Quarks and Gluons in Nuclear Collisions

1. Quarks and Gluons in Nuclear Collisions Rainer Fries Texas A&M University & RIKEN BNL INPC 2007 Tokio, June 7, 2007

2. Overview • QCD & High Energy Nuclear Collisions • Baryon Puzzles • Recombination and the Rediscovery of Valence Quarks • Light from Shooting Bullets through Quark Matter INPC 2007

3. Hot Nuclear Matter • Early universe: • Microseconds after the Big Bang: a hot soup of quarks & gluons • Phase transition to hadrons • Temperature T ~ 1012 K • What are the properties of hot quark matter? Hadrons melt: quark gluon plasma (QGP) Too hot for nucleons to be bound inside nuclei u  u s d g u g u u d d p g d + Molecules, atoms dissolve: EM plasma Today’s universe: 2.7 K INPC 2007

4. QCD Energy density (T) • Asymptotic freedom for T  • Deconfinement • Weakly interacting quark gluon plasma. • Lattice QCD: • Phase transition at Tc ~ 170 – 192 MeV (@ B = 0) • Cross over for B = 0,  critical point ? • Thermodynamics: 20% deviation from SB-limit • Strongly interacting system above Tc? [Karsch et al.] INPC 2007

5. High Energy Nuclear Collisions • Evolution of High Energy Nuclear Collisions • Measuring bulk hadrons (low PT): latest times • Measuring jets or hadrons from jets (high PT) • Photons and dileptons: penetrating probes • Approximate hierarchy: larger PT or M ~ larger sensitivity to early times INPC 2007

6. Jet Quenching • RHIC: strong quenching of high-PT pions and kaons. • Energy loss of leading parton. Nuclear modification factor INPC 2007

7. Jet Quenching & Baryon Puzzle • RHIC: strong quenching of high-PT pions and kaons. • Energy loss of leading parton. • No jet quenching for baryons? (RAA , RCP ~ 1) • Seen for PT ~ 1.5 … 5 GeV/c. • “Baryon Anomaly” at intermediate PT. PHENIX INPC 2007

8. Baryon Puzzle • Proton/pion ratio > 1 at PT ~ 4 GeV in Au+Au collisions. • Expectation from parton fragmentation: p/ ~ 0.1 … 0.3 • As measured in p+p and e++e- PHENIX INPC 2007

9. Baryon vs Meson • General baryon/meson pattern: p, , ,  versus K, , , K*,  INPC 2007

10. Hadrons from Fragmentation • Conventional wisdom: hadrons at large PT produced through fragmentation of single partons • E.g. in e++e-, p+p • Fragmentation mechanism: • Single parton starts gluon radiation + pair production in a jet cone • Alternative picture: QCD string breaking • QCD Factorization: • Fragmentation functions • Measured in e++e; hadron yields:  >> p INPC 2007

11. Hadrons from Dense Parton Systems • Fragmentation = limit of hadronization for dilute systems (parton density  0) • Possible corrections at finite : • Fragmentation + pick-up • 2, 3, n partons  hadrons • What happens in the opposite limit, very dense phase of partons? • E.g. thermalized system just above Tc? • Immediate problem: no hard scale in the problem (T  QCD) • Recombine partons! INPC 2007

12. Quark Recombination • Simple realization of a recombination model • Recombine valence quarks of hadrons • Dressed quarks, no gluons • Instantaneous projection of quark states on hadron states Meson Wigner function Product of quark distributions Production hypersurface INPC 2007

13. Recombination & Fragmentation • Dual model of hadron production: • Recombination + pQCD/fragmentation. • Competition between Recombination and Fragmentation • Fragmentation dominates for power law at high PT (recovering the dilute limit). • Recombination dominates for thermal quarks. • Using thermal quark spectra for recombination. • Describes hadron production at RHIC for PT> 1…2 GeV/c. fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph INPC 2007

14. Phenomenological Success • Recombination of thermal partons dominates up to 4 GeV/c for mesons, 6 GeV/c for baryons [Greco, Ko & Levai] [RJF, Müller, Nonaka & Bass] INPC 2007 [RJF, Müller, Nonaka & Bass]

15. Phenomenological Success • Summary so far: • Describe about a dozen spectra of identified hadrons with one distribution function of quarks (3 parameters) • Good news: • Solved baryon puzzle. • Hint of common partonic origin of hadrons. • But: • Very limited insights into dynamics of hadronization. • There is more … INPC 2007

16. z y x Anisotropy • Finite impact parameter b > 0: • Spatial anisotropy in the initial state • Momentum anisotropy in the final state INPC 2007

17. z y x Anisotropy • Finite impact parameter b > 0: • Spatial anisotropy in the initial state • Momentum anisotropy in the final state • Space  momentum space translation High-PT particles Bulk/soft particles Test EOS of the system Test density/opacity of the medium INPC 2007

18. z y x Elliptic Flow • Finite impact parameter b > 0: • Spatial anisotropy in the initial state • Momentum anisotropy in the final state • Space  momentum space translation • Fourier analysis • v2 = elliptic flow INPC 2007

19. Elliptic Flow Scaling • Assume universal elliptic flow v2p of the partons before the phase transition • Recombination prediction (simplest case!): • Scaling works! • Caution: • hydro-like behavior at low PT. • Resonance decay + hadronic phase [Greco et al.] INPC 2007

20. Elliptic Flow Scaling • Scaling of kinetic energy: • Implied by hydrodynamics. • Scaling close to perfect. INPC 2007

21. Elliptic Flow Scaling • Scaling of kinetic energy: • Implied by hydrodynamics • Scaling close to perfect. • Heavy  meson (m ~ mp) scales with other mesons INPC 2007

22. Quark Counting Rule for the QGP • Quark counting rules: quark substructure in hadrons • Classic example: counting valence quarks • RHIC 2003: a new quark counting rule • Subhadronic degrees of freedom are at work. • They act collectively: observable v2 describes collective effect • Equilibrium / hydrodynamic behavior of this matter (?) • Deconfinement is reached. INPC 2007

23. Zooming in on v2 Scaling • We proposed a new variable: baryon/meson v2 asymmetry (B-M)/(B+M) for scaled v2. • Small deviations visible • Wave function effects. • Small contribution of higher Fock states could be accommodated. [P. Sorensen, STAR] INPC 2007

24. Hadron Correlations • Jet like correlations seen in data even at intermediate PT. • How to reconcile with recombination? • Correlations induced by Soft/Hard Reco (pick up reactions) • Hadron correlations arise from correlations between soft partons • Hot spots: fully or partially thermalized jets INPC 2007

25. Hadron Correlations • Jet like correlations seen in data even at intermediate PT. • How to reconcile with recombination? • Correlations induced by Soft/Hard Reco (pick up reactions) • Hadron correlations arise from correlations between soft partons • Hot spots: fully or partially thermalized jets Near side Away side Meson trigger Baryon trigger [RJF, Bass & Müller] INPC 2007

26. Hadronization in Other Systems • An old story: recombination in very forward direction • E.g. to explain D+/D asymmetries • Clear signs of interactions with beam remnants • d+Au / p+A at midrapidity • Cronin enhancement: initial state broadening • At RHIC: large final state effect seen • Pick-up reactions (soft/hard recombination) important E791  beam [Braaten, Jia & Mehen] [Hwa, Yang] INPC 2007

27. Hadronization in Other Systems • We don’t need a QGP, just a finite parton density • Fragmentation is very ineffective for baryons! • It might just be easier to pick up soft partons to make baryons instead of creating them, even in cold nuclear matter. AA pA pp e+e- INPC 2007

28. Photon Sources • Prompt photons from initial hard processes • e.g. initial Compton q + g  q +  • No final state effects at all. • Fragmentation/vacuum bremsstrahlung • Possible large contribution from q, g   + X • Sensitivity to medium effects in the final state ~ similar to hadrons • Thermal/bulk photons (QGP + hadronic phase) • Measure temperature T • Difficult measurement, need to subtract background at low PT INPC 2007

29. Photon Sources • Prediction: jets interacting with the medium will produce photons [RJF, Müller, Srivastava], [Zakharov] • Jet-photon conversion • Induced photon bremsstrahlung • This idea seems compelling: • RHIC data: jets do interact strongly with the medium. • Rates sensitive to physics of the medium • Complementary information • Temperature measurement with GeV Photons? • Sensitivity to energy loss: different path integral INPC 2007

30. Jet-Photon Conversion • Annihilation and Compton scattering in the fireball • Cross sections forward/backward peaked: • “Conversion”: very effective process. • Photon yields approximately proportional to the jet distributions. • How bright is the new source? • Same order of magnitude as hard photons at RHIC. • Steeper PT spectrum  important at intermediate PT FMS PRL 90 (2003) INPC 2007 [RJF, Müller & Srivastava]

31. Direct Photons & Dileptons • Photons at RHIC and dileptons at LHC. • Very good description of RHIC direct photons. • But: no proof of jet induced medium photons yet. [Turbide, Gale, Jeon & Moore] [Turbide, Gale, Srivastava & RJF] INPC 2007

32. Elliptic Flow • Jet-medium photons show azimuthal anisotropy • Hadrons: longer path leads to more suppression • Conversion photons: longer path leads to increased production! • Prediction: conversion photons have negative v2 optical v2. • Other photon sources: • Initial hard processes: v2 = 0 • Fragmentation, thermal photons: v2 > 0 (like hadrons) • Expect delicate cancellations. Hadrons Conversion Photons INPC 2007

33. Elliptic Flow • No clear signal in the data so far. • Strong dependence on parameters in the calculation. • Wait for more photon data & dileptons. PHENIX [Turbide, Gale, RJF] INPC 2007

34. Summary • Recombination is a very simple model to describe a very complex process. • And it does a remarkable job on RHIC data! • Provides a clear signal for the existence of collective effects in partonic matter  deconfinement. • Revival for recombination models in other systems? • Jet induced medium-photons: promising new penetrating probe. • Optical v2 < 0 for induced photons, maybe observable. INPC 2007

35. Acknowledgements Many thanks to my collaborators on these projects: M. Asakawa S. A. Bass C. Gale R. C. Hwa B. Müller C. Nonaka D. K. Srivastava S. Turbide C. B. Yang INPC 2007

36. Backup INPC 2007

37. u  u s d g u g u u d d p g d + Hadronization • Formation of bound states is non-perturbative in QCD. • Hadrons look differently, depending on how we probe them • Probe different matrix elements of different operators. • If we were able to solve QCD completely, we could compute all of them. How we see a hadron depends on … … which process we use to probe … the resolution of the process INPC 2007 … the reference frame.

38. What is in the Parton Phase? • Recombination: low Q, no hard scattering • No perturbative plasma at hadronization • Effective degrees of freedom; no gluons • Constituent quarks? • We need a field theoretic description including chiral symmetry breaking. • cf. dynamical masses from instantons, lattice, DSE Diakonov & Petrov Bowman et al. INPC 2007

39. Soft (T) partons Shower (S) pT Soft/Hard Recombination • Attempts to treat reco + fragmentation consistently • Hwa and Yang: jets  parton showers; fitted to fragmentation functions • Majumder et al.: 2- and 3- quark constituent quark fragmentation + recombination ( Q2 evolution) • Recombine all partons: • Partons = soft/thermal + showers from jets • Two parton distribution function: Partons from 1 jets soft-soft Partons from 2 jets soft-shower INPC 2007

40. Dileptons • Very similar for dileptons • Same sources: • Thermal emission • Jet-induced emission • Initial hard scattering (Drell-Yan) • Background from charm & bottom INPC 2007

41. Dileptons • Dilepton PT spectrum at small mass, central collisions. [Turbide,Gale, Srivastava, RJF] INPC 2007

42. Dileptons • Dilepton mass spectrum at high PT, central collisions. [Turbide,Gale, Srivastava, RJF] INPC 2007