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Recent Results from RHIC

Recent Results from RHIC. Workshop on Physics at Hadron Collider KIAS, June 24th, 2005 Ju Hwan Kang Yonsei University. Overview. Introduction : Why high-energy A+A collisions ? RHIC and Experiments at RHIC Major Findings at RHIC:

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Recent Results from RHIC

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  1. Recent Results from RHIC Workshop on Physics at Hadron Collider KIAS, June 24th, 2005 Ju Hwan Kang Yonsei University

  2. Overview Introduction: • Why high-energy A+A collisions ? • RHIC and Experiments at RHIC Major Findings at RHIC: • Reaches chemical equilibrium at (or before) hadronization • Collective flow from nearly ideal (hydrodynamical) fluid which is consistent with strongly coupled “perfect fluid” • Suppressionof high pThadrons (Jet Quenching) • Disappeareanceof back-to-back(di)jet correlations • Degrees of freedom consistent with constituent quarks Summary and Outlook:

  3. as(Q2) ~1/ln(Q2/L2), L~200 MeV High-energy heavy-ion physics program (in 4 plots) 2. Study the phase diagram of QCD matter: esp. produce & study the QGP • Learn about 2 basic properties of strong interaction: (de)confinement, • chiral symm. breaking (restoration) _ e/T4 <qq> (as=g2/4p) T/Tc 3. Probe quark-hadron phase transition of the primordial Universe (few μsec after the Big Bang) 4. Studythe regime of non-linear (high density) many-body parton dynamics at small-x (CGC)

  4. Final state probes Penetrating probes The "Little Bang" in the lab. • High-energy nucleus-nucleus collisions: fixed-target reactions (√s=20 GeV, SPS) or colliders (√s=200 GeV, RHIC. √s=5.5 TeV, LHC) • QGP expected to be formed in a tiny region (~10-14 m) and to last very short times (~10-23 s). • Collision dynamics: Diff. observables sensitive to diff. react. stages Time t ~ 107 fm/c t ~ 10 fm/c Penetrating probes t~0.1 fm/c

  5. Relativistic Heavy-Ion Collider (RHIC) @ BNL Specifications: 3.83 km circumference 2 independent rings: • 120 bunches/ring • 106 ns crossing time A + A collisions @√sNN = 200GeV Luminosity: 2·1026 cm-2 s-1 (~1.4 kHz) p+p collisions @ √smax=500 GeV p+A collisions @ √smax= 200 GeV 4 experiments: BRAHMS, PHENIX, PHOBOS, STAR Runs 1 - 5 (2000 – 2005): Au+Au @ 200, 130, 62.4GeV p+p @ 200 GeV d+Au @ 200 GeV Cu+Cu @ 200, 62.4 GeV PHOBOS BRAHMS PHENIX STAR

  6. The 4 RHIC experiments BRAHMS detector

  7. maximumprojection physics target minimumprojection RHIC Au+Au luminosities Luminosities • RHIC (Au+Au) is currently running at ~2x design luminosity

  8. pR2 t0 ~ 1 fm/c Energy density (Au+Au @ 200 GeV, y=0) • Bjorken estimate: (longitudinally expanding plasma) • dET/dηat mid-rapidity measured by calorimetry (using PHENIX EMCal as hadroniccalorimeter: EThad = (1.17±0.05) ETEMCal) Au+Au @ 200 GeV • <dET/dh> ~ 650 GeV (top 5% central) (~70% larger than at CERN-SPS) Bjorken ~5.0 GeV/fm3 > QCD critical density (~1 GeV/fm3) PHENIX Collab. PRL 87, 052301 (2001) nucl-ex/0104015

  9. Soft particle spectra • Bulk p±, K±, p(pbar) spectra reproduced by hydro w/ QGP EOS att0= 0.6 fm/c Au+Au central (b = 2.6 fm) Strong radial collective flow built-up at freeze-out: <bT> » 0.6 Hydro (quenched) pQCD Solid lines: QGP+HG Dashed lines: HG D.d'E. & Peressounko nucl-th/0503054

  10. Ratios of particle yields • Ratios of hadron yields consistent w/ system at chemical equilibrium at hadronization (Tchem.freeze-out ~ Tcrit) : • Hadron composition (even for strange had., gs=1) “fixed” at hadronization 157 MeV 9.4 MeV • Assume all distrib. described by one T and one m: • 1 ratio (e.g. p/p) determines m/T • 2nd ratio (e.g. K/pi) provides T,m. • Then predict all other hadronic yields and ratios dN ~ e - (E- m )/T d3p _ _ p/p ~ e – (E+m )/T/e –(E- m )/T = e - 2m /T PBM, Redlich, Stachel nucl-th/0304013 Kaneta, Xu nucl-th/0405068

  11. Elliptic flow • Initial anisotropy in x-space in non-central collisions (overlap) translates into finalazimuthal asymmetry in p-space (transverse to react. plane) 1. Truly collective effect (absent in p+p collisions). 2. Early-state phenomenon: develops only in 1st instants of reaction. Strongly self-quenches after t~1 fm/c Elliptic flow = v2 2nd Fourier coefficient of dN/df Time evolution of ellipsoid eccentricity: Hydro calculations Kolb, Sollfrank, Heinz PRC62, 054909 (2000)

  12. Elliptic flow at RHIC • Mass dependence of v2 consistent w/ hydrodynamics too: • Large v2 signal at RHIC: Exhausts hydro limit for pT<1.5 GeV/c  Strong (collective) pressure grads.  Large & fast parton rescattering: early thermalization. PHENIX . PRL 91, 181301 (2003) nucl-ex/0305036 PHENIX . PRL nucl-ex/0411040 • √s-dependence of v2: ~50% increase from CERN-SPS (apparent saturation within 62-200 GeV)

  13. Anisotropic Flow • Same phenomena observed in gases of strongly interacting atoms • M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. ThomasScience 298 2179 (2002) strongly coupled weakly coupled The RHIC fluid behaves like this, that is, a strongly coupled fluid

  14. V2 requires ultra-low viscosity Relativistic viscous fluid dynamics: Elliptic flow from hydro with early thermalization requires h/s  0.1 ( D.Teaney; nucl-th/0301099) Quantum lower bound on h/s : h/s = 1/4p(P.K. Kovtun, D.T. Son, A.O. Starinets; hep-th/0405231) RHIC data suggest that the fluid is “as perfect as it can be”, that is, it approaches the (conjectured) quantum mechanical limit Realized in strongly coupled N = 4 SUSY YM theory, also in QCD ? D. Teaney QGP(T≈Tc) = sQGP

  15. Viscosity Primer • Remove your organic prejudices • Don’t equate viscous with “sticky” • Think instead of a not-quite-ideal fluid: • “not-quite-ideal”  “supports a shear stress” • Viscosity hthen defined as • Dimensional estimate: • Viscosityincreases withtemperature • Largecross sections  small viscosity

  16. RHIC Scientists Serve Up “Perfect” Liquid • New state of matter more remarkable than predicted -- raising many new questions • April 18, 2005 • TAMPA, FL -- The four detector groups conducting research at the Relativistic Heavy Ion Collider (RHIC) -- a giant atom “smasher” located at the U.S. Department of Energy’s Brookhaven National Laboratory -- say they’ve created a new state of hot, dense matter out of the quarks and gluons that are the basic particles of atomic nuclei, but it is a state quite different and even more remarkable than had been predicted. In peer-reviewed papers summarizing the first three years of RHIC findings, the scientists say that instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC’s heavy ion collisions appears to be more like a liquid. High-Energy Physics: An emptier emptiness? Nature 435, 152-153 (May 12, 2005) by Frank Wilczek At the RHIC, collisions between heavy ions create a fireball in which temperature exceeding 1.5x1012K are achieved. Impressive evidence has accumulated that a qualitative new state of matter has been created, a liquid-like plasma of quarks and gluons. Could something even more dramatic - a qualitative change in the properties of empty space - be occurred as well? Theoretical calculations indicate that at such temperatures the pairs that make up the chiral condensate will break apart. When the condensate vaporizes, the full underlying chiral symmetry of QCD becomes operative.

  17. Hard QCD probes Production yields theoretically calculable via perturbative-QCD: “Factorization theorem”: A B Independent scattering of “free” partons: A+B = “simple superposition of p+p collisions” Nuclear Modification Factor: dAB → hard= A·B·dpp → hard At impact parameter b: AB dNAB → hard (b) = TAB(b)·dpp → hard AB geom. nuclear overlap at b production is “shadowed” TAB~ # NN collisions (“Ncoll scaling”)

  18. Suppressed high pT hadroproduction in Au+Au @ RHIC ! Au+Aup0 X (peripheral) Au+Aup0 X (central) Peripheral data agree well with Strong suppression in p+p (data & pQCD) plus Ncoll-scaling central Au+Au collisions D.d'E, nucl-ex/0401001

  19. Suppressed high pT hadroproduction @ RHIC PHENIX Collab. PRL 88, 022301 (2002) nucl-ex/0109003 Ncoll scaling (“hard” production) x5 suppression D.d'E., HP'04 nucl-ex/0504001 Npart scaling (surface emission) Discovery of high pT suppression (one of most significant results @ RHIC so far) RAA << 1: well below pQCD (collinear factorization) expectations for hard scattering cross-sections

  20. P+A (or d+A): The control Experiment Proton/deuteron nucleus collision Nucleus- nucleus collision • Jet Quenching interpretation; interaction with medium produced in final state suppresses jet. • Gluon Saturation interpretation, gluons are suppressed in initial state resulting in suppression of initial jet production rate. • If these initial state effects are causing the suppression of high-PT hadrons in Au+Au collisions, we should see suppression of high-PT hadrons in d+Au collisions.

  21. Unquenched d+Au production at high pT PHENIX. PRL91, 072303(2003) • Conclusion: High pTsuppression in central Au+Au due to final-state effects (absent in “control” d+Au experiment) D.d'E., nucl-ex/0401001

  22. “NN scaling” in Au+Au @ 200 GeV: Direct Photons • Direct photon production in Au+Au (all centralities) consistent w/ p+p incoherent scattering (“NN-scaled” pQCD) predictions: Direct photon production in Au+Au unmodified by QCD medium. Submitted to PRL nucl-ex/0503003

  23. High- pT hadron Jet Gluon bremsstrahlung QGP “Jet quenching” predictions • Multiple final-state non-Abelian (gluon) radiation off the produced hard parton inducedby the traversed dense medium. • Parton energy lossµmedium properties: DEloss ~ gluon(gluon density) DEloss~ DL2(medium length) • Energy is carried away by gluonsstrahlung inside jet cone: dE/dx ~ sk2T • Correction for expanding (1-D) plasma : E1-D =(20/RA) ·Estatic ~ 15·Estatic (0=0.2 fm/c, RA=6 fm) “gluonstrahlung” • Prediction I: Suppression of high pT leading hadrons • Prediction II: Disappeareance of back-to-back (di)jet correlations

  24. “Near Side Jet” Escapes “Far Side Jet” Lost Jet Correlations: 2-particle Correlations Parton exiting on the periphery of the collision zone should survive while partner parton propagating through the collision zone is more likely to be absorbed if Jet-Quenching is the correct theory. d+Au Au+Au 60-90% Min Bias 0-10% Near Far Near Far PHENIX Preliminary PHENIX Preliminary Far-side Jet is suppressed in Central Au+Au : Further indication of suppression by produced medium.

  25. GONE GONE Pedestal&flow subtracted Df Further evidence 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

  26. Unsuppressed baryon production • Rcp(ratio central/peripheral) at intermediate pT =2 – 4 GeV/c: • Particle composition inconsistent with known (universal) fragmentation functions. • Additional production mechanism for baryons in the intermediate pT range _ _ Baryons:p, p, Λ, ΛNOT (or much less) suppressed in central Au+Au. Mesons: p0, K0s, η, equally suppressed. PHENIX Collab. PRL91:172301(2003) STAR Collab. subm. to PRL, nucl-ex/0306007 D.d'E. J.Phys. G30, S677 (2004)

  27. Enhanced (anti)proton/pion ratio • Central Au+Au: p/π~ 0.8 (at pT = 2 - 4 GeV/c) at variance with perturbative production mechanisms (favour lightest mesons). • Periph. Au+Au: p/π ~ 0.2 as found in p+p (ISR,FNAL) & e+e- jet fragmentation (anti)proton/pion PHENIX Collab. PRL91:172301(2003)

  28. Enhanced baryonic elliptic flow • Different v2 saturation for mesons and baryons: v2meson > v2baryon at low pT v2meson ≈ v2baryon at pT≈ 2 GeV/c v2meson < v2baryon at higher pT • Simple v2 scaling behaviour if v2 and pT are normalized by number of constituent quarks: n = 2 mesons n = 3 baryons (“universal” parent quark flow ?) PHENIX Collab. to appear in PRL nucl-ex/0305013

  29. “Quark recombination” models vs. data Anomalous baryon enhancement & quark number scaling of v2 at pT= 2--5 GeV/c explained by “quark recombination” (coalescence) in dense (thermal) medium: • Via quark momenta addition, recombination dominates for pT ~ 1- 4 GeV/c: pT(baryons)> pT(mesons)> pT(quarks) • Fragmentation dominates for pT > 5 GeV/c: pT(hadrons)= z pT(partons) , with z<1 • Constituent-quark scaling of v2 naturally explained Fries, Mueller Nonaka, Bass PRL 90, 202303 fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph • However ... Near-side azimuthal correlations of high pT trigger (leading) baryons is jet-like • No pure thermal + thermal parton recombination produces such jet-like correlations ... (thermal+hard ?) Greco, Ko, Levai PRL 90, 202302 • Rethink hadronization at interm. pT at RHIC ! Phase space filled with partons Recombine quarks into hadrons

  30. Summary 1. Energy densities: Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > εcrit 2. Elliptic flow: Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c 3. Soft particle spectra: Shapes & yields consistent w/ hydrodyn. (thermal+coll. velocity) source emission Particles ratios consistent w/ chemically equilibrated system before hadronization 4. Hard particle spectra: Strong high pT suppression in central A+A (compared to p+p, p+A & pQCD) consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100 5. Intermediate pT spectra: Enhanced baryon yields & v2 (compared to meson) consistent w/ quark recombination mechanisms in a thermal and dense system All observations consistent with formation of thermalized dense partonic matter in central Au+Au collisions

  31. RHIC Open Questions • Is the quark-gluon plasma being formed in RHIC collisions? To be determined: • Does charmonium show the expected suppression from (color) Debye screening? • Is there direct (photon) radiation from the plasma? • Do the suppression effects extend to the highest pT’s? • What are the properties of the produced matter (sound speed, heat capacity, viscosity, etc.) • What are the gluon and sea-quark contributions to the proton spin? (polarized proton running)

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