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The Highest Energy Density Matter: Quark Gluon Plasma

The Highest Energy Density Matter: Quark Gluon Plasma. Colloquium Vanderbilt University Barbara V. Jacak Stony Brook Dec. 2, 2004. The physics of the quark gluon plasma. Energy densities characteristic of the early universe Why expect a quark gluon plasma? Experimental study at RHIC

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The Highest Energy Density Matter: Quark Gluon Plasma

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  1. The Highest Energy Density Matter: Quark Gluon Plasma Colloquium Vanderbilt University Barbara V. Jacak Stony Brook Dec. 2, 2004

  2. The physics of the quark gluon plasma • Energy densities characteristic of the early universe • Why expect a quark gluon plasma? • Experimental study at RHIC • Experiments and probes of the hot, dense matter • What happens? • Energy loss of fast quarks, gluons • Collective motion: hydrodynamic flow • First glimpse of the matter’s properties • Underlying degrees of freedom & hadronization • Conclusions & open questions

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

  4. Quarks, gluons and hadrons • 6 quarks: • 2 light (u,d), 1 sort of light (s), 2 heavy (c,b), 1very heavy (t) • Besides flavor, also have color quantum number • In normal life: quarks are bound into hadrons • Baryons (e.g. n, p) have 3 • Mesons (e.g. p, K, f) have 2 (1 quark + 1 anti-quark) • Colored quarks interact by exchange of gluons (also have color) • Quantum Chromo Dynamics (QCD) • Field theory of the strong interaction • Parallels Quantum Electrodynamics (QED) • but in EM interactions, exchanged photons electrically uncharged • gluons carry color charge

  5. At high temperature and density: force is screened by produced color-charges expect transition to free gas of quarks and gluons + +… QCD phase transition Color charge of gluons  gluons interact among themselves • theory is non-abelian • curious properties at large distance: • confinement of quarks in hadrons

  6. Karsch, Laermann, Peikert ‘99 ~15% from ideal gas of weakly interacting quarks & gluons e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 non-perturbative QCD – lattice gauge theory required conditions to study quark gluon plasma

  7. So, collide BIG ions at v ~ c • Reach T  170 MeV; e > 3 GeV/fm3 • Characterize the hot, dense medium • Evidence for QCD phase transition to quark gluon plasma? • does medium behave as a plasma? coupling weak or strong? • What’s the density, temperature, radiation rate, collision frequency, conductivity, opacity, Debye screening length? • Probes • passive (radiation) • those created in the collision (“external”)

  8. RHIC at Brookhaven National Laboratory Collide Au + Au ions for maximum volume s = 200 GeV/nucleon pair, p+p and d+A to compare

  9. STAR 4 complementary experiments

  10. The Scope of the Tools (!) STAR specialty: large acceptance measurement of hadrons PHENIX specialty: rare probes, leptons, and photons

  11. Hard scattered q,g (short wavelength) probes of plasma formed p, K, p, n, f, L, D, X, W, d, Hadrons reflect (thermal) properties when inelastic collisions stop (chemical freeze-out). g, g* e+e-, m+m- Real and virtual photons emitted as thermal radiation. probing early stage of heavy ion collision e, pressure builds up we focus on mid-rapidity (y=0), as it is the CM of colliding system 90° in the lab at collider

  12. So what happens? • Lots of information collected over past 4 years • I’ll focus on a few important results • Start with the “external” probes • Made in the first step in the collision • Rate and spectrum calculable with (perturbative) QCD • look for differences… • Start by benchmarking the probe!

  13. p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions Start simple p+p collisions p QCD works for high p transfer processes!  Have a handle on initial NN interactions by scattering of q, g inside N We also need: p0

  14. ( pQCD x Ncoll) / background Vogelsang/CTEQ6 ( pQCD x Ncoll) / (background x Ncoll) [w/ the real suppression] [if there were no suppression] pQCD in Au+Au? direct photons g+q g+q gTOT/gp0 Au+Au 200 GeV/A: 10% most central collisions Preliminary pT (GeV/c) []measured / []background = measured/background

  15. Energy  to beam direction per unit velocity || to beam pR2 2ct0 Is the energy density high enough? PRL87, 052301 (2001) Colliding system expands: • e 5.5 GeV/fm3 (200 GeV Au+Au) well above predicted transition! value is lower limit: longitudinal expansion rate, formation time overestimated

  16. schematic view of jet production hadrons leading particle q q hadrons leading particle AA AA AA nucleon-nucleon cross section <Nbinary>/sinelp+p “external” probes of the medium Hard scattering of q,g early. Observe fast leading particles, back-back correlations Before creating hadron jets, scattered quarks induced to radiate energy (~ GeV/fm) by the colored medium -> jet quenching

  17. Au-Au s = 200 GeV: high pT suppressed! PRL91, 072301(2003)

  18. near side away side Medium is opaque! peripheral central look for the jet on the other side STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au

  19. Suppression: a final state effect? Hot, dense medium causes Such a medium is absent in d+Au collisions! But Au provides all initial state effects of q, g wavefunction inside a Au nucleus d+Au is the “control” experiment

  20. Experiments show NO suppression in d+Au! PHENIX Preliminary p0 STAR Preliminary PHOBOS Preliminary

  21. Centrality Dependence Au + Au Experiment d + Au Control PHENIX preliminary • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. • Jet Suppression is clearly a final state effect.

  22. leading particle suppressed hadrons Pedestal&flow subtracted q q ? Are back-to-back jets there in d+Au? Yes! So this is the right picture for Au+Au

  23. Almond shape overlap region in coordinate space Collective motion? Pressure: a barometer called “elliptic flow” Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy  momentum anisotropy v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

  24. The data show Particle emission really is azimuthally anisotropic Magnitude of the anisotropy grows with beam energy, then flattens c.m. beam energy

  25. Hydro. Calculations Huovinen, P. Kolb, U. Heinz Hydrodynamics assumes early equilibration Initial energy density is input Equation of state from lattice QCD Solve equations of motion v2 reproduced by hydrodynamics • see large pressure buildup • anisotropy  happens fast •  early equilibration ! STAR PRL 86 (2001) 402 central

  26. Kolb, et al Collective effect probes early phase Hydrodynamics can reproduce magnitude of elliptic flow for p, p. BUT correct mass dependence requires softer than hadronic EOS!! NB: these calculations have viscosity = 0 medium behaves as an ideal liquid

  27. So, what do E loss & collectivity tell us? • Medium is “sticky” or opaque to colored probes • Thermalization must be very fast (< 1fm/c) • Constrain hydrodynamic, energy loss models with data:

  28. Lattice QCD shows qq resonant states at T > Tc, also implying high interaction cross sections Is s enough for fast equilibration & large v2 ? Parton cascade using free q,g scattering cross sections underpredicts pressure must increase x50

  29. Plasma coupling parameter? • Very high gluon density • estimate G = <PE>/<KE> • using QCD coupling strength • <PE>=g2/d d ~1/(41/3T) • <KE> ~ 3T • g2 ~ 4-6 (value runs with T) • G ~ g2 (41/3T)/ 3T  so plasma parameter G ~ 3 • NB: such plasmas known to behave as a liquid! • May have bound q,g states, but not color neutral • So the quark gluon plasma is a strongly coupled plasma • As in warm, dense plasma at lower (but still high) T • But strong interaction rather than electromagnetic

  30. Underlying degrees of freedom? • Should be quarks and gluons in the early (QGP) stage • Experimental evidence for this? • Look at production of final state particles with different number of quarks • Baryons have 3, mesons have 2 • Recall comparison of proton and pion collective flow constrained equation of state

  31. More energetic baryons in Au+Au PRL 91 (2003) 172301 • p/ ~1 at high pT • in central collisions • > p+p, d+Au > peripheral Au+Au >jets in e+e- • collisions

  32. R. Fries, et al pQCD spectrum shifted by 2.2 GeV Teff = 350 MeV Are extras from the thermal medium? Hydrodynamic expansion common  velocity boost heavier particles boosted to higher pT enhances mid pT hadrons baryons especially Do the hadrons get boosted? or Coalescence of boosted quarks into hadrons?

  33. Elliptic flow scales as number of quarks • v2 scales ~ with # of quarks! • evidence for quarks as relevant d.o.f. when pressure built up

  34. equilibrated final state, including s quarks Hadron yields in agreement with Grand Canonical ensemble T(chemical freeze-out) ~ 175 MeV (multi)strange hadrons enhanced over p+p, but QGP hypothesis not unique explanation

  35. How about J/Y suppression? • J/Y • Test confinement: • do bound c + c survive? • or does QGP screening kill them? Need (a lot) more statistics (currently being analyzed) But can take a first look…

  36. 0-20% most central Ncoll=779 40-90% most central Ncoll=45 20-40% most central Ncoll=296 Lattice predictions for heavy quarks Color screening? This data inconclusive

  37. conclusions Evidence is mounting that RHIC creates a strongly coupled, opaque plasma still is debate in community about standard of proof • With aid of hydrodynamic, l-QCD and p-QCD models: • e ~ 15 GeV/fm3 • dNgluon/dy ~ 1000 • sint large for T < 2-3 Tc • Equation of state not hadronic! • Have evidence that thermalized q,g produce the particles observed in the final state • Will learn screening properties via cc bound states • Are poised to characterize this new kind of plasma • T, radiation rate, conductivity, collision frequency

  38. r/ ggg d + Au collisions cent/periph. (~RAA) Saturation of gluons in initial state(colored glass condensate) Mueller, McLerran, Kharzeev, … Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse. Saturation at higher x at RHIC vs. HERA due to nuclear size  suppressed jet cross section; no back-back pairs

  39. Open charm: baseline is p+p collisions PHENIX PRELIMINARY Measure charm s via semi-leptonic decay to e+ & e- p0, h, photon conversions are measured and subtracted fit p+p data to get the baseline for d+Au and Au+Au.

  40. Charm production s scales as Ncoll! The yellow band represents the set of alpha values consistent with the data at the 90% Confidence Level.

  41. No large suppression as for light quarks! PHENIX PRELIMINARY Curves are the p+p fit, scaled by the number of binary collisions

  42. Evidence for equilibrated final hadronic state BRAHMS • Simple quark counting: K-/K+ = exp(2ms/T)exp(-2mq/T) = exp(2ms/T)(pbar/p)1/3 = (pbar/p)1/3 • local strangeness conservation K-/K+=(pbar/p)a a = 0.24±0.02 BRAHMS a = 0.20±0.01 for SPS • Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV From y=0 to 3 At y=0 PRL 90 102301 Mar. 2003

  43. Implications of the results for QGP • Ample evidence for equilibration • initial dN(gluon)/dy ~ 1000, energy density ~ 15 GeV/fm3, energy loss ~ 7-10 GeV/fm • Very rapid, large pressure build up requires • parton interaction cross sections 50x perturbative s

  44. How to get 50 times pQCD s? spectral function • Lattice indicates that hadrons don’t all melt at Tc! • hc bound at 1.5 Tc Asakawa & Hatsuda, PRL92, 012001 (2004) • charmonium bound states up to ~ 1.7 Tc Karsch; Asakawa&Hatsuda • p, s survive as resonances Schaefer & Shuryak, PLB 356 , 147(1995) • q,g have thermal masses at high T. as runs up at T>Tc? (Shuryak and Zahed) • would cause strong rescattering qq  meson

  45. E. Shuryak

  46. Implications of the results for QGP • Ample evidence for equilibration • v2 & jet quenching measurements constrain initial gluon density, energy density, and energy loss • parton interaction cross sections 50x perturbative s • parton correlations at T>Tc • complicates cc bound states as deconfinement probes! • Hadronization by coalescence of thermal,flowing quarks • v2 & baryon abundances point to quark recombination as hadronization mechanism • Jet data imply must also include recombination between quarks fromjets and the thermalized medium •  medium modifies jet fragmentation!

  47. early universe 250 RHIC 200 quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Locate RHIC on phase diagram From fit of yields vs. mass (grand canonical ensemble): Tch = 176 MeV mB = 41 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

  48. But baryons show jet-like properties too… Baryons scale  Ncoll ! ~RAA f follows the mesons  Kinematics determined by quark content

  49. Do see Cronin effect! (h++h-)/2 “Cronin” enhancement more pronounced in the charged hadron measurement Possibly larger effect in protons at mid pT Implication of RdAu? RHIC at too high x for gluon saturation… p0

  50. No CGC signal at mid-rapidity So, perhaps Rda G-sat. Rda pQCD >2 BFKL, DGLAP Xc(A) RHIC Pt (GeV/c) Pt (GeV/c) Log Q2 How about Color Glass Condensate? Central: Enhanced not suppressed PHENIX preliminary y=0 Peripheral d+Au (like p+p)

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