重子衰变$Λ_b \to pπ, p K$的pQCD方法剖析
Anatomy of the pQCD Approach to the Baryonic Decays $Λ_b \to pπ, p K$
摘要：
用微扰QCD（pQCD）方法计算了两体无粲强子衰变$α_s$的CP平均分支比和CP破坏不对称性.因子分解公式中涉及的重子分布振幅被考虑到领先扭转精度，质子的分布振幅被扩展到次领先共形自旋（即，“P”波），其矩由QCD求和规则确定。我们的工作表明，在$Λ_b \to p π, pK$衰变中的可因式分解图的贡献比传统的pQCD方法中的不可因式分解图小得多。我们认为，这反映了估计的$Λ_b \to p$过渡形式的因素，在$k_T$因式分解的方法，这是发现通常的数量级小于那些估计的光锥总和规则和非相对论夸克模型。作为替代方案，我们采用混合pQCD方法，其中我们计算的可因式分解的贡献与$Λ_b \to p$形状因子取自光锥QCD求和规则。利用传统的pQCD形式主义，这是自由的端点奇点的不可因式分解图进行评估。在这里工作的预测面临的分支比和直接CP不对称的衰变$ Λ_b \to p π$和$Λ_b \to p K$的CDF合作最近可用的数据。计算了极化$Λ_b$重子衰变中与质子各向异性角分布有关的非对称性参数$α$。
英文摘要：
We calculate the CP-averaged branching ratios and CP-violating asymmetries for the two-body charmless hadronic decays $Λ_b \to p π, pK$ in the perturbative QCD (pQCD) approach to lowest order in $α_s$. The baryon distribution amplitudes involved in the factorization formulae are considered to the leading twist accuracy and the distribution amplitudes of the proton are expanded to the next-to-leading conformal spin (i.e., "P" -waves), the moments of which are determined from QCD sum rules. Our work shows that the contributions from the factorizable diagrams in $Λ_b \to p π, pK$ decays are much smaller compared to the non-factorizable diagrams in the conventional pQCD approach. We argue that this reflects the estimates of the $Λ_b \to p$ transition form factors in the $k_T$ factorization approach, which are found typically an order of magnitude smaller than those estimated in the light-cone sum rules and in the non-relativistic quark model. As an alternative, we adopt a hybrid pQCD approach, in which we compute the factorizable contributions with the $Λ_b \to p$ form factors taken from the light cone QCD sum rules. The non-factorizable diagrams are evaluated utilizing the conventional pQCD formalism which is free from the endpoint singularities. The predictions worked out here are confronted with the recently available data from the CDF collaboration on the branching ratios and the direct CP asymmetries for the decays $ Λ_b \to p π$, and $Λ_b \to p K$. The asymmetry parameter $α$ relevant for the anisotropic angular distribution of the emitted proton in the polarized $Λ_b$ baryon decays is also calculated for the two decay modes.
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