Hard Scattering Processes in QCD

量子色动力学（QCD）是强相互作用的微观理论，使我们能够理解强子，如质子，中子和π介子，是如何通过部分子形成的，部分子是QCD的基石。高能散射过程可以用来探索部分子（夸克和胶子）如何从高能散射中跃迁到强子。虽然这些过程的各个方面都很清楚，但仍然存在许多令人震惊的谜团。横向单自旋不对称性（SSA）就是这样一个难题。早期的QCD计算预测基本上消失的影响，但实验显示巨大的SSA高达40%。尽管有几十年的研究，QCD中单自旋不对称性的详细理解仍然缺失。在此背景下，本项目将探讨不同单自旋不对称性的部分子碎裂所扮演的角色。研究生和博士后研究员将参与当前强子物理学前沿的这一非常活跃的领域。该项目将使质子-质子碰撞中单强子产生的横向SSA的广泛数值研究成为可能，其中一个入射质子垂直于反应平面极化。这些研究将利用研究人员和他的一名研究生以前开发的现有分析结果。这项研究不仅有望澄清部分子碎裂是否确实可以解释实验观察到的巨大SSA，而且还可以同时描述强子碰撞和轻子诱导过程中的SSA。该项目与托马斯杰斐逊国家加速器设施、布鲁克海文国家实验室的相对论重离子对撞机以及世界各地的其他加速器设施进行的实验密切相关。

Quantum Chromodynamics (QCD) is the microscopic theory of the strong interactions and allows us to understand how hadrons, such as the proton, neutron and pion, are formed through partons, the building blocks of QCD. High-energy scattering processes can be used to explore how partons (quarks and gluons) that come out of a high-energy scattering make a transition into hadrons. While various aspects of these processes are well understood, many striking puzzles remain. Transverse single-spin asymmetries (SSAs) represent one such puzzle. Early QCD calculations predicted basically vanishing effects, but experiments revealed huge SSAs up to 40%. Despite decades of research, the detailed understanding of single-spin asymmetries in QCD is still missing. In this context, this project will explore the role played by the fragmentation of partons for different single-spin asymmetries. Graduate students and postdoctoral fellows will be involved in this very active field at the frontier of current hadronic physics.This project will make possible extensive numerical studies of transverse SSAs for single-hadron production in proton-proton collisions, where one of the incoming protons is polarized perpendicular to the reaction plane. These studies will make use of existing analytical results that were developed previously by the investigator and one of his graduate students. Not only is this investigation expected to clarify if parton fragmentation can indeed explain the huge SSAs observed experimentally, but also whether a simultaneous description of SSAs in hadronic collisions and in lepton-induced processes can be obtained. This project is closely related to experiments conducted at the Thomas Jefferson National Accelerator Facility, the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, and other accelerator facilities worldwide.