Studying Hot and Dense QCD Matter in the LHC era

1. Studying Hot and Dense QCD Matter in the LHC era Urs Achim Wiedemann CERN, PH-TH Department Quark Matter 2009 30 March Knoxville

2. <<Many in the RHIC community … are interested in the LHC heavy-ion program, but have several questions … What can we learn at the LHC that is qualitatively new? Are collisions at LHC similar to RHIC ones, just with a somewhat hotter/denser initial state? If not, why not? … These questions are asked in good faith.>> Glenn Young, 25 January e-mail inviting me for a plenary talk

3. Testing QCD thermodynamics at RHIC and LHC SPS RHIC LHC • Large quantitative gains at LHC? • YES • - bigger • - longer • - denser Karsch, Laermann, Peikert, NPB605 (2001) 579 • Qualitatively the same system @ LHC • “just hotter and denser”? • No • QCD thermodynamics indicates • characteristic qualitative differences • in going from ~1.5 Tc to ~5 Tc. • Which ones? p.t.o. RBC-Bielefeld, PoS LAT2007:217,2007.

4. Trace of Wilson line The renormalized Polyakov loop Limit reached above 3-5 Tc - transforms under Z(3) - interpretation RBC-Bielefeld, PoS LAT2007:217,2007. • Good order parameter for pure SU(3) A.Kurkela

5. Relation to the validity of quasi-particle pictures • The effective potential at T>>Tc • - height of potential barrier • - fluctuations between minima • negligible only above 3-5 Tc Gross, Pisarski, Yaffe Rev.Mod.Phys.53:43,1981 • Quasi-particle models • - generally rely on expanding Veff around A0=(0,0) • - this breaks Z(3) invariance • - the complex minima lie outside domain of validity of description • - no sound basis for T< 2-3 Tc

6. a quasi-particle model Pressure/T4 SB • Quasi-particle models vs. Lattice • - significant deviations/uncertainties • below Lattice data • Strong fluctuations between • different Z(3) minima • deviates from • asymptotic value above J.P.Blaizot, QM06 Indicative of qualitative novel physics above 2-3 Tc

7. QCD thermodynamics is qualitatively different above 2-3 Tc • Beware: - Lattice shows • - naïve quasi-particle models • - LHC reaches well beyond T=400 MeV, • where QCD thermodynamics is different

8. QCD vs. AdS/CFT N=4 SYM in vacuum QCD in vacuum N=4 SYM finite T QCD at finite T • chiral condensate NO YES NO melted • asymptotic • freedom less important NO YES NO • confinement NO NO YES deconfined • superymmetric broken NO YES NO • conformal YES NO YES approx at T >>Tc • degrees of freedom Very different, but may be taken care of by normalization Physics near vacuum and at high energy is very different EXP: Conceptual questions in reach of LHC TH: Which field theoretical tools are best suited? Conceivable for T>2-3 Tc only • quasi-particles NO Zero Small for T>2-3 Tc only

9. Elliptic flow Hydro curves at CERN SPS • To establish hydrodynamic behavior, • reliable theoretical baseline needed • - close to perfect liquid ? • - sensitivity to ? • - sensitivity to dissipative properties • such as ? • - is the prefect liquid more like • ketchup or custard? TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page Shear thickening? or Shear thickening? NA45: PRL92 (2004) 032301

10. “True” Jets Talks by S. Salur T. Renk, K. Zapp, I. Vitev …

11. Leading hadron suppression “True” jet quenching Hadrons • branching of leading parton based on • BDMPS-Z-ASW-GLV-WDHG-etc • Exact energy conservation indispensable - sufficient for leading fragment? => Monte Carlo needed TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page • leading and subleading branchings • must be treated on equal footing • branching of subleading partons • not needed => Monte Carlo • perturbatrive (vacuum) baseline • analytical calculation or MC • perturbatrive (vacuum) baseline => Monte Carlo needed

12. Requirement: reproduce all analytically known limits Constraining Models of Jet Quenching Vacuum Parton Shower Perturbative Baseline + Hadronization Model Monte Carlo Model of Jet Quenching Elastic scattering limit BDMPS- limit Collisional Energy Loss Radiative Energy Loss Talk by K. Zapp

13. Needed: probabilistic implementation of quantum interference Solution: in vacuum => angular ordering only in medium => formation time constraint only (follows analytically from BDMPS-ASW) Monte Carlo including LPM-effect MC reproduces BDMPS-limits K. Zapp, J. Stachel, UAW arXiv:0812.3888 Talk by K. Zapp

14. In the BDMPS-limit: From limiting cases to full MC K. Zapp, J. Stachel, UAW in prep (lines denote MC results) • Relax soft scattering • approximation Extreme incoherent • Exact E-p-conservation • Realistic cross sections • Main messages: • more realistic => more incoherent • => larger e-loss • quadratic L2 supression • due to coherence • linear L-enhancement • due to incoherence Talk by K. Zapp H

15. Jet shapes, energy flows • Thrust – baseline • Jet multiplicity distributions • Thrust – medium above baseline Recent jet quenching MCs: JEWEL (K.Zapp et al.) Q-Pythia (Santiago group) YaJEM (T.Renk) Talks by C.Salgado, T.Renk

16. Jet Finding Algorithms • Tremendous recent progress • on jet finding algorithms • - novel class of IR and collinear safe • algorithms satisfying SNOWMASS accords • kt(FastJet) • anti-kt(FastJet) • SISCone • - new standard for p+p@LHC • - fast algorithms, suitable for heavy ions! Runtime [sec] Event multiplicity M. Cacciari, G. Salam, G. Soyez, JHEP 0804:005,2008 • Catchment area of a jet • - novel tools for separating soft • fluctuations from jet remnants • - interplay with MCs of jet • quenching needed

17. Jürgen’s mini-serving of the QGP: light ion physics with protons @ LHC? 60,000 MB events dNch/dh ~ 50 ! Abundant rate of p+p events with J. Schukraft, QM06 • comparable to semi-peripheral Cu+Cu @ RHIC • includes events not dominated by hard jets Does multiplicity drive collectivity? • Hadrochemistry • Is strangeness undersaturated in high-multp+p? • FSI/Hadronization • Does phase-space density drive recombination? • Femtoscopy • What is the dynamical origin of HBT radii? • Flow in mesoscopic systems • How does it arise as a function of multiplicity and • system size • Jet quenching? • (Bjorkens original proposal was for p+p!!!) • - … M.A.Lisa et al., Ann.Rev.Nucl.Part.Sci.55:357-402,2005

18. @ LHC, even proton wave functions are dense • Double hard cross section - scale factor at Tevatron for - hard partons closer localized/ correlated than average soft ones - at LHC S.Domdey, H.J.Pirner, UAW, in prep LHC is first collider to give access to scale dependence of scale factor (transverse growth of hard components in hadron wave functions) Kovner, UAW Phys.Rev.D66:051502,2002. Central issue since Saturation Unitarization of hadronic cross section

19. The LHeC-project • 70 GeV electron on 7 TeV proton • or 2.75 TeV Pb • Ring-ring or linac-ring • LHeC physics program • - discovery machine • - precision in the era of sLHC • * pdfs at large x and Q2 • separating all flavors, valence and sea • * strong coupling at per-mille • accuracy to test grand unification • * … • - QCD at extreme parton densities http://www.ep.ph.bham.ac.uk/exp/LHeC/

20. Summary LHC heavy ion program is a giant leap into the unexplored regime of high temperature QCD. We have all reasons to expect that this will allow for major advances on fundamental open questions in QCD.