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大横距下手征动力学和部分子结构

Chiral dynamics and partonic structure at large transverse distances

M. Strikman, C. Weiss

备注：
24 pages, 11 figures. Minor corrections in discussion of time-ordered formulation; numerical results unchanged
发表信息：
Phys.Rev.D80:114029,2009
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摘要：

在基于广义部分子分布（GPD）的横坐标（碰撞参量）表象中，研究了大距离对核子部分子密度的贡献。手征动力学产生了一个独特的部分子结构，位于动量分数x ~< M_pi/M_N和横向距离b ~ 1/M_pi处。我们计算这个组件使用唯象π交换的物理下限b（横向“核心”半径估计从核子的轴向形状因子，R_core = 0.55 fm），并证明其普适性。这个公式保留了核子海夸克分布的“π介子云”模型的基本图像，同时限制了它在手征动力学实际控制的区域的应用。结果表明：（a）在x ~ 0.1处，大距离分量只占反夸克味不对称度dbar-ubar的1/3，（b）奇异海夸克s和sbar比轻反夸克海明显局域化;（c）x < 0.1时核子的单重夸克尺寸大于胶子尺寸，<b^2>_{q + qbar} > <b^2>_g，正如在HERA和FNAL测量的深度虚康普顿散射和排他性J/psi产生的t斜率所表明的那样。由于N和Delta中间态在大N_c极限下的简并，我们的方法再现了QCD中部分子密度的一般N_c标度。我们还评论的作用，在大的纵向距离和它们的适用性在小x的限制。

英文摘要：

We study large-distance contributions to the nucleon's parton densities in the transverse coordinate (impact parameter) representation based on generalized parton distributions (GPDs). Chiral dynamics generates a distinct component of the partonic structure, located at momentum fractions x ~< M_pi/M_N and transverse distances b ~ 1/M_pi. We calculate this component using phenomenological pion exchange with a physical lower limit in b (the transverse "core" radius estimated from the nucleon's axial form factor, R_core = 0.55 fm) and demonstrate its universal character. This formulation preserves the basic picture of the "pion cloud" model of the nucleon's sea quark distributions, while restricting its application to the region actually governed by chiral dynamics. It is found that (a) the large-distance component accounts for only ~1/3 of the measured antiquark flavor asymmetry dbar - ubar at x ~ 0.1; (b) the strange sea quarks, s and sbar, are significantly more localized than the light antiquark sea; (c) the nucleon's singlet quark size for x < 0.1 is larger than its gluonic size, <b^2>_{q + qbar} > <b^2>_g, as suggested by the t-slopes of deeply-virtual Compton scattering and exclusive J/psi production measured at HERA and FNAL. We show that our approach reproduces the general N_c-scaling of parton densities in QCD, thanks to the degeneracy of N and Delta intermediate states in the large-N_c limit. We also comment on the role of pionic configurations at large longitudinal distances and the limits of their applicability at small x.

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