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arXiv 2202.04804hep-phnucl-th
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微扰QCD中的$Λ_b\to p$跃迁形状因子

$Λ_b\to p$ transition form factors in perturbative QCD

Jia-Jie Han, Ya Li, Hsiang-nan Li, Yue-Long Shen, Zhen-Jun Xiao, Fu-Sheng Yu

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37 pages, 7 figures, version to be published in Eur.Phys.J.C
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摘要：

在微扰QCD（PQCD）方法中，我们重新分析了一个$Λ_b$重子和一个质子的高扭曲光锥分布振幅（LCDAs）的$Λ_b\to p$跃迁形状因子。几十年前进行的PQCD评估，只有领先扭曲的$Λ_b$重子和质子LCDAs给出了形状因子，这是两个数量级小于实验数据表明。我们发现扭转4的$Λ_b$重子LCDAs以及扭转4和扭转5的质子LCDAs占主导地位，增强的形状因子与晶格QCD和其他非微扰方法的结果一致。半轻子衰变$Λ_b\to p\ell\barν_\ell$和强子衰变$Λ_b\to pπ$的分支比的估计值也与实验数据接近。这意味着$b$夸克的质量并不足够重，更高次幂的贡献起着至关重要的作用，类似于在分析$B$介子跃迁形状因子时所观察到的。在此基础上，我们准备用PQCD方法系统地研究各种唯一重子衰变。

英文摘要：

We reanalyze the $Λ_b\to p$ transition form factors in the perturbative QCD (PQCD) approach by including higher-twist light-cone distribution amplitudes (LCDAs) of a $Λ_b$ baryon and a proton. The previous PQCD evaluation performed decades ago with only the leading-twist $Λ_b$ baryon and proton LCDAs gave the form factors, which are two orders of magnitude smaller than indicated by experimental data. We find that the twist-4 $Λ_b$ baryon LCDAs and the twist-4 and -5 proton LCDAs contribute dominantly, and the enhanced form factors become consistent with those from lattice QCD and other nonperturbative methods. The estimated branching ratios of the semileptonic decays $Λ_b\to p\ell\barν_\ell$ and the hadronic decay $Λ_b\to pπ$ are also close to the data. It implies that the $b$ quark mass is not really heavy enough, and higher-power contributions play a crucial role, similar to the observation made in analyses of $B$ meson transition form factors. With the formalism established in this work, we are ready to study various exclusive heavy baryon decays systematically in the PQCD approach.

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