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Quantum Chromodynamics at Work

Final Report Summary - QWORK (Quantum Chromodynamics at Work)

Summary of ERC Advanced Grant Project: Quantum Chromodynamics at Work (QWORK), project no. 320389

From 2013 - 2018 the project 'Quantum Chromodynamics at Work' (QWORK) has looked into novel aspects of parton distribution functions. These distribution functions represent momentum space densities depending on the longitudinal fraction as well as on the transverse components of the momentum carried by the quarks or gluons, the partons making up protons. In particular the inclusion of transverse components is the novel aspect. The transverse momentum dependence (TMD) is an intrinsic part of the nonperturbative partonic structure of the nucleons but cannot be decoupled from the dynamics governed by the strong interactions described by Quantum Chromodynamics (QCD).

In the QWORK project the main emphasis was on the gluons. Including transverse momentum one can look at linearly polarized gluons in a proton. Since gluons dominate the region of small momentum fractions x (the small-x region) one will hit a region in which transverse momenta become dominant. This has been studied using in the small-x limit Wilson loop correlators, a novel procedure. This approach incorporates the non-abelian gauge character of QCD. In the project attention was given to technical aspects such as factorization, evolution and resummation, while the community was provided with several of the methodologies and results via a web utility. The technical aspects related to resummation, evolution and factorization also were central in a new series of workshops that has been established. Within the project a close collaboration between theorists and experimentalists was foreseen and also realized, which contributed greatly to the success of the project and led to interesting excursions, even beyond the original plans. An example of this were multi-parton interactions. The project contributed to discussions and plans developed for future accelerators (e.g. RHIC/Brookhaven or upgrades at LHC/CERN). The project has resulted in over 50 publications, including contributions to research reports and reviews as well as three theses of Ph.D. students.

Although it is a gradual process, there is growing awareness of the role of transverse momenta of quarks and gluons in the proton. It is an essential part towards understanding the internal structure of the proton or when using the proton as a tool to study the physics beyond and underlying the standard model of particle physics. It becomes an ingredient in effective theories and novel concepts like emergence may play a central role.