Early hydrodynamisation, energy loss and small systems in holographic heavy ion collisions

Abstract

Abstract. There has been much recent progress in describing heavy ion collisions using strongly coupled methods. We start by introducing a few caveats in applying holography to QCD, after which we will highlight recent progress on the initial stage, by simulating shock collisions in the dual gravitational theory. Secondly, we describe an attempt at describing jet evolution through strongly coupled plasma, where we specifically also included the width of the jet, and the interplay of this width with the energy lost as the jet propagates through the plasma. Lastly, there has been much recent discussion about the formation of quark-gluon plasma in small systems at low multiplicies, such as produced in proton-nucleus or even proton-proton collisions. At infinitely strong coupling it is possible to estimate that this may be possible, provided that the particle multiplicity satisfies dN tot /dy 4. OutlineCare must always be taken when interpreting results from holography in relation to heavy ion collisions. Especially around the QCD confinement/deconfinement transition pure N = 4 SYM is different from QCD, although much progress can be made by considering more realistic holographic models including a running of the coupling constant and a confinement/deconfinement transition [1] . At high energy scales a direct comparison is also complicated, since asymptotic freedom implies that the coupling becomes weak at (very) high energies. The coupling, however, runs only logarithmically with energy, and hence at LHC energy scales there is always a need of including some strongly coupled physics.It is hence a sensible approach to model heavy ion collisions at weak coupling, using perturbative QCD, as well as to model the collisions at strong coupling, using holography. Combining insights from both approaches can then lead to a realistic understanding of heavy ion collisions. Here, we will give a recent overview of holographic attempts to describe the initial stage as well as the propagation of jets. Lastly, we comment on recent excitement where flow-like signals were observed in small systems at relatively low multiplicities. a

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