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.

Research themes

Flow, anisotropies and fluctuations

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.

Flow from microscopic dynamics

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.

In-medium modification of probes

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.

Gluon saturation

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

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.

Researchers involved

Permanent and emeritus researchers

PhD students

Postdoctoral researchers

Former staff members

Former Postdoctoral researchers

Former graduate students

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Networking, collaborations & fundings

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Seminars

Our weekly seminar takes place every Tuesday at 16:00.

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Events

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Jobs

Postdoctoral positions are available each year in the Fall. Check this page or contact any staff member of the group.

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Contact

Each member of the group can be contacted via email at name.surname@ipht.fr . 

The full postal adress of IPhT is: Institut de Physique Théorique,  CEA/Saclay, Bat 774 Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France.  

Here are directions to the IPhT.