DECOLHC · Probing strongly coupled deconfined matter at the LHC

Probing strongly coupled deconfined matter at the LHC

At temperatures one hundred thousand times larger than the temperature at the center of the sun (or 170 MeV in nuclear physics units), matter experiences a rapid crossover transition to a new state of matter. In this phase, the quarks and gluons that at everyday temperatures are confined within the atomic nucleus are liberated and the color degrees of freedom become apparent. By analogy with electromagnetic plasmas, in which the degrees of freedom of matter are charged particles (as opposed to neutral molecules), this high temperature phase is called Quark Gluon Plasma. Microseconds after the Big Bang, the Quark Gluon Plasma filled the entire universe. Nowadays, only small amounts of this plasma exist in the debris of the high-energy collisions of heavy ions at hadronic colliders such as the LHC. Therefore, those collisions provide us with a window of opportunity to study the properties of matter at the early universe. And what these collisions have taught us is that this matter behaved as the most perfect fluid that, up to date, we know of. However, we do not yet know how the microscopic dynamics of this plasma work. This is, in part, because the constituents of the plasma are strongly coupled, which makes the quark gluon plasma a very interesting system, but also hard to describe. This project is aimed at providing new theoretical developments to understand two key aspects of the quark gluon plasma as produced in heavy ion collisions. The first aspect is concerned with how the plasma itself is generated in the debris of those collisions. This is important to understand, since a precise determination of its transport properties demands theoretical control over this process. The second aspect is the interaction of the plasma with energetic particles. This is important since these are used as tomographic probes of the plasma, which give us information about the microscopic degrees of freedom. To understand the formation of plasma at strong coupling, in this project we use a new technique called holography. This is a mathematical relation, which allows us to solve complicated problems in non-abelian field theories (like QCD) by translating them into simpler problems in gravity. In this way, we have translated the complicated quantum mechanical problem of plasma formation from the collision of energetic objects to the problem of black hole formation in classical gravity after the collision of shock waves. By numerically solving Einstein equations, in this project we have addressed how strongly coupled plasma forms after the collision of energetic lumps of energy. By comparing our simulations with data, we have established that this type of computations capture essential qualitative features of the energy dependence of matter distribution along the direction of collisions. We have also investigated how these features can leave and imprint of strong coupling dynamics. We have also investigated in more detail the process by which matter is formed. By extending our computations to gauge theories with non-trivial equation of state we have been able to describe a novel process, which we called eosization, by which the energy and pressure of a collision system become related via the equilibrium equation of state, even though the system is still far from equilibrium. We have also stressed the success of hydrodynamics, which can successfully describe the far-from-equilibrium dynamics we simulate even when the equation of state is not satisfied. To understand the interaction of the plasma with probes, we have developed a hybrid strong/weak-coupling model for those interactions. This model puts together the most up-to-date understanding of how energetic probes interact with strongly plasma with modern event generators to be able to address jet measurements at the LHC in heavy ions. We have tested successfully the model with existing heavy ion jet data and we have also use it to predict the distribution of several new jet-boson correlations. Some of those predictions have been already fulfilled. We have also addressed how the energy distributions of the energy lost by jets allow us to understand global properties of the medium. This analysis has allowed us to show that a consistent description of the plasma as a strongly coupled fluid is possible both for low energy and high-energy particles. By studying the processes of plasma formation and its reaction to probes, this project has contributed to devising new analysis strategies which will allow us to better use LHC data to learn about the state matter was at an early phase of the universe. Additionally, since this is the high temperature phase of one of the fundamental forces of Nature, by studying the plasma we are also learning about how the Strong Force works. Furthermore, since the quark gluon plasma is the only state of matter made of elementary particles and bound by a force other than electromagnetism that can be produced in a laboratory, understanding how collective properties emerge in this system provides us with complementary information on how matter properties appear. Finally, the strong coupling among the quarks and gluons of the plasma and the techniques we use to describe it put the quark gluon plasma into the context of many other systems appearing at very different energy regimes, such as cold atoms systems and some types of high Tc superconductors, which are also strongly coupled. Therefore, by learning about the quark gluon plasma we are also learning about the dynamics of those interesting systems.

Data: CORDIS, © European Union