HeavyIon Collisions at RHIC Search for Quark Gluon

Heavy-Ion Collisions at RHIC ~Search for Quark Gluon Plasma~ Takao Sakaguchi Brookhaven National Laboratory 米国ブルックヘブン国立研究所 坂口貴男 Outline of talk • Motivation of Quark Gluon Plasma Search • Accelerator and detectors • Dynamics and Global feature of Heavy ion collisions • Hard scattering as a new probe • Direct Photon, Jet and Heavy Quark results • Summary and Future

Why do we carry out Heavy Ion Collisions? Why Quark Gluon Plasma (QGP) ? • Believe it or not! • It existed in the early universe. • Understanding fundamental QCD problem • Quark confinement • Origin of proton (hadron) Mass • Both questions rely on ｌow Q 2 region, where as(Q 2)>1 • QGP is a phase where bare strong interaction plays significant role • Quarks and gluons are free from hadron “bag” • Study dynamical behavior of strongly interacting system

RHIC at BNL Long Island, New York PHENIX PHOBOS STAR BRAHMS Axial Field Solenoidal fieldtarget geometry 2 Spectrometers - fixed Resolution & Rates. Rings, TOF Large Tracking Solid Angle Tracking. TOF, RICH Magnets, Chambers, Si High -Strips, Si Multiplicity 2 Central Arms, 2 Forward Arms TPC’s, every Si-Vertex Run started in 2000. Around 6 months running year. Tracking TEC, RICH, EM Cal, Si, TOF, -ID RICH, EM Cal, TOF Statistics aka PHENIX experiment. “Table-top” 2 Arm Spectrometer Magnet, Paddle Trigger Counter Species Au+Au s 1/2 [Ge. V ] TOF 130 200 Ldt Ntot (sampled) 1 b-1 265 b Spectrometer -1 9. 1 b Octagon+Vertex -1 Data Size 10 M 3 TB 1. 8 G 120 TB 4 TB Ion collider • First Heavy • 3. 83 km circumference p+p 200 0. 5 pb-1 10 G 50 TB • 106 ns bunch crossing d+Au 200 2. 74 nb-1 5. 5 G 46 TB • Top Energy: -1 Cu+Cu 200 3. 06 nb 1. 1 G • 500 Ge. V for p+p Cu+Cu 63 0. 16 nb-1 • 200 Ge. V for Au+Au • Luminosity Leptons, Photons, and Hadrons in Measurements of Hadronic observables 26 cm-2 s-1 • Au+Au: 2 x 10 selected solid angles (especially muons) using a large acceptance spectrometer 32 cm-2 s-1 • production p+p : 2 x 10 Inclusive particle over Low p charged hadrons t Simultaneous detection of phase transition Event-by-event analyses of global a large Multiplicity in 4 & Particle Correlations rapidity and pt range (polarized) phenomena (e– coincidences) Ring Counters Au+Au 63 58 M observables, hadronic spectra and jets

Time profile of heavy ion collisions Gluon Plasma • QGP phase Gold ions “pass through” each other • • Large-x partons fly over. Mid-rapidity region is full of small-x gluons • High energy heavy ion collisions = Gluonic matter collisions • Turns into Gluon plasma • Gluon -> quark + anti-quark -> QGP • Cooling QGP -> Mixed phase -> Hadronic stage • Global Feature • • Energy density: 5. 7 Ge. V/fm 3 @ Au+Au s. NN=200 Ge. V (ref. LQCD: reaches plateau at 2 -3 Ge. V/fm 3) Temperature T=178 Me. V (Threshold) Mixed phase Hadronization + Expansion

New probe to HEHIC: Hard scattering • • Hard scattering process well described by NLO p. QCD calculation at high Q 2 Unique Signature at high energy: Hard scattering cross-section is large • Jet and Direct photon • Heavy Quark production: Charm(onium), Bottom(onium) In A+B collisions---TAB Scaling • C Cross section in A+B collisions = TAB(b) p+p collisions • TAB(b): Overlap integral of nuclear profile functions Number of binary collisions • =ABP 1 • Can be calculated by Geometrical description of Nucleus x. P 1 1 P 2 A c X x 2 P 2 d B Centrality 0% (Central) Centrality 100% (peripheral) A B looking from top view along beam axis Proportional to number of participated nucleons

Calibrating Hard scattering • High p. T Direct photon in Au+Au at s. NN=200 Ge. V Leading Order (LO) • Electromagnetic probes bring out information on the stage it is emitted • Direct access to hard scattering (p. T>4 Ge. V/c) • Yellow bands show error due to three different cutoff scale of NLO p. QCD scaled by number of binary collisions (Ncoll) Next-to-Leading Order (NLO) • NLO p. QCD agrees very well with measurement • First measurement of hard scattered direct photon in heavy ion collisions! PHENIX, nucl-ex/0503003 Hard scattering cross-section in nucleus-nucleus collisions has been calibrated

Jet as a probe of dense medium • Parton may change its momentum in hot dense medium • Energy loss through Gluon radiation, etc. • Reconstruction of Jet in Au+Au is impossible • Trigger Leading particle of Jet • Angle correlation, Energy, momentum, etc. may reflect Jet kinematics Fragmentation: Yield [Ge. V-1 c] 0 without energy loss 0 with energy loss Energy loss =Yield suppress p. T [Ge. V/c] X. -N. , Wang, PRC 58 (1998)2321

High p. T Identified hadron spectra(I) • Produced in initial hard scattering process • Should scale with Ncoll if no additional process exists • In peripheral Au+Au collisions, yield is consistent with p+p collisions scaled by Ncoll 70 -80% Centrality 0 -10% Centrality (peripheral) (Central) • In Central Au+Au collisions, yield is significantly lower than p+p • Energy loss of hard scattered parton in hot and dense medium? pp. T [Ge. V/c] PHENIX, PRL 91, 072301 (2003) s. NN=200 Ge. V

High p. T Identified hadron spectra(II) • Nuclear Modification Factor: RAA • Ratio of per-collision-yield to p+p • Hard scattering only: ratio is 1 =1 << 1 • Is suppression due to final state interaction? • Au+Au Direct photon: RAA= 1 • Suppression is final state effect d+Au Minimum bias 200 Ge. V 0 RAA /Rd. A • Peripheral Au+Au, Minimum bias d+Au: • Central Au+Au: Au+Au Peripheral • Energy loss of parton in hot dense medium BRAHMS, PRL 91, 072305 (2003) Au+Au Central • Medium expands in longitudinal direction as well? • Suppression at high rapidity region • Answer is Yes! PHENIX, PRL 91, 072301 (2003) PHENIX, PRL 91, 072303 (2003)

Modification of away side Jet • Correlation of back-to-back jets through high p. T hadrons • Trigger leading high p. T (4<p. T<6) hadrons STAR PRL 90, 082302 (2003) Trigger particles sit at 0. Au + Au peripheral near • Angle correlation of lower p. T (2<p. T<trig) particles with triggered hadrons • p+p and peripheral Au+Au: • Near side yield = Away side yield • Central Au+Au: away side particles suppressed. • Energy loss of away side Jet • Near side jet produced almost at surface of medium away Au + Au central away Coll im regi ated on • Near side: In Same Cone of leading • Away side: In Cone of associated jet Histogram: p+p, Black Points: Au+Au Blue Line: Mixed background

Where is away side Jet ? Near(Trigger) Side Away Side (Folded into 0 -p ) Interpretation. . Wake effect or “sonic boom” hep-ph/0411315 Casalderrey-Solana, Shuryak, Teaney Correlations of Jets with flowing medium W. Holtzmann for PHENIX, WWND, 2005 Even lower p. T associated particles (1. 0<p. T<2. 5) hep-ph/0411341 Armesto, Salgado, Wiedemann Strong Modification of away-side Jet observed! Dawn of Jet tomography

Heavy Quark(onium) • • • Charm or bottom produced in hard scattering process Energy loss of light quark is mostly due to gluon radiation (analogous to Bremsstrahlung) • How about heavier quarks? Collisional energy loss? Charmonium in hot dense medium will be: • Suppressed due to dissociation (debye screening) • D mesons , Y’, c Large Q value needed (>≈3 Ge. V) Enhanced due to coalescence of c-cbar p. QCD should work better! J/ (M=3. 1 Ge. V/c 2)

Single heavy quark measurement • Experimentally observe the decay products of Heavy Flavor particles (e. g. D-mesons) • STAR • PHENIX • Direct D mesons hadronic • Singledecay electron • Hadronic channels D K , D 0 + decay - 0 channels in d+Au measurements in p+p, D e( ) d+Au, K n • D 0 K • Semi-leptonic decays e Au+Au s. NN = 130, 200, 62. 4 Ge. V Phys. Rev. Lett. 88, 192303 (2002) Meson D±, D 0 Mass 1869(1865) Ge. V BR D 0 --> K+ - (3. 85 ± 0. 10) % BR D --> e+ +X 17. 2(6. 7) % BR D --> + +X 6. 6 % • D± K • D*± D 0 • Single electron measurements in p+p, d+Au

Single electron Result • Strong modification of the spectral shape in Au+Au is observed at high p. T • Statistics insufficient to quantify centrality dependence • Possibility of different energy loss mechanism? RAA of Integrated CS (2. 5<p. T<5. 0 Ge. V/c). PHENIX PRELIMINARY T. Tabaru for PHENIX, ICPAQGP 05, 2005

Where is suppressed Charm? • Particles boosted by pressure gradients in collision area • Elliptic shape turns into anisotropic flow • Positive flow(v 2) = Collective motion with expanding system = Hint of equilibrium d. N/d =v 0/(2 )+v 2 cos(2 ) / +… • Energy loss of charm implies interaction of charm with the medium • Charm participate in collective motion! Strong indication of “quark level” early equilibrium STAR, nucl-ex/0411007, Theory curves from: Greco, Ko, Rapp: Phys. Lett. B 595 (2004) 202

Charmonium results First J/ ->ee measurement in heavy ion collisions! Phys. Rev. C 69, 014901, 2004 Not enough statistics to make definitive conclusions J/ ->ee R. L. Thews, M. Schroedter, J. Rafelski, Phys Rev C 63, 054905 Plasma Coalescence Model y = 1. 0 Au+Au s. NN=200 Ge. V y = 4. 0 Binary Scaling Stat. Model Andronic et al nucl-th/0303036 Absorption (Nuclear + QGP) + final-state coalescence Absorption (Nuclear + QGP) L. Grandchamp, R. Rapp, Nucl Phys A 709, 415; Phys Lett B 523, 60

• Summary Hard scattering (p. QCD) as new probe for Np. QCD QGP • • Cross section is significantly large at RHIC. Calculable by p. QCD Calibrated by Direct photon • First measurement in high energy heavy ion collisions • Jet modification • High p. T particle yield (fragment of Jet) suppression in central Au+Au collisions • Hot dense medium expanding both transverse and longitudinal direction • • Away side jet is strongly modified Hint of Charm suppression and flow in Au+Au collisions • Needs more statistics to conclude Future Outlook • High Statistics Run 4 Au+Au data is now in analysis • • • 10 times statistics: ~ 1. 5 G events accumulated Thermal radiation on top of p. QCD photon -> Direct emission from QGP 700 J/ ’s expected -> precise measurement of charmonium Complemented by Run 5 Cu+Cu data on system size dependence Hard scattering will be even more powerful probe at LHC

RHIC Collaborations STAR