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arXiv 0709.0901hep-ph
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包含双光子交换的DIS中的横向靶自旋不对称性

Transverse target spin asymmetry in inclusive DIS with two-photon exchange

A. Afanasev, M. Strikman, C. Weiss

摘要:

研究了非极化电子与核子碰撞的截面与横向靶自旋的关系。这种依赖性在单光子交换近似(Christ-Lee定理)中是不存在的,并且只出现在QED展开的高阶中,来自单光子和吸收性双光子交换振幅的干涉以及来自真实光子发射(韧致辐射)。我们证明了横向自旋相关的双光子交换截面是免费的QED红外和共线发散。我们认为,在DIS运动学的横向自旋的依赖性应该由一个“部分子”的机制,其中的双光子交换耦合主要是一个单一的夸克。我们计算正常的自旋不对称性的近似,其中的主要贡献来自夸克螺旋翻转由于与非微扰真空场(组分夸克图片)的相互作用,并在核子中的夸克横向分布成正比。如果光子-夸克子过程中胶子发射的红外标度是手征对称性破缺标度的量级,即μ_chiral ^2>> Lambda_QCD ^2,那么这种螺旋度翻转过程就不会受到显著的Sudakov抑制。我们估计计划中的杰斐逊实验室A厅实验的运动学不对称性为10^{-4}的数量级,质子和中子的符号不同。我们还评论了软高能散射极限下的自旋依赖性。

英文摘要:

We study the transverse target spin dependence of the cross section for inclusive electron-nucleon scattering with unpolarized beam. Such dependence is absent in the one-photon exchange approximation (Christ-Lee theorem) and arises only in higher orders of the QED expansion, from the interference of one-photon and absorptive two-photon exchange amplitudes as well as from real photon emission (bremsstrahlung). We demonstrate that the transverse spin-dependent two-photon exchange cross section is free of QED infrared and collinear divergences. We argue that in DIS kinematics the transverse spin dependence should be governed by a "parton-like" mechanism in which the two-photon exchange couples mainly to a single quark. We calculate the normal spin asymmetry in an approximation where the dominant contribution arises from quark helicity flip due to interactions with non-perturbative vacuum fields (constituent quark picture) and is proportional to the quark transversity distribution in the nucleon. Such helicity-flip processes are not significantly Sudakov-suppressed if the infrared scale for gluon emission in the photon-quark subprocess is of the order of the chiral symmetry breaking scale, mu_chiral^2 >> Lambda_QCD^2. We estimate the asymmetry in the kinematics of the planned Jefferson Lab Hall A experiment to be of the order 10^{-4}, with different sign for proton and neutron. We also comment on the spin dependence in the limit of soft high-energy scattering.

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