2nd ECFA HL-LHC Workshop “Heavy Ion Theory” Urs Achim Wiedemann CERN PH-TH 2nd ECFA HL-LHC Workshop Aix-les-Bains, 21 Oct 2014

Heavy Ion Physics – Future Directions Luminosity* Higher Energy (FHC) pPb and PbPb data Status Oct 2014 pA physics* A-dependence* (Ar, Xe, …) If HL-LHC = LHC after LS3 Then three important directions * to go Machine preparation & beam use should allow for exploration of these three directions.

Focus of this talk is on soft probes Heavy Ions in the 2020’s Higher Luminosity in A+A to study rarer probes Quarkonium dissociation (vs. centrality, pt, flow …) Jet quenching studies Electromagnetic probes … Logic as in high-lumi pp … Higher Luminosity in pA to study rarer probes Constraining nuclear pdfs Searching for non-linear QCD evolution (saturation) … Higher Luminosity to characterize soft probes, e.g. Correlations of EbyE flow distributions Flow of rarer probes (quarkonium, open HF, … Logic different from high-lumi pp … 15 minutes = 1 argument, Focus of this talk is on soft probes

The early years of heavy ion physics Early Main Perspective: QGP reveals itself via kinks in data. e.g. J. Harris, B. Muller, Ann.Rev.Nucl.Part.Sci. 46 (1996) 71-107 Today: no kinks in data unambiguously related to QGP, irrespective of whether plotted against cms energy, system size (p-Pb, Pb-Pb, centrality), ... Early Critique/Doubt: Thermal QGP equilibrium state not accessible in short-lived, mesoscopic, rapidly expanding system. Today: It is exactly via the rapid collective expansion that QGP properties become experimentally accessible. Central role of Hydrodynamics in modern QGP research

Hydro = QGP-physics + conservation laws Evolution equations of dissipative hydrodynamics are: based on: E-p conservation: 2nd law of thermodynamics: sensitive to properties of matter that are calculable from first principles in quantum field theory - EOS: and sound velocity - transport coefficients: shear , bulk viscosity, conductivities … - relaxation times: , , … Lattice QCD => Finite Temp pQCD => AdS/CFT => Establishing hydro behavior is the door to the QGP.

Elliptic Flow: hallmark of a collective phenomenon Compilation ALICE, PRL 105, 252302 (2010)

Hydro-like phenomena at LHC Idea: Matter flows along spatial density gradients (that can vary Event by Event). Fig from M.Luzum, arXiv:1107.0592 Flow in Pb+Pb @ LHC … and in p+Pb? B. Schenke, MUSIC Hydro simulations can account for flow data. Applicability of fluid dynamics to small systems?

How can hydro apply to the smallest systems? The strong coupling paradigm: Strong coupling limit of N=4 SYM Kovtun, Son, Starinets, hep-th/0309213 Arnold, Moore, Yaffe, JHEP 11 (2000) 001 Fluid dynamics applies at In perturbative scenario: hydro valid if but => non-perturbative thermalization Heller, Janik Witaszczyk, PRL 108 (2012) 201602 Chesler, Yaffe, PRL 102 (2009) 211601 No quasi-particles in medium This interpretation lends itself to further experimental tests! Tertium datur: applicability of strong coupling expansion in does not invalidate perturbative expansion in => Quasi-particles in medium Short thermalization times can be obtained in perturbative equilibration scenarios, too. A. Kurkela & E. Lu, arXiv:1405.6318

From averages to EbyE statistics of flow measurements The current frontier in EX and TH: relating statistics of initial distributions to statistics of measured flow distributions. Distributions follow approximately Bessel-Gaussian distributions ATLAS JHEP 1311 (2013) 183 One of the next frontiers: Cross-correlating these distributions (can we test hydro-interactions between different azimuthal modes?) Are these flow correlations in smaller systems (pA) also hydro-like?

Why open heavy flavor at low pt is interesting Within the strong coupling paradigm,‘no-quasiparticle conjecture’implies that light low-momentum dressed quarks do not exist (i.e. do not propagate beyond ) In contrast, charm & bottom propagate (consequence of flavor conservation). How? Dragged along as external test particles of flow. At low pt, Langevin dynamics determines how charm & beauty quarks move: The perfect liquid is source of random forces PRL 98, 172301 (2007) calculable from 1st principles in quantum field theory, e.g. in strong coupling limit: Can we separate mass and T-dependence of ? From Langevin dynamics, expect that heavy Q is dragged along with flow but does not necessarily show the same flow as medium. => motivates precise determination.

How can LHC experiments continue to test hydro? All flow measurements are correlation measurements. Correlations can originate from initial conditions, from hydro evolution and/or from hadronization. Correlations that allow to disentangle these sources will be most useful. Example: initial conditions do not specify flow dependence on particle species. That makes particle identified flow (and its mass-ordering) a strong pillar of the hydrodynamic interpretation of data Does charm (beauty) flow? Both TH and EXP are still far from an answer of textbook quality! What is missing? => Statistics There are important problems in the soft physics sector that warrant high luminosity running. => Correlation tests Is for event distributions? => Systematics Does triangular, quadrangular … charm flow exist? Does it scale as hydro flow? And p-Pb ..

Instead of a summary What I have presented is a personal view. It is neither a summary of the future opportunities of heavy ion collisions, nor of the current understanding of flow. Rather, it is an example to illustrate a very general point: Also in the HL-LHC era, the improved characterization of soft physics phenomena will remain a driver of the experimental heavy ion programme. What is at stake (amongst others) is to further exploit the possibility of establishing matter properties via fluctuation analysis.