mesons $\pi^-$, $\rho^-$, $a_1^-(1260)$, $K^-$, $K^{\ast -}$, $K_1^-(1270)$ and $K_1^-(1400)$, in the perturbative QCD approach to order $\alpha_s^2$ in the strong coupling. Various topological amplitudes are obtained by incorporating subleading-twist hadron distribution amplitudes, which exhibit reasonable hierarchical patterns, sizable strong phases, and non-negligible higher-power corrections. The predicted direct CPVs in $\Lambda_b\to p\pi^-,pK^-$, different from those in similar $B$ meson decays, are as small as the current data. The low CPV in $\Lambda_b\to p\pi^-$ results from the cancellation between the $S$- and $P$-wave CPVs, while the one in $\Lambda_b\to pK^-$ is determined by the tiny $S$-wave CPV. However, individual partial-wave CPVs can exceed $10\%$, consistent with direct CPVs in $B$ meson decays. The CPVs in the $\Lambda_b\to pK_1^-(1270),pK_1^-(1400)$ channels are relatively larger. In particular, CPVs above $20\%$ appear in the up-down asymmetries associated with the final-state angular distributions of $\Lambda_b\to pK_1^-(1270),pK_1^-(1400)$, followed by the secondary $K_1\to K\pi\pi$ decays. These observables offer promising prospects for firmly establishing baryon CPVs. The decay asymmetry parameters of $\Lambda_b\to ph$ are also predicted for future experimental confrontations.