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Heavy Ion Collisions at the LHC: Strong coupling techniques for high density QCD

Final Report Summary - HICLHC (Heavy Ion Collisions at the LHC: Strong coupling techniques for high density QCD)

The research work carried out during the fellowship is directly linked to the theoretical interpretation of the experimental results obtained at the Large Hadron Collider (LHC). The LHC is the biggest and most powerful particle collider on earth and, by all measures, and most complex machine ever built. The LHC is located at CERN, in Geneva, and started operation in 2009 thus placing Europe at the forefront of experimental high-energy physics. In particular we have been mainly interested in the study of lead-lead collisions at the LHC. The ultimate goal of experimental programs on heavy ion collisions is to produce and characterise a Quark Gluon Plasma (QGP), a new state of matter where quarks and gluons move freely over macroscopically large distances. By doing so, we expect to advance in our understanding of Quantum Chromodynamics (QCD), the theory of strong interactions. We have also considered data on electron-proton collisions collected in HERA (DESY, Hamburg) and on deuteron-gold collisions from RHIC (BNL, USA).

A main positive outcome of this project is having achieved an unified and consistent description of high-energy phenomena in such very different colliding systems - electron+proton, proton+proton, deuterium+gold and lead+lead collisions- from different accelerator complexes - HERA, RHIC and the LHC - performed at different collision energies. The starting point of our studies was the Color Glass Condensate (CGC), an effective theory to QCD, which goal is to describe the 'small-x' part of hadronic / nuclear wave functions, made of a dense partonic system and therefore prone to collective non-linear QCD dynamics. Thus, by using the most up-to-date theoretical tools available in the CGC and an innovative global fit procedure we provided an excellent description of structure functions measured in HERA. The obtained parametrisations of the 'dipole amplitude' were made publicly available in the form of easy-to-use routines (please see online). Taking these results as a baseline we provided an excellent description of RHIC data on single-inclusive and double-inclusive forward particle production in d+Au collisions.

All the accumulated knowledge in the study of simpler systems was then used to model the initial state of heavy ion collisions. Our model merges CGC theory tools with a Monte Carlo treatment of the initial geometry of the collision. Our predictions for the energy and centrality dependence of total charged multiplicities in Pb+Pb collisions were in excellent agreement with the data later on collected at the LHC. By releasing all our code for public use (please see online) we provided the wider heavy ion community with an essential tool for further data analyses. Therefore, the proposal objective of building accurate and predictive phenomenological tools has been fully met.

Other important aspect of the project has been the cooperation with other researchers. We have participated in the analyses of physics prospects for the Large Hadron electron Collider (LHeC), proposed as a future upgrade of the LHC.

We also joined the the network for the Electron Ion Collider and the FP7 EU funded ReteQuarkonii for the phenomenological studies of heavy quark production. Our results have been presented in numerous international conferences. Javier L Albacete has given nine invited seminars and taught two courses on high-energy scattering in European Union-based institutions (Universidad de Oviedo, Spain, and Universita de Torino, Italy). Also, he has established permanent collaborations with researchers based in other European institutions (CERN, Universidad de Santiago de Compostela, IST Lisbon). All these collaborative and outreach efforts certainly contribute to the realization of the European Research Area and to position of Europe as a leading agent in theoretical and high energy physics in the LHC era, which was other major goal of the project.