Ultrarelativistic heavy ion collisions from discovery to detailed

Ultra-relativistic heavy ion collisions: from discovery to detailed analysis Urs Achim Wiedemann CERN PH-TH 7 th France China Particle Physics Laboratory (FCPPL) Workshop Clermont-Ferrand, 8 -10 April 2014

From elementary interactions to collective phenomena 1968: electroweak theory = leptons + gauge invariance 1973: asymptotic freedom QCD = quark model + gauge invariance Today: mature theory with a precision frontier How do collective phenomena and macroscopic properties of matter emerge from fundamental interactions? Standard Model is much richer than QED: • non-abelian quantum field theories • for QCD, degrees of freedom change with

Phase transitions of elementary quantum fields Properties of the Higgs field(s) ATLAS, 1207. 7214 Morrissey, Ramsey-Musolf 1206. 2942 determine elwk. phase transition in the hot & dense Early Universe - Mass generation - Baryon asymmetry requires understanding the Standard Model at finite temperature.

Are the high-T phases of the Standard Model experimentally accessible? • Electroweak sector not within exp. reach. • Strong sector Wuppertal-Budapest, ar. Xiv: 1005. 3508, ar. Xiv: 1007. 2580 Within experimental reach.

How to test thermodynamic QGP properties? Fluid dynamics is the fundamental theory of a locally equilibrated system. • based only on: E-p conservation: 2 nd law of thermodynamics: • sensitive only 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: , , …

Question: Why do we need collider energies to test properties of dense QCD matter which arise on typical scales ?

Answer 1: Large quantitative gains Increasing the center of mass energy implies Denser initial system Longer lifetime Bigger spatial extension Stronger collective phenomena More statistics per event to test collectivity A large body of experimental data from the CERN SPS, RHIC and LHC supports this argument.

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

How do we know that v 2 signals collectivity? Consider particle production w. r. t. reaction plane: • Single 2 ->2 process • Maximal asymmetry • NOT correlated to the reaction plane • Many 2 ->2 or 2 -> n processes • Reduced asymmetry • NOT correlated to the reaction plane • final state interactions • asymmetry caused not only by multiplicity fluctuations • collective component is correlated to the reaction plan • limiting case: zero mean free path => fluid dynamics

Particle production w. r. t. reaction plane ● Want to measure particle production as function of angle w. r. t. reaction plane But reaction plane is unknown. . . ● Have to measure particle correlations: “Non-flow effects” But this requires signals ● Improve measurement with higher cumulants: This requires signals Borghini, Dinh, Ollitrault, PRC (2001)

v 2 @ LHC ● Several independent methods to measure ● Momentum space Reaction plane • Signal implies 2 -1 asymmetry of particles production w. r. t. reaction plane. • ‘Non-flow’ effect for 2 nd order cumulants do not characterize solely collectivity. Strong Collectivity !

1 st conclusions from fluid dynamic modeling Assumptions: - fluid with shear viscous term - ‘realistic’ initial conditions & decoupling Results: - initial transverse pressure gradient - dependence of flow field elliptic flow - size and pt-dependence of accounted for by fluid of minimal shear viscosity data - characteristic mass dependence, since all particle species emerge from common flow field H. Song et al. PRL 106 (2011) 192301 Z. Qiu et al. , Phys. Lett. B 707 (2012) 151 P. Romatschke ar. Xiv. 0902. 3663

1 st conclusions continued • Value of shear viscosity minimal, => perfect liquid, Arnold, Moore, Yaffe, JHEP 11 (2000) 001 strongly coupled plasma • Shear viscosity minimal => expansion is close to isentropic For 1 -dim expanding fluid (Bjorken boost -invariant), entropy density s increases like Isentropic “perfect liquid” applies if Strong coupling limit of N=4 SYM Kovtun, Son, Starinets, hep-th/0309213

1 st conclusions continued … • Fluid dynamics applies at In perturbative scenario: hydro valid if Heller, Janik Witaszczyk, PRL 108 (2012) 201602 but => non-perturbative thermalization Chesler, Yaffe, PRL 102 (2009) 211601 • • Very fast, non-perturbative isotropization Perturbatively require but Such a plasma is unique in that it does not carry quasi-particle excitations Establishing this conjecture is one of the main drivers for future EXP&TH H-T. Ding et al, ar. Xiv: 1012. 4963 Quasi-particle peak melts

Minimal dissipation Maximal Transparency to Fluctuations • Fluctuation damping controlled by sound attenuation length final Pics by B. Schenke Much to be learnt from varying scale of fluctuation e. g. initial • Experimental control on properties of Eby. E fluctuations Fluid dynamical response to spatial eccentricities given by vn in momentum distributions Alver and Roland, 2009 Fig from M. Luzum, ar. Xiv: 1107. 0592

Flow as linear response to spatial asymmetries Characterize spatial eccentricities, e. g. , via moments of transverse density ALICE, ar. Xiv: 1105. 3865, PRL LHC data indicate: Spatial eccentricity is related approx. linearly to (momentum) flow for n=2, 3

Sensitivity to non-linear fluid dynamic response • Non-linear response seen e. g. in reaction plane correlations • Fluid dynamics as perturbation theory of fluctuations on non-perturbative background S. Flörchinger, UAW et al. ar. Xiv: 1312. 5482 Flow measurements are correlation measurements. Long term goal is to disentangle sources of correlations: i) Fluctuating initial conditions (geometrical origin, quantum origin) ii) Hydrodynamic evolution (dynamical origin) iii) Hadronization (kinematic origin) Teaney, Yan, ar. Xiv: 1312. 3689

A (valid) analogy From a signal … via fluctuations … to properties of matter Progress via more differential measurements & analysis Slide adapted from W. Zajc

A field with fundamental open questions • How does/Does fluid dynamics extend to smaller systems? (p-A and light-ion collisions, analysis of CMS p-p ridge) => test of thermalization/isotropization time • What is the p. T-range of fluid dynamics? (at sufficiently high resolution scale, particle-like excitations must exist – what sets this scale? ) => test of ‘no quasiparticle conjecture’ • On which scale do hard probes such as jets and heavy quarks ‘flow’ with the medium? test of ‘no quasiparticle conjecture’ • Can we constrain bulk viscosity? test of ‘non-conformality’ at finite T • … … … T. Schäfer, QM 12

Open heavy flavor at low pt • ‘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? • At low pt, Langevin dynamics determines how charm & beauty quarks move: The perfect liquid is source of random forces calculable from 1 st principles in quantum field theory, e. g. in strong coupling limit: • This hard probe is unique in that we have first experimental indications of flow. Much more differential characterization needed to constrain Langevin dynamics. (High luminosity requirement!).

Finally, a minimal comment on high-p. T: “Jet Quenching” Jet “=“ characterization of hadronic final state of a QCD parton shower. Can this parton shower be modified in heavy ion collisions? Hard production process: Occurs on time & distance scale d ~1/ET << mean free path Unaffected by surrounding QCD matter QCD (final state) Parton Shower: Must traverse an in-medium path length up to O(10 fm/c) On the QCD scale, a long time to interact with QCD matter. Jet quenching = in-medium modification of final state QCD parton showers. Provides unique access to thermal history of the collision.

How do high-momentum partons propagate? - theory In a perfect liquid (Ad. S/CFT view) In system with finite mean free path • Light partons/jets thermalize (no collinear structure remains) • Light hard partons fragment in medium (Energy moved to softer scales/larger angles but collinear dynamics persists) • Heavy quarks loose momentum via sound modes and wake Chesler, Yaffe ar. Xiv: 0712. 0050 • Heavy quarks fragment with smaller branching probabilities (dead-cone effect) testable hierarchy in mass and color charge • For , mass unimportant. In high energy limit (eikonal limit) determined by Q: How does the rich phenomenology of jet quenching relate to the fluid dynamic nature of soft particle production?

Take-home message • Strong evidence for perfect liquid behavior in heavy ion collisions at RHIC and at the LHC • To turn this discovery into a chapter in the ultimate QCD textbook, we need to understand How is minimal dissipation realized in a strongly coupled non-abelian medium without quasi-particle? • This requires - much more differential experimental tests (small fraction of them mentioned in this talk) - theory developments