Cosmology and QCD: Exploring the Fundamental Questions of High Energy Heavy Ion Physics

1. Link to cosmology A QCD/QGP/RHIC primer The discovery of the sQGP Towards the most fundamental questions High Energy Heavy Ion PhysicsQuo Vadis ? Rene Bellwied Wayne State University (bellwied@physics.wayne.edu)

2. Two big connections: cosmology and QCD Motivation for Relativistic Heavy Ion Collisions

3. Going back in time… Age Energy Matter in universe 0 1019 GeV grand unified theory of all forces 10-35 s 1014 GeV 1st phase transition (strong: q,g + electroweak: g, l,n) 10-10 s 102 GeV2nd phase transition (strong: q,g + electro: g + weak: l,n) 10-5 s 0.2 GeV 3rd phase transition (strong:hadrons + electro:g + weak: l,n) 3 min. 0.1 MeVnuclei 6*105 years0.3 eVatoms Now3*10-4 eV = 3 K (15 billion years) RHIC, LHC & FAIR RIA & FAIR

4. Evolution of Forces in Nature

5. Connection to Cosmology • Baryogenesis ? Separation of Matter and Antimatter – can it happen at the phase transition ? • Dark Matter Formation ? – can it happen at the phase transition ? • Dark Energy Formation – can it happen at the phase transition ? • Is matter generation in cosmic medium (plasma) different than matter generation in vacuum ? • Can fluctuations at the phase transition explain an anisotropic matter distribution in the universe ?

6. Sakharov (1967) – three conditions for baryogenesis • Baryon number violation • C- and CP-symmetry violation • Interactions out of thermal equilibrium • Currently, there is no experimental evidence of particle interactions where the conservation of baryon number is broken: all observed particle reactions have equal baryon number before and after. Mathematically, the commutator of the baryon number quantum operator with the Standard Model Hamiltonian is zero: [B,H] = BH - HB = 0. This suggests physics beyond the Standard Model • The second condition — violation of CP-symmetry — was discovered in 1964 (direct CP-violation, that is violation of CP-symmetry in a decay process, was discovered later, in 1999). If CPT-symmetry is assumed, violation of CP-symmetry demands violation of time inversion symmetry, or T-symmetry. Under investigation • The last condition states that the rate of a reaction which generates baryon-asymmetry must be less than the rate of expansion of the universe. In this situation the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation.

7. A mass problem of universal proportion • The stars and gas in most galaxies move much quicker than expected from the luminosity of the galaxies. • In spiral galaxies, the rotation curve remains at about the same value at great distances from the center (it is said to be ``flat''). • This means that the enclosed mass continues to increase even though the amount of visible, luminous matter falls off at large distances from the center. • Something else must be adding to the gravity of the galaxies without shining. We call it Dark Matter ! According to measurements it accounts for > 80% of the mass in the universe.

8. The cosmic connection of RHI physics Witten’s ‘Cosmic Separation of phases’(Phys.Rev.D 30 (1984) 272) basic parameter: mass Originally: strange quark matter was a prime candidate for dark matter (as recent as SQM 2003)

9. Dark Matter vs. Luminous Matter distributionBullet Cluster, 3.4 Billion Lightyears from EarthX-ray image vs. gravitational lensing

10. The universe is accelerating…. Based on supernovae measurements You need DARK ENERGY as an explanation (!?)

11. Dark Energy does not kick in at the time of the Big Bang !

12. Let’s understand mass generation in the luminous matter The cosmic connection of RHI physics

13. What do we know about quark masses ? • Why are quark current masses so different ? • There is no answer to this questions. • There likely will be no answer to this question ! • Nature’s constants: • speed of light, electric charge, • quark current masses (?)

14. Very little is known, very little can be explained Standard model is symmetric All degrees of freedom are massless Electro-weak symmetry breaking via Higgs field (Dm of W, Z, g) Mechanism to generate current quark masses (but does not explain their magnitude) Chiral symmetry breaking via dynamical quarks Mechanism to generate constituent quark masses (but does not explain hadronization)

15. We can’t answer the question of massgeneration at the most fundamental level,but can we answer the question of massgeneration at the nuclear level ? Theory: Quantum Chromo Dynamics The fundamental problem: how is baryonic mass generated Based on quark interactions (5+10+10 = 935 MeV/c2) ?

16. Theoretical and computational (lattice) QCD In vacuum: - asymptotically free quarks have current mass - confined quarks have constituent mass - baryonic mass is sum of valence quark constituent masses Masses can be computed as a function of the evolving coupling strength or the ‘level of asymptotic freedom’, i.e. dynamic masses. But the universe was not a vacuum at the time of hadronization, it was likely a plasma of quarks and gluons. Is the mass generation mechanism the same ?

17. The main features of Quantum Chromodynamics (QCD) • Confinement • At large distances the effective coupling between quarks is large, resulting in confinement. • Free quarks are not observed in nature. • Asymptotic freedom • At short distances the effective coupling between quarks decreases logarithmically. • Under such conditions quarks and gluons appear to be quasi-free. • (Hidden) chiral symmetry • Connected with the quark masses • When confined quarks have a large dynamical mass - constituent mass • In the small coupling limit (some) quarks have small mass - current mass

18. To understand the strong force and the phenomenon of confinement: Create and study a system of deconfined colored quarks (and gluons) quark-antiquark pair created from vacuum Analogies and differences between QED and QCD to study structure of an atom… electron …separate constituents Imagine our understanding of atoms or QED if we could not isolate charged objects!! nucleus neutral atom Confinement: fundamental & crucial (but not understood!) feature of strong force - colored objects (quarks) have  energy in normal vacuum quark Strong color field Force grows with separation !!! “white” 0 (confined quarks) “white” proton (confined quarks) “white” proton

19. A mechanism of hadronization in vacuum:String Fragmentation High momentum current mass quark pair forms flux tube in a collision = string of energy (string tension) i.e. dynamical quark field which fragments into hadrons when string tension becomes too large. Describes e+e- and p-pbar and p-p collisions well. Hadronization in medium (i.e. during universe expansion) could be different because medium might affect the mechanism.

20. 1.05 Tc 1.5 Tc 3 Tc 6 Tc 12 Tc The temperature dependent running coupling constant as and its effect on mass generation above Tc O.Kaczmarek et al. (thermal mass, LQCD) (hep-lat/0406036) Massive partons above Tc e.g. P.Levai and U.Heinz (hep-ph/9710463) in an expanding system: interplay between distance and temperature

21. Lattice QCD:Chiral Symmetry is restored at Tc

22. One goal: Proving asymptotic freedom in the laboratory. • Measure deconfinement and chiral symmetry restoration under the conditions of maximum particle or energy density. Nobel Prize 2005 D. Gross H.D. Politzer F. Wilczek QCD Asymptotic Freedom (1973)

23. Rolf Hagedorn German Hadron bootstrap model and limiting temperature (1965) Before QCD Density of hadron mass states dN/dM increases exponentially with mass. Broniowski, et.al. 2004 TH ~ 21012oK Energy diverges as T --> TH Maximum achievable temperature? “…a veil, obscuring our view of the very beginning.” Steven Weinberg, The First Three Minutes (1977)

24. QCD to the rescue! Replace Hadrons (messy and numerous) by Quarks and Gluons (simple and few) e/T4  g*S Thermal QCD ”QGP”(Lattice) “In 1972 the early universe seemed hopelessly opaque…conditions of ultrahigh temperatures…produce a theoretically intractable mess. But asymptotic freedom renders ultrahigh temperatures friendly…” Frank Wilczek, Nobel Lecture (RMP 05) Hadron gas Karsch, Redlich, Tawfik, Eur.Phys.J.C29:549-556,2003

25. Nobel prize for Physics 2005 “Before [QCD] we could not go back further than 200,000 years after the Big Bang. Today…since QCD simplifies at high energy, we can extrapolate to very early times when nucleons melted…to form a quark-gluon plasma.” David Gross, Nobel Lecture (RMP 05) g*S Thermal QCD -- i.e. quarks and gluons -- makes the very early universe tractable; but where is the experimental proof? n Decoupling Nucleosynthesis e+e- Annihilation Heavy quarks and bosons freeze out QCD Transition Mesons freeze out Kolb & Turner, “The Early Universe”

26. Generating a deconfined state • Present understanding of Quantum Chromodynamics (QCD) • heating • compression •  deconfined color matter ! Hadronic Matter (confined) Nuclear Matter (confined) Quark Gluon Plasma deconfined !

27. Expectations from Lattice QCD /T4 ~ # degrees of freedom confined: few d.o.f. deconfined: many d.o.f. TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core] C  0.7 GeV/fm3

28. Suggested Reading • October 2006 issue of Nature: “Did the Big Bang Boil ? ” by F. Wilczek • …the answer as far as the quark-hadron transition is concerned is ‘No’. QCD evolves smoothly with temperature there is no thermodynamic phase transition. • Heavy Ion collisions at RHIC and the LHC can produce fireballs with a significant excess of baryons over anti-baryons, or different effective temperatures for quarks and gluons – possibilities that did not occur in the cosmic Big Bang. In those new circumstances do true phase transitions occur ?

29. A phase transition into what ? • With the liquid-gas phase transition established (ground state liquid drop nuclei transition to a hadron gas) the question was: What comes next ? A weakly interacting plasma. • Edward Shuryak (1971) : name it the Quark Gluon Plasma Cabibo-Parisi, PLB59 (1975) G.Baym, NSAC-LRP (1983)

30. The phase diagram of QCD Early universe quark-gluon plasma critical point ? Tc Temperature colour superconductor hadron gas nucleon gas nuclei CFL r0 Neutron stars vacuum baryon density

31. Study all phases of a heavy ion collision If the QGP was formed, it will only live for 10-22 s !!!! BUT does matter come out of this phase the same way it went in ???

33. QGP energy density • > 1 GeV/fm3 i.e. > 1030 J/cm3 Energy density of matter high energy density: e > 1011 J/m3 P > 1 Mbar I > 3 X 1015W/cm2 Fields > 500 Tesla

34. Step 1: Measuring a reference systemIn order to prove that we form a phase of matter thatbehaves different than the vacuum we need to understand our results in pp collisions ?

35. Hadronization in QCD (the factorization theorem) hadrons Parton Distribution Functions hadrons Hard-scattering cross-section leading particle Fragmentation Function High pT (> 2.0 GeV/c) hadron production in pp collisions: ~ Jet: A localized collection of hadrons which come from a fragmenting parton c a Parton Distribution Functions Hard-scattering cross-section Fragmentation Function b d “Collinear factorization”

36. Thermally-shaped Soft Production “Well Calibrated” Hard Scattering p0 in pp: well described by NLO (& LO) p+p->p0 + X • Ingredients (via KKP or Kretzer) • pQCD • Parton distribution functions • Fragmentation functions • ..or simply PYTHIA… hep-ex/0305013 S.S. Adler et al.

37. pp at RHIC:Strangeness formation in QCD nucl-ex/0607033

38. How strong are the NLO correctionsin LO calculations (PYTHIA) ? • K.Eskola et al. (NPA 713 (2003)): Large NLO corrections not unreasonable at RHIC energies. Should be negligible at LHC (5.5 or 14 TeV). STAR LHC

39. New NLO calculation based on STAR data (AKK, hep-ph/0502188, Nucl.Phys.B734 (2006)) K0s apparent Einc dependence of separated quark contributions.

40. Mt scaling in pp

41. Breakdown of mT scaling in pp ?

42. mT slopes from PYTHIA 6.3 Gluon dominance at RHIC PYTHIA: Di-quark structures in baryon production cause mt-shift Recombination: 2 vs 3 quark structure causes mt shift

43. Collision Energy dependence of baryon/meson ratio - baryon production in pp is simply not well understood Ratio vs pT seems very energy dependent (RHIC < < SPS or FNAL), LHC ? Not described by fragmentation ! (PYTHIA ratios at RHIC and FNAL are equal) Additional increase with system size in AA Both effects (energy and system size dependence) well described by recombination

44. Conclusions for RHIC pp data • We are mapping out fragmentation and hadronization in vacuum as a function of flavor. • What we have learned: • Strong NLO contribution to fragmentation even for light quarks at RHIC energies • Quark separation in fragmentation function very important. Significant non-valence quarks contribution in particular to baryon production. • Gluon dominance at RHIC energies measured through breakdown of mt-scaling and baryon/meson ratio. Unexpected small effect on baryon/antibaryon ratio • Is there a way to distinguish between fragmentation and recombination ? Does it matter ? • What will happen at the LHC ? What has happened in AA collisions (hadronization in matter) ?

45. The future: unprecedented physics reach at LHC (ALICE – pp)(charged particle spectra) enormous reach in multiplicity and transverse momentum. Could this system behave collectively ??

46. Step 2: Proving the existence of a new phase of matterCan we prove that we have a phase thatbehaves different than elementary pp collisions ? Three steps: a.) prove that the phase is partonic b.) prove that the phase is collective c.) prove that the phase characteristics are different from the QCD vacuum

47. Fate of jets in heavy ion collisions? idea: p+p collisions @ same sNN = 200 GeV as reference p p ?: what happens in Au+Au to jets which pass through medium? • Prediction: scattered quarks radiate energy (~ GeV/fm) in the colored medium: • “quenches” high pT particles • “kills” jet partner on other side ? Au+Au

48. Major discoveries in AuAu collisions‘The Big Three’(leading to the discovery of the sQGP = the Perfect Quark Gluon Liquid = AIP Science Story of 2005)

49. # I: The medium is dense and partonic STAR, nucl-ex/0305015 pQCD + Shadowing + Cronin energy loss pQCD + Shadowing + Cronin + Energy Loss • Deduced initial gluon density at t0 = 0.2 fm/c dNglue/dy ≈ 800-1200 • ≈ 15 GeV/fm3, eloss = 15*cold nuclear matter (compared to HERMES eA or RHIC dA)(e.g. X.N. Wang nucl-th/0307036)

50. Experiment: there are baryon/meson differences Theory: there are two types of e-loss: radiative and collisional, plus dead-cone effect for heavy quarks Flavor dependencies map out the process of in-medium modification An important detail: the medium might not be totally opaqueThere are specific differences to the flavor of the probe