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Georgia State University, Atlanta, Georgia 30303, USA.
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA.
Phys Rev Lett. 2021 Oct 15;127(16):162001. doi: 10.1103/PhysRevLett.127.162001.
Studying spin-momentum correlations in hadronic collisions offers a glimpse into a three-dimensional picture of proton structure. The transverse single-spin asymmetry for midrapidity isolated direct photons in p^{↑}+p collisions at sqrt[s]=200 GeV is measured with the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton. This is the first time direct photons have been used as a probe of spin-momentum correlations at RHIC. The uncertainties on the results are a 50-fold improvement with respect to those of the one prior measurement for the same observable, from the Fermilab E704 experiment. These results constrain gluon spin-momentum correlations in transversely polarized protons.
研究强子碰撞中的自旋-动量关联有助于深入了解质子结构的三维图像。在相对论重离子对撞机（RHIC）上，利用PHENIX探测器测量了(\sqrt{s}=200) GeV的(p^{↑}+p)碰撞中，快度中间孤立直接光子的横向单自旋不对称性。由于直接光子特别是由硬散射产生的，并且不通过强力相互作用，所以该测量是对质子内部初始态自旋-动量关联的一种纯净探测，并且对质子内的胶子干涉效应特别敏感。这是首次在RHIC上使用直接光子作为自旋-动量关联的探测手段。与费米实验室E704实验对同一可观测量的前一次测量相比，结果的不确定性提高了50倍。这些结果限制了横向极化质子中的胶子自旋-动量关联。
