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High Energy QCD for Heavy Ions

Final Report Summary - QCDHI (High Energy QCD for Heavy Ions)

The Large Hadron Collider (LHC) at CERN is the most important project in high-energy physics worldwide. With a program on heavy-ion (HI) physics in which the dedicated experiment - ALICE - and the other large ones - ATLAS, CMS and LHCb - participate, the LHC focuses on the unique possibility of creating and studying a new state of matter, the quark-gluon plasma (QGP), at temperatures and energy densities similar to those of the early Universe. The quest for QGP is also the driving force behind the currently operating (though at much smaller energies) Relativistic Heavy Ion Collider (RHIC) at the BNL. The fundamental theory underlying the proton structure as well as the one responsible for nuclear forces is Quantum ChromoDynamics (QCD), the theory of strong interactions. The study of QCD under extreme conditions of high energies, temperatures and densities has been for long one of the most challenging problems in physics, capturing increasing experimental and theoretical attention. Understanding QCD at high energies is an essential ingredient for the success of the LHC program. It is also one of the most active frontiers both in particle phenomenology and nuclear physics, and one of the topics where both fields clearly profit from close collaboration. In particular, a theoretical description of HI collisions and the formation of QGP from first principle calculations is still missing. This project is aimed at achieving a qualitatively new level of understanding of HI collisions, both theoretically and phenomenologically, through the development of a new QCD-based description of these processes.

Both searches for new physics at the LHC in proton-proton and investigations of high-energy medium effects in HI collisions require a solid understanding of multiple gluonic interactions. The same effects should be present, though less pronouncedly, at other currently operating accelerators. Gluon densities rise rapidly with energy as was discovered at the Hadron-Electron Ring Accelerator (HERA) at the German Electron Synchrotron DESY. When probed at very high energies, the proton appears like a very dense cloud of gluons. Part of this project aims at developing a new theory for high-energy collisions of dense objects. This is an essential problem at the LHC, especially for its HI program where the effects of high gluonic densities get enhanced not only by the very high energies but also by the number of nucleons in nuclei.

Today, exploring QCD under extreme conditions - such as at high energy or density - is more important than ever due to its relevance for the LHC and future collider programs. The main objective of our research project has been to further develop the theory of high-energy collisions of dense objects from first-principle calculations, named Color Glass Condensate (CGC), and to apply it to phenomenology at the LHC and at the proposed Electron-Ion Collider in the US and Large Hadron-electron Colliders at CERN, focusing on collisions involving heavy nuclei.

The objectives of this project have been achieved via strengthening and broadening of a long-lasting collaboration between research centres from the EU and the US. They aimed at the development of a unified, QCD-based theoretical framework to describe the structure and collisions of hadrons and nuclei.

The results of our study facilitate a better understanding of a quantum field theory, such as QCD, beyond naive perturbation theory - specifically, in a weak-coupling regime but non-perturbative as the field strength becomes very large. Besides the theoretical developments, in this project we have addressed questions fundamental to understanding the experimental results.

The results obtained involve a throughout understanding of a realization of QCD at high energies, the QCD Reggeon Field Theory (RFT). Particularly, next-to-leading order corrections in the QCD coupling constant have been derived. The study of exclusive and inclusive particle, vector meson, heavy quark and dijet production within the effective theory, has been addressed. Furthermore, we have performed a throughout study of two-particle correlations produced in proton-ion and HI collisions. This led to a better understanding of collective phenomena which emerge from the initial state of high-energy collisions. Finally, we have made a noticeable progress in understanding collective phenomena in terms of quasi-thermodynamics and hydrodynamics of fluids produced in HI collisions.

Overall, during the project period our collaboration published close to 30 peer reviewed publications. Our results were presented at dozens of international conferences, workshops and seminars in many countries worldwide. A complete listing of our publications can be found at the collaboration webpage