Parton distributions and fragmentation functions

核子的部分子含量和末态部分子碎裂的知识是当今粒子对撞机精确预测的关键因素。在QCD的标准因式分解方法中，它们提供了关于长距离部分子动力学的基本信息，这是微扰计算无法获得的。现代QCD理论的一个中心任务是通过比较实验数据和高阶微扰理论中短距离因子（通常是部分子截面）的理论计算，精确提取这些长距离因子（通常是部分子分布和碎裂函数）。这些理论预测的典型精度目前在QCD中对大多数感兴趣的截面的次-次-领先阶（N2 LO），对完全包含的可观测量的精度甚至更高。受理论进展和深度非弹性散射和LHC的可用数据以及未来EIC前景的推动，P2项目提出了一种统一的方法，以新的精度水平提供非微扰部分子动力学信息。为此，它结合了所有关于质子以及较轻和较重的核中部分子的动量分布的努力，解决了有和没有极化的散射过程，目标碎裂函数，并旨在推动QCD中N2 LO的精度。因此，该项目与所有其他项目有许多相互关系，要么利用新的成果，要么分享成分。

The parton content of nucleons and knowledge on fragmentation of final state partons are key ingredients in precision predictions at today’s particle colliders. In the standard factorization approach to QCD, they provide the essential information on the long-distance parton dynamics, which is inaccessible to perturbative computations. A central task for modern QCD theory consists of providing precision extractions of these longdistance parts (typically, parton distributions and fragmentation functions) from comparisons of experimental data with the corresponding theoretical calculations of the short-distance factors (typically, partonic cross sections) at high orders of perturbation theory. The typical accuracy of those theoretical predictions is currently at next-to-next-to-leading order (N2LO) in QCD for most cross sections of interest and even higher for fullyinclusive observables. Motivated by the theoretical progress and the available data from deep-inelastic scattering and the LHC and the prospects of the future EIC, project P2 presents a unified approach to deliver information on the non-perturbative parton dynamics at a new level of precision. To that end, it combines all efforts concerning momentum distributions of partons in the proton as well as lighter and heavier nuclei, addresses scattering processes with and without polarization, targets fragmentation functions and aims at pushing the accuracy beyond N2LO in QCD. As such, the project has numerous interrelations to all other projects, either by exploiting new results to be derived or by sharing ingredients.