Heavy Ion Collisions at RHIC and at the LHC: Theoretical Overview

1. Heavy Ion Collisions at RHIC and at the LHC: Theoretical Overview Urs Achim Wiedemann CERN PH-TH

2. 1973: asymptotic freedom QCD = quark model +gauge invariance Today: mature theory with a precision frontier • background in search for new physics • TH laboratory for non-abelian gauge theories QCD much richer than QED: • non-abelian theory • degrees of freedom change with From elementary interactions to collective phenomena How do collective phenomena and macroscopic properties of matter emerge from fundamental interactions ?

3. Quark Gluon Plasma Hadron Gas 2SC CFL • What is the QCD equation of state? How can we test it? Open questions • What is the origin of mass in the universe?

4. Confinement: How does hadronization proceed dynamically? How is it changed in dense QCD matter? • Why is there no strong CP violation? Or is there at finite temperature? • What are the properties of matter at the highest temperatures and densities? • Degrees of freedom? • Viscosity? • Heat Conductivity? • Transport of conserved quantum numbers? • What are the dominant microscopic mechanisms of QCD non-equilibrium dynamics and thermalization? • Parton energy loss? • Plasma Instabilities, color chaos? … and more questions… … and many more …

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

6. Increasing the center of mass energy implies Denser initial system, higher initial temperature Longer lifetime Bigger spatial extension Stronger collective phenomena A large body of experimental data from RHIC supports this argument. Answer 1: Large quantitative gains

7. For a detailed experimentation with dense QCD matter, one ideally wants to do DIS on the QGP. … and we can by using auto-generated probes at high Large allows us to embed well-controlled large- processes (hard probes) in dense nuclear matter. Q: How sensitive are hard probes? Answer 2: Qualitatively novel access to properties of dense matter

8. Bjorken’s original estimate and its correction Bjorken 1982: consider jet in p+p collision, hard parton interacts with underlying event collisional energy loss (Bjorken realized later that this estimate was numerically erroneous.) Bjorken conjectured monojet phenomenon in proton-proton Today we know (th): radiative energy loss dominates Baier Dokshitzer Mueller Peigne Schiff 1995 • p+p: Negligible ! Monojet phenomenon! • A+A: observed at RHIC, see talk by Bill Zajc

9. 0-5% 70-90% Centrality dependence: L large L small High pT Hadron Spectra

10. partonic energy loss Centrality dependence: Au+Au vs. d+Au • Initial state enhancement • Final state suppression

11. Leading hadron suppression at RHIC: Abundant yield at collider energies (detailed differential study of experimental signal possible) + robust and large signal (medium effect much larger than theoretical uncertainties) = Basis for controlled experimentation and controlled theoretical interpretation

12. Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ... The medium-modified Final State Parton Shower Medium characterized by transport coefficient: • pt-broadening of shower • energy loss of leading parton Salgado,Wiedemann PRD68:014008 (2003)

13. Why is RAA = 0.2 natural ? • Surface emission limits sensitivity to ? The fragility of leading hadrons Eskola, Honkanen, Salgado, Wiedemann NPA747 (2005) 511, hep-ph/0406319 • The quenching is anomalously large (I.e. exceeds the perturbative estimate by ~ 5)

14. How can we understand the size of ? • Are there other classes of measurements sensitive to ? • to test the microscopic dynamics of parton energy loss • on which extraction of is based. • to confirm and further constrain the large value of . • defines short-distance behavior of expectation value of two light-like Wilson lines • Well-defined but difficult problem in QCD. • Is this calculable from 1st principles • in a thermal quantum field theory?

15. Where does this associated radiation go to ? How does this parton thermalize ? What is the dependence on parton identity ? How can we better gauge ‘hard probes’?

16. Vacuum and medium radiation is suppressed due to quark mass • To test this at the LHC, exploit: light-flavored mesons - gluon parents D - mesons - quark parents (mc~0) B - mesons - quark parents (mb>0) Dokshitzer, Kharzeev, PLB 519 (2001) 199 Armesto, Dainese, Salgado, Wiedemann, PRD71:054027, 2005 Massless “c,b” Massive c,b • Color charge dependence dominates • Mass dependence dominates Parton energy loss depends on parton identity

17. Jet modifications in dense QCD matter • ‘Longitudinal Jet heating’: • The entire longitudinal jet • multiplicity distribution softens • due to medium effects. Borghini,Wiedemann, hep-ph/0506218 • Jets ‘blown with the wind’ • Hard partons are not produced • in the rest frame comoving with the medium Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301

18. Question: How to relate experimental data to fundamental properties of dense QCD matter? Approaches include: Perturbative QCD Lattice QCD Saturation Physics String theory … Approach discussed here

19. AdS/CFT correspondence relates Strong coupling problems Classical Problem in a curved in non-abelian QFT higher-dimensional space String Theory Calculations of Properties of Matter Maldacena, 1997 String coupling and string tension T’Hooft coupling • Translation into field theoretic quantities Black hole horizon Curvature radius • Finite T Lattice QCD is difficult to apply to • problems involving real-time dynamics • (moving QQbar pair, light-like Wilson loops, …) • hydrodynamic properties (Problem of analytical continuation to • if lowest Matsubara frequency in imaginary time formalism is .) • ….

20. Wilson loop C in our world Our (3+1)-dim world : area of string world sheet with boundary C. horizon AdS/CFT Calculation of Quenching Parameter • AdS/CFT Recipe Maldacena (1998) Rey and Lee (1998) • Result for the quenching parameter Liu, Rajagopal, Wiedemann, Phys. Rev. Lett. 97:182301, 2006 • “Numerology”: relate N=4 SYM to QCD by fixing for T = 300 MeV Is this comparison meaningful ?

21. Comment on: Is comparions meaningful? N=4 SYM theory • conformal Physics near vacuum and at very high energy is very different from that of QCD • no asmptotic freedom no confinement • supersymmetric • no chiral condensate • no dynamical quarks, 6 scalar and 4 Weyl fermionic fields in adjoint representation

22. At finite temperature: Is comparions meaningful? N=4 SYM theory at finite T QCD at T ~ few x Tc • conformal • near conformal (lattice) • no asymptotic freedom no confinement • not intrinsic properties of QGP at strong coupling • supersymmetric (badly broken) • not present • no chiral condensate • not present • no dynamical quarks, 6 scalar and 4 Weyl fermionic fields in adjoint representation • may be taken care of by proper normalization Is there a new form of “universality class” at high temperature? In solid state physics, materials of different microscopic composition and interaction show similar thermal properties. Here: non-abelian gauge theories of different particle content and symmetries show similar thermal properties above Tc.

23. At highest , there is a qualitatively novel regime of QCD, in which • Parton densities are maximal up to large scales • Coupling constant is small • Semi-classical methods apply This requires that the action is large Need weak coupling and strong fields, satisfied at sufficiently small Bjorken x,where hard processes develop over long distance QCD Saturation Physics:QCD at the highest parton densities Venugopalan McLerran; Jalilian-Marian,Kovner,Leonidov,Weigert; Balitsky; Kovchegov;… Can we test this novel QCD regime in the laboratory?

24. Small x higher initial parton density qualitatively different matter produced at LHC mid-rapidity? tests of saturation phenomena? - bulk observables - pt-spectra in scaling regime - rapidity vs. dependence - … • Large abundant yield of hard probes precise tests of properties of produced matter - color field strength - collective flow - viscosity - … The kinematical range accessible

25. Abundant yield + robust signal + theory = understanding of hard probes (e.g. jet quenching >> uncertainties) The Next-to-last Slide • This presentation was not comprehensive. • I missed to mention: and how they relate to • first principle calculations in a QFT: • - collective flow - transport coefficients, relaxation times • - electromagnetic probes - spectral functions • - rapidity and -dependence - saturated non-linear QCD evolution • Instead of being comprehensive, I emphasized • - how controlled experimentation with dense QCD matter is possible • - how the field makes progess in relating measurements to • fundamental properties of matter calculable in QCD.

26. The probes: • Jets • identified hadron specta • D-,B-mesons • Quarkonia • Photons • Z-boson tags The wide kin. range: ,x, A, luminosity Abundant yield of hard probes + robust signal (medium sensitivity >> uncertainties) = detailed understanding of dense QCD matter The RHICness of the LHC