Quantum Chromodynamics (QCD) is the study of strong interactions. It is a vast domain of research in fundamental theoretical physics and our lab is at the forefront of a broad spectrum of activities in QCD with several major contributions over the reporting period. An area of QCD actively studied in our lab is the physics of nuclear matter in extreme conditions, mostly in heavy ion collisions. High-energy heavy-ion collisions at RHIC and at the LHC produce a quark-gluon plasma (QGP) similar to what was created shortly after the Big Bang. We study this fundamental, but complex, QCD system at various stages of its evolution.
In many respects, the QGP behaves like a perfect fluid, as can be shown by the study of azimuthal particle distributions based on a hydrodynamical description. Recently, we have put a lot of effort into the description of anisotropies based on the initial shape fluctuations and the corresponding hydrodynamical response of the system to those fluctuations. Amongst our important findings, we have highlighted universal non-Gaussian fluctuations in small systems -- the best signature of collectivity in small systems to-date --- and in large systems. The predictions we have obtained have then been successfully compared to measurements by the CMS and Alice collaborations at the LHC.
Understanding in terms of fundamental QCD degrees of freedom why the QGP is interacting sufficiently strongly to sustain hydrodynamical flow despite a rapid expansion remains an outstanding question. We have made substantial progress on this question, first by setting up the formalism and obtaining numerical results from a first-principles NLO-resummed calculation, then by showing in kinetic theory that purely classical approximations fail to correctly describe the expansion of the system. We have also studied the relative importance of elastic and inelastic collisions for the thermalisation of the QGP, and used simple moments of the distribution functions to address the onset of hydrodynamical evolution.
Another important approach to extract properties of the QGP is to study how high-energy probes such as jets or heavy quarkonia are modified by their interaction with the QGP. We have studied the formation and dissociation of heavy-quark bound states in the QGP, based on a generalised Langevin equation, first for an Abelian plasma then extending the approach to QCD. We have extensively studied how jets propagate through the QGP. In particular, we have shown that the radiation induced by collisions off the QGP can be described by a classical stochastic process (yielding to wave turbulence), and we have provided the first description of vacuum-like emissions in the QGP showing that they factorise from the medium-induced emissions.
Another longstanding activity of the lab on fundamental properties of QCD is the question of the high-energy limit of QCD and gluon saturation phenomena. Evolution equations towards high energy are known up to next-to-leading order (NLO) accuracy. Several pathologies appear at NLO and our main activity has been to cure these pathologies. We identified the source of the instability of the NLO evolution equation and cured this problem via an all-order resummation of the leading perturbative corrections. Then, we reformulated the high-energy factorisation so as to guarantee a positive-defined production cross-sections.
Nuclear matter at high density is also encountered in objects such as in neutron stars. Effective field theories may be used to study the possible onset of strong-coupled quark-gluon degrees of freedom at high baryonic density.
|Miguel Angel Escobedo
Our weekly seminar takes place every Tuesday at 16:00.
Postdoctoral positions are available each year in the Fall. Check this page or contact any staff member of the group.
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The full postal adress of IPhT is: Institut de Physique Théorique, CEA/Saclay, Bat 774 Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France.
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