Final Report Summary - HOTLHC (Hot and dense QCD in the LHC era) Some microseconds after the Big Bang, a liquid formed by the quarks and gluons that are nowadays confined inside the protons and neutrons filled the whole Universe. This primordial liquid is a universal state of matter: any material heated up to some hundred thousand times the temperatures at the core of stars like the Sun will be in exactly the same state of matter, called Quark Gluon Plasma (QGP). This temperature is in the range of five trillion Kelvin.Interestingly, this QGP can be produced in Earth laboratories when smashing large atomic nuclei, like lead or gold, at the highest collider energies. The main problem for its detection is, however, that this material lasts only a tiny fraction of time, approximately the same time that the light takes to cover a distance of the size of a lead nucleus. Studying the best signals to unveil the properties of the QGP is, hence, crucial. One of the best tools is the study of “jet quenching”: in some lead against lead collisions at the LHC, together with the QGP, very energetic quarks or gluons are produced and subsequently decay into a spray of particles called a jet. These quarks or gluons traverse the QGP losing energy and radiating gluons in a process similar to bremsstrahlung. We have discovered that color coherence, a manifestation of quantum mechanical interference in QCD, produce very characteristic features in the measured signals of jets. This mechanism leads to a new picture of jet quenching that leads to an excellent quantitative understanding of the experimental findings at the LHC. Using these techniques we have also extracted one of the transport coefficients of the medium that, surprisingly, when systematically compared to other collision systems and energies, lead to the unexpected result that this quantity seems to depend on non-local properties of the medium. This result was later confirmed by independent analyses.A large part of the activity was also devoted to study the initial stages of the collision, before the creation of the QGP. The knowledge of the initial conditions is a prerequisite for a correct interpretation of the data. We have performed a new analysis that determines to unprecedented accuracy the structure of the nucleus in terms of quarks and gluons. We have also studied the mechanism of thermalization using different techniques: how a system as out-of-equilibrium as a particle collision at the LHC rapidly evolves into an (apparently) equilibrated QGP. Several of the most important observations in these subjects were performed in collisions of protons against lead nuclei at the LHC.
