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The Quest for Old Physics at RHIC

This article discusses the search for new states of matter and the ultimate temperature at ultrahigh temperatures and densities at RHIC. It also explores the concept of the Quark-Gluon Plasma (QGP) and its formation in energetic collisions of heavy nuclei.

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The Quest for Old Physics at RHIC

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  1. The Quest for Old Physics at RHIC W.A. ZajcColumbia University W.A. Zajc

  2. Is There An Ultimate Temperature? No.7Are there new states of matter at ultrahigh temperatures and densities? From the National Research Council Committee on The Physics of the Universe report W.A. Zajc

  3. ~1970: An Ultimate Temperature? • The very rapid increase of hadron levels with mass ~ equivalent to an exponential level density • and would thus imply a “limiting temperature”TH ~ 170 MeV Hagedorn, S. Fraustchi, Phys.Rev.D3:2821-2834,1971 W.A. Zajc

  4. QCD is not QED • QED (Abelian): • Photons have do not carry charge • Flux is not confined  1/r potential  1/r2 force • QCD (Non-Abelian): • Gluons carry charge (red, green, blue)(anti-red, anti-green, anti-blue) • Flux tubes form  potential ~ r  constant force + +… W.A. Zajc

  5. 0.66 TC T =0 0.90 TC 1.06 TC ~2000: TH  TC That is: The ‘Hagedorn temperature’ TH is now understood as a precursor of TC TC = Phase transition temperature of QCD Study confining potentialin Lattice QCD at various temperatures Current estimates from lattice calculations:TC ~ 150-170 MeVL ~ 0.70.3 GeV / fm3(latent heat) F. Karsch, hep-ph/0103314 W.A. Zajc

  6. Making Something from Nothing • Explore non-perturbative “vacuum” that confines color flux by melting it • Experimental method: Energetic collisions of heavy nuclei • Experimental measurements:Use probes that are • Auto-generated • Sensitive to all time/length scales • Particle production • Our ‘perturbative’ region is filled with • gluons • quark-antiquark pairs which screen the “bare” interaction • A Quark-Gluon Plasma (QGP) W.A. Zajc

  7. Energy density for “g” massless d.o.f 8 gluons, 2 spins;  2 quark flavors, anti-quarks, 2 spins, 3 colors 37 (!) “Reasonable” estimate Relevant Thermal Physics Q. How to liberate quarks and gluons from ~1 fm confinement scale? A. Create an energy density • Need better control of dimensional analysis: W.A. Zajc

  8. Pressure in plasma phase with “Bag constant” B ~ 0.2 GeV / fm3 Pressure of “pure” pion gas at temperature T Slightly More Refined Estimate • Compare • Select system with higher pressure: Phase transition at T ~ 140 MeV with latent heat ~0.8 GeV / fm3 Compare to best estimates (Karsch, QM01)from lattice calculations:T ~ 150-170 MeV latent heat ~ 0.70.3 GeV / fm3 W.A. Zajc

  9. early universe 250 RHIC quark-gluon plasma 200 Lattice QCD SPS 150 AGS deconfinement chiral restauration Chemical Temperature Tch [MeV] thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryon Potential B [MeV] The Landscape of QCD W.A. Zajc

  10. g The Early Universe, Kolb and Turner Previous Attempts First attempt at QGP formation was successful (~1010 years ago) ( Effective number of degrees-of-freedom per relativistic particle ) W.A. Zajc

  11. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) W.A. Zajc

  12. How is RHIC Different? • Different from p-p, e-p colliders Atomic number A introduces new scale Q2 ~ A1/3 Q02 • Different from previous (fixed target) heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction)decreases by ~ same factor • Access to perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons W.A. Zajc

  13. STAR RHIC’s Experiments W.A. Zajc

  14. PHENIX during last 10 days: 24 (mb)-1/week Lave(week) = 0.4  1026 cm-2 s-1 Lave(week)/Lave(store) = 27 % FY2001 – 02 100 GeV/amu ~Most results presented here are from the RHIC Run-1 data set ~Most results presented here are those Run-1 data in the refereed literature: 40 publications to date (31 PRL’s) FY2000 (66 GeV/amu) RHIC Runs to date • Run-1, Summer 2000: • Au+Au √sNN = 130 GeV, ~1 mb-1 • Run-2, 2001-2: • Au+Au √sNN = 200 GeV, ~ 24 mb-1 • p+p √sNN = 200 GeV, ~0.13 pb-1, P ~ 25% • Run-3, 2003: • d-Au (ongoing) √sNN = 200 GeV • p+p √sNN = 200 GeV (polarized, to begin 22-Mar-03) W.A. Zajc

  15. 1 RHIC Event Data Taken June 25, 2000. Pictures from STAR Level 3 online display. Q. How to take the measure of such complexity?? (Is it possible?) A. (Yes.) Begin with single-particle momentum spectra W.A. Zajc

  16. Questions • Do those many particles in the final state have anything to do with a state of matter? • For example: Is there a well-defined • Energy density e • Temperature T • Chemical potential m • Size R • Transport coefficient l • Answer: Yes (apparently) • The first round of RHIC experiments have determined ~all of these parameters (and more) W.A. Zajc

  17. Kinematics Dynamics Kinematics 101 Fundamental single-particle observable: Momentum Spectrum W.A. Zajc

  18. BRAHMS Acceptance (PID) Acceptances STAR Acceptance PHOBOS Acceptance W.A. Zajc

  19. t Dynamics 101 Q. How to estimate initial energy density? A. From rapidity density of transverse energy ET • “Highly relativistic nucleus-nucleus collisions: The central rapidity region”, J.D. Bjorken, Phys. Rev. D27, 140 (1983). • Assumes • ~ 1-d hydrodynamic expansion • Invariance in y along “central rapidity plateau”(I.e., flat rapidity distribution) • Then since boost-invariance of matter  where t ~ 1 fm/c W.A. Zajc

  20. Determining Energy Density EMCAL • What is the energy density achieved? • How does it compare to the expected phase transition value ? PHENIX For the most central events: Bjorken formula for thermalized energy density time to thermalize the system (t0 ~ 1 fm/c) ~6.5 fm eBjorken~ 4.6 GeV/fm3 pR2 ~30 times normal nuclear density ~1.5 to 2 times higher than any previous experiments W.A. Zajc

  21. Results on Particle Composition PHOBOS and BRAHMS:ratios of p - / p + K- /K+p / p mid-rapidity PHENIX, STAR: spectra of p- , p0 , p+ , K- , Ks0 , K+ , p , p , f ,L , L , … W.A. Zajc

  22. STAR preliminary Systematic errors ~10-20% 130 GeV RHIC : STAR / PHENIX / PHOBOS /BRAHMS 17.4 GeV SPS : NA44, WA97 Central K+/K- BRAHMS PHENIX PHOBOS STAR X+/X- p-/p+ p/p L/L Ratio (chemical fit) K+/p+ K-/p- K+/h- p/p+ p/p- K-/h- K0s/h- K*0/h- L/h- L/h- f/h- X-/h- X+/h- Model:N.Xu and M.Kaneta, nucl-ex/0104021 Ratio (data) Is there a ‘Temperature’? • Apparently: • Assume distributions described by one temperature T and one ( baryon) chemical potential m: • One ratio (e.g., p / p ) determines m / T : • A second ratio (e.g., K / p ) provides T m • Then predict all other hadronic yields and ratios: W.A. Zajc

  23. STAR preliminary Systematic errors ~10-20% 130 GeV RHIC : STAR / PHENIX / PHOBOS /BRAHMS 17.4 GeV SPS : NA44, WA97 Locating RHIC on Phase Diagram • Previous figure  RHIC has net baryon density ~ 0: • TCH = 179 ± 4 MeV, B = 51 ± 4 MeV (M. Kaneta and N. Xu, nucl-ex/0104021) • RHIC is as close as we’ll get to the early universe for some time Previous Heavy Ion Experiments (CERN SPS) W.A. Zajc

  24. Hydrodynamic Behavior • Superimposed on the thermal (~Boltzmann) distributions: • Collective velocity fields from • Momentum spectra ~ • ‘Test’ by investigating description for different mass particles: • Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061) W.A. Zajc

  25. z Hydrodynamic limit STAR: PRL86 (2001) 402 PHOBOS preliminary (scaled) spatial asymmetry y x (PHOBOS : Normalized Paddle Signal) Compilation and Figure from M. Kaneta Hydrodynamics of Elliptic Flow Parameterize azimuthal asymmetry of charged particlesas 1 + 2 v2cos (2 f) Evidence that initial spatial asymmetry is translated quickly to momentum space ( as per a hydrodynamic description) W.A. Zajc

  26. RSIDE p1 ROUT Beamaxis p2 Measuring Space-Time Dimensions • Can we “image” the particle source? • No – But Hanbury-Brown—Twiss (HBT) measurements provide 3-D measure of spatial dimensions F.B. Yano and S.E. Koonin, Phys.Lett.B78:556-559,1978 W.A. Zajc

  27. Sizes • Measurements of RSIDE • ‘Large’ source • Transverse momentum dependence  strongly expanding source • In rough agreement with hydrodynamic description • But • Complete disagreement with predictions for ROUT / RSIDE • ‘Freeze-out’ time development not understood? • Problems in formalism? • An outstanding open question W.A. Zajc

  28. PHENIX Preliminary pT dependent systematic error Q. Why did we build RHIC? A: To gain access to ‘small’ cross-sections* that are A) Fundamental B) Calculable C) Interesting which then allow us to use Ncoll ( aka A*B or “binary” or “point-like”) scaling of yields as our baseline hypothesis for probing a new state of matter • (This of course one of many possible answers…) } W.A. Zajc

  29. Luminosity • Consider collision of ‘A’ ions per bunchwith ‘B’ ions per bunch: • Luminosity Cross-sectional area ‘S’ A B W.A. Zajc

  30. Change scale by ~ 109 • Consider collision of ‘A’ nucleons per nucleuswith ‘B’ nucleons per nucleus: • ‘Luminosity’ Cross-sectional area ‘S’ A B • Provided: • No shadowing • Small cross-sections W.A. Zajc

  31. Binary Collisions Spectators Participants Participants Spectators b (fm) Systematizing our Knowledge • All four RHIC experiments have carefully developed techniques for determining • the number of participating nucleons NPART in each collision(and thus the impact parameter) • The number of binary nucleon-nucleon collisions NCOLL as a function of impact parameter • This effort has been essential in making the QCD connection • Soft physics ~ NPART • Hard physics ~ NCOLL • Often express impact parameter b in terms of “centrality”, e.g., 10-20% most central collisions W.A. Zajc

  32. g conversion         Open Charm as a Rare Process • Via analysis of single-electron spectrum: • Measure electron pT spectrum • Quantify (or bound) all other sources of e’s • Remaining excess: from semi-leptonic decays of D’s W.A. Zajc

  33. Better Example of Ncoll Scaling • Q: Are there rare probes at RHIC that scale as the number of binary collisions? • A: Yes, charm production (for Ncoll from 71 to 975) PHENIX Run-2 Preliminary Data presented at Quark Matter 2002 W.A. Zajc

  34. Jet Axis R ‘Jets’ at RHIC • Tremendous interest in hard scattering (and subsequent energy loss in QGP) at RHIC • Production rate calculable in pQCD • But strong reduction predicted due to dE/dx ~ path-length (due to non-Abelian nature of medium) • However: • “Traditional” jet methodology very difficult at RHIC • Dominated by the soft background • Investigate by (systematics of) high-pT single particles W.A. Zajc

  35. Another Example of Ncoll Scaling • PHENIX (Run-2) data on p0 production in peripheral collisions: • Excellent agreement between PHENIX measured p0’s in p-pandPHENIX measured p0’s in Au-Au peripheralcollisions scaled by the number of collisionsover ~ 5 decades PHENIX Preliminary W.A. Zajc

  36. Extending Ncoll Scaling(?) Q: Do all processes that should scale with Ncoll do just that? A: No! Central collisions are different .(Huge deficit at high pT) • This is a clear discoveryof new behavior at RHIC • It is presumably a resultdue to formation of unusually denseand opaque matter earlyin the collision • Intense theoretical activity to understand dependence on • Medium properties(?) • Deconfinement(??) PHENIX Preliminary W.A. Zajc

  37. Energy Loss of Fast Partons • Many approaches • 1983: Bjorken • 1991: Thoma and Gyulassy (1991) • 1993: Brodsky and Hoyer (1993) • 1997: BDMPS- depends on path length(!) • 1998: BDMS • Numerical values range from • ~ 0.1 GeV / fm (Bj, elastic scattering of partons) • ~several GeV / fm (BDMPS, non-linear interactions of gluons) W.A. Zajc

  38. Is the suppression new? • Yes- all previous measurements see enhancement, not suppression. • This is qualitatively new physics made accessible by RHIC’s ability to produce • (copious) perturbative probes • (New states of matter?) • Run-2 results show that this effect persists (increases) to the highest available transverse momenta • Describe in terms of scaled ratio RAA= 1 for “baseline expectations” ‘Understood’ enhancement from Cronin effect Ncoll baseline PHENIX Preliminary W.A. Zajc

  39. Centrality dependence of change • suppression stronger with centrality & increased pT W.A. Zajc

  40. Df Extinction of back-to-back jets • STAR azimuthal correlation function shows ~ complete absence of “away-side” jet • Surface emission only (?) • That is, “partner” in hard scatter is absorbed in the dense medium W.A. Zajc

  41. PAUSE W.A. Zajc

  42. Making the QCD Connection • A surprising connection has emerged between softphenomena (charged multiplicity) and QCD • The measured multiplicities at RHIC are low compared to (pre-data) calculations • This is (now) understood* as a manifestation of saturation in the initial state gluon distributions • Nch ~ Nuclear Gluon Density ~ A xG (x, Q2)not ~ A xG (x, Q2)  xG (x, Q2) • ‘Understood’ in the sense that this is an area of intense theoretical activity and discussion W.A. Zajc Eskola, QM2001

  43. Production via gluon fields • Apply the “method of virtual quanta” to each nucleus • Particle “production” is the realization of “pre-existing” (dense) gluon fields • But with essentialmodifications from QCD: • Non-linear saturation in gluon densities • Saturation scale QS2in momentum space increases with nuclearsize and/or overlap • Strong fields ~classical dynamics W.A. Zajc

  44. Gluon saturation in Nuclei / xpz 2R dT =  /Q 2R m/pz Longitudinal Transverse When do the gluons overlap significantly? 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987) At rest Moving with pz So for  /mx ~ 2R , ~ all constituents contributeParton density r ~ A xG(x,Q2) /R2 Parton cross section s ~ aS2/ Q2 Saturation condition r s ~ 1 QS2 ~ aS A xG(x,Q2) /R2 ~ A1/3 W.A. Zajc D. Kharzeev, nucl-th/0107033

  45. dN/dh / .5Npart Npart Saturation in Multiplicity • Large nucleus (A) at low momentum fraction x gluon distribution saturates ~1/as(QS2) with QS2~ A1/3 • A collision* puts these gluons ‘on-shell’ r ~ A xg(x,Q2) / R2 • Parton-hadron duality maps gluons directly to charged hadrons • Each collision varies the effectiveA , i.e, the number of participants NPART • Shattering the ‘Color Glass Condensate’) W.A. Zajc

  46. pp Extensions of Saturation Approach* • Use HERA data, counting rules • x G(x,Q2) ~ x-l (1-x)4 • Describe rapidity dependence: • y ~ ln(1/x)  QS2(s,y) = QS2(s,y=0)ely • Predict energy dependence: • x = QS / s QS2(s,y) = QS2(s0,y) (s/s0) l/2 • Predict10-14% increase betweens = 130 and 200 GeV • Versus 146% reported by PHOBOS * D. Kharzeev and E. Levin, nucl-th/0108006 W.A. Zajc

  47. RHIC and HERA (?) • The gluon densities at low x and their Q2 evolution are the same as those used in saturation models applied at HERA in e-p collisions: • A.M. Stasto, K. Golec-Biernat, J. Kwiecinski, Phys. Rev. Lett. 86, 596 (2001) • J. Bartels, K. Golec-Biernat, H. Kowalski hep-ph/0203258 • More detailed understanding of • the precise connection • implications for other RHIC observables an area of intense investigation W.A. Zajc

  48. Making the Connection(?) • “Final-state descriptions” provide excellent description of the data in terms of • Input: • Temperature T • Pressure Tmn • Transport properties of medium R/l • Output • Species abundances • Spectra • Collective flow • “jet quenching” • “Initial-state descriptions” provide excellent description of the data in terms of • Input: • Saturation scale QS2 • Measured gluon structure functions xG(x,Q2) • Output • Multiplicities • Spectra • Collective flow • “jet quenching” • Can these be reconciled via “parton-hadron duality”? • Possible explanation of (mysterious) “pre-existing equilibrium” required in old Hagedorn approach to statistical production of particles W.A. Zajc

  49. RHIC Dynamics From (just) the first run: • Thermodynamics • Established • Hydrodynamics • Validated • Chromodynamics • In progress W.A. Zajc

  50. Coupling Constant Number Limit Weak Strong Particle Bulk Gravity  X X  Weak  X  X QED  X   QCD     Experimental Gauge Theory • QCD is the only fundamental gauge theory amenable to experimental study in both • Weak and strong coupling limits • Particle and bulk limits • RHIC • (Strong, bulk ) limit : heavy ion collisions • (Strong, particle) limit : spin physics • (Weak , particle) limit : W’s as helicity probes • (Weak , bulk ) limit : high pT probes of plasma state W.A. Zajc

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