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Direct CP violation in $Λ_b$ decays

We study direct CP violating asymmetries (CPAs) in the two-body $Λ_b$ decays of $Λ_b\to pM(V)$ with $M(V)=K^-(K^{*-})$ and $π^-(ρ^-)$ based on the generalized factorization method. After simultaneously explaining the observed decay branching ratios of $Λ_b\to (p K^-\,,\; p π^-)$ with ${\cal R}_{πK}\equiv {\cal B}(Λ_b\to p π^-)/{\cal B}(Λ_b\to p K^-)$ being $0.84\pm 0.09$, we find that their corresponding direct CPAs are $(5.8\pm 0.2,\,-3.9\pm 0.2)\%$ in the standard model (SM), in comparison with $(-5^{+26}_{-~5},\, -31^{+43}_{-~1})\%$ and $(-10\pm8\pm4,\, 6\pm7\pm3)\%$ from the perturbative QCD calculation and the CDF experiment, respectively. For $Λ_b\to ( p K^{*-},\, pρ^-)$, the decay branching ratios and CPVs in the SM are predicted to be $(2.5\pm0.5,\,11.4\pm2.1)\times 10^{-6}$ with ${\cal R}_{ρK^*}=4.6\pm0.5$ and $(19.6\pm1.6,\, -3.7\pm0.3)\%$, respectively. The uncertainties for the CPAs in these decay modes as well as ${\cal R}_{πK,\,ρK^{*-}}$ mainly arise from the quark mixing elements and non-factorizable effects, whereas those from the hadronic matrix elements are either totally eliminated or small. We point out that the large CPA for $Λ_b\to p K^{*-}$ is promising to be measured by the CDF and LHCb experiments, which is a clean test of the SM.