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Jets and high p T hadrons at RHIC: recent results from STAR

Jets and high p T hadrons at RHIC: recent results from STAR. Inclusive spectra Single-particle azimuthal asymmetry (“elliptic flow”) Two-particle azimuthal distributions Results from the d+Au run What have we learned at RHIC?. Peter Jacobs Lawrence Berkeley National Laboratory.

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Jets and high p T hadrons at RHIC: recent results from STAR

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  1. Jets and high pT hadrons at RHIC:recent results from STAR • Inclusive spectra • Single-particle azimuthal asymmetry (“elliptic flow”) • Two-particle azimuthal distributions • Results from the d+Au run • What have we learned at RHIC? Peter Jacobs Lawrence Berkeley National Laboratory HIC03

  2. The STAR detector E-M Calorimeter Projection           Chamber Time of    Flight HIC03

  3. Partonic energy loss in dense matter Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,… Multiple soft interactions: Gluon bremsstrahlung Opacity expansion: • Strong dependence of energy loss on gluon density glue: • measure DE color charge density at early hot, dense phase HIC03

  4. jet parton nucleon nucleon Jets at RHIC Find this……….in this p+p jet+jet (STAR@RHIC) Au+Au ??? (STAR@RHIC) HIC03

  5. Binary collision scaling p+p reference Partonic energy loss via leading hadrons Energy loss  softening of fragmentation  suppression of leading hadron yield HIC03

  6. Au+Au and p+p: inclusive charged hadrons nucl-ex/0305015 PhysRevLett 89, 202301 p+p reference spectrum measured at RHIC HIC03

  7. Suppresion of inclusive hadron yield RAA Au+Au relative to p+p RCP Au+Au central/peripheral nucl-ex/0305015 • central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT HIC03

  8. py px Azimuthal anisotropy wrt reaction plane (“Elliptic Flow”) Coordinate space: initial asymmetry Momentum space: final asymmetry multiple collisions (pressure) asymmetry generated early in collision, quenched by expansion  observed asymmetry emphasizes early time Second Fourier coefficient v2: HIC03

  9. jet jet Azimuthal anisotropy at high pT: theory Jet propagation through anisotropic matter (non-central collisions) Snellings; Gyulassy, Vitev and Wang (Phys Rev Lett 2537 (2001) ) Finite v2: high pT hadron correlated with reaction plane from “soft” part of event (pT<2 GeV/c) HIC03

  10. STAR Azimuthal anisotropy at high pT: data Phys Rev Lett 90, 032301 STAR preliminary • pT<2 GeV: detailed agreement with hydrodynamics • pT >4 GeV: finite v2 in qualitative agreement with expectations from partonic energy loss HIC03

  11. trigger Jets and two-particle azimuthal distributions p+p  dijet • trigger: highest pT track, pT>4 GeV/c • Df distribution: 2 GeV/c<pT<pTtrigger • normalize to number of triggers Phys Rev Lett 90, 082302 N.B. shifted horizontally by p/2 relative to previous STAR plots! HIC03

  12. Azimuthal distributions in Au+Au Au+Au peripheral Au+Au central pedestal and flow subtracted Phys Rev Lett 90, 082302 Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au collisions HIC03

  13. ? What might all this mean? Conjecture: core of reaction volume is opaque to jets  surface emission • Consequences: • near-side fragmentation independent of system • suppression of back-to-back jets • suppression of inclusive rates • strong elliptic flow at high pT Compelling picture, but is it right? HIC03

  14. Is suppression an initial or final state effect? Initial state? Final state? partonic energy loss in dense medium generated in collision strong modification of Au wavefunction (gluon saturation at low Bjorken x?) HIC03

  15. RCP nucl-ex/0305015 Inclusive suppression: theory vs. data pQCD-I: Wang, nucl-th/0305010 pQCD-II: Vitev and Gyulassy, PRL 89, 252301 Saturation: KLM, Phys Lett B561, 93 Final state Initial state pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching HIC03

  16. partonic energy loss Is suppression an initial or final state effect? Initial state? Final state? gluon saturation How to discriminate? Turn off final state d+Au collisions HIC03

  17. Inclusive spectra RAB If Au+Au suppression is final state 1.1-1.5 1 If Au+Au suppression is initial state (KLM saturation: 0.75) ~2-4 GeV/c pT High pT hadron pairs broadening? pQCD: no suppression, small broadening due to Cronin effect saturation models: suppression due to mono-jet contribution? 0 suppression? /2  0  (radians) d+Au vs. p+p: Theoretical expectations All effects strongest in central d+Au collisions HIC03

  18. FTPC-Au ZDC-Au ZDC-d Au d d+Au event selection Minimum bias trigger: ZDC-Au (953% of shadronic) Centrality tags: 1. FTPC-Au: charged particle multiplicity in -3.8<h<-2.8 2. ZDC-d: neutron spectator from deuteron HIC03

  19. Centrality tags • ZDC-d neutron spectator tag: • measured: 191% of shadronic • Glauber: 183% of shadronic •  peripheral events FTPC-Au cut selects more central collisions Modeling FTPC-Au multiplicity: convolute Au-participant (Glauber) with UA5 mult. distribution (pbar+p @ 200 GeV, 2.5<h<3.5) min. bias <Nbinary>=7.00.3; central 20% <Nbinary>=12.91.0 HIC03

  20. Inclusive charged particle spectra HIC03

  21. Inclusive yield relative to binary-scaled p+p • d+Au : enhancement • Au+Au: strong suppression • pT=4 GeV/c: • cent/minbias= 1.110.03 • central collisions enhanced wrt minbias Suppression of the inclusive yield in central Au+Au is a final-state effect HIC03

  22. pedestal and flow subtracted Azimuthal distributions Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back correlation in central Au+Au is a final-state effect HIC03

  23. ? X STAR collaboration, nucl-ex/0306024: The strong suppression of the inclusive yield and back-to-back correlations at high pT previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions. HIC03

  24. Have we found the Quark Gluon Plasma at RHIC? We now know that Au+Au collisions generate a medium that • is dense (pQCD theory: many times cold nuclear matter density) • is dissipative • exhibits strong collective behavior This represents significant progress in our understanding of strongly interacting matter We have yet to show that: • dissipation and collective behavior both occur at the partonic stage • the system is deconfined and thermalized • a transition occurs: can we turn the effects off ? Not yet, there is still work to do HIC03 We have developed the tools necessary to complete this program

  25. The STAR Collaboration Brazil: Universidade de Sao Paulo China: IHEP – Beijing IMP - Lanzou IPP – Wuhan USTC SINR – Shanghai Tsinghua University Great Britain: University of Birmingham France: IReS Strasbourg SUBATECH - Nantes Germany: MPI – Munich University of Frankfurt India: IOP - Bhubaneswar VECC - Calcutta Panjab University University of Rajasthan Jammu University IIT - Bombay VECC – Kolcata Poland: Warsaw University of Tech. Russia: MEPHI - Moscow LPP/LHE JINR - Dubna IHEP - Protvino U.S. Laboratories: Argonne Berkeley Brookhaven U.S. Universities: UC Berkeley UC Davis UC Los Angeles Carnegie Mellon Creighton University Indiana University Kent State University Michigan State University City College of New York Ohio State University Penn. State University Purdue University Rice University Texas A&M UT Austin U. of Washington Wayne State University Yale University HIC03

  26. Extra slides HIC03

  27. K0s STAR preliminary L+Lbar STAR preliminary 200 GeV Au+Au: L and K0s Power-law shape except for central L HIC03

  28. Central/peripheral: dependence on particle species STAR preliminary • 1.5<pT<3.5 GeV/c: L scales ~Nbinary, K0s suppressed • pT~ 6 GeV/c:L, K0s, charged hadrons scale similarly • “baryon enhancement”: partonic flow? modified fragmentation? Cronin effect? … HIC03

  29. STAR Preliminary Important complication: non-flow • Non-flow: any two-particle correlation not due to correlation with the reaction plane: jets, resonances, momentum conservation, …. • Borghini, Ollitraut et al: measure non-flow via higher order correlations (cumulants) Non-flow Finite flow at pT~7 GeV/c HIC03

  30. Two-particle azimuthal distributions • near-side and back-to-back peaks seen in minbias and central d+Au • p+p  d+Au minbias  d+Au central: • growth of off-jet pedestal • small, if any, growth of back-to-back width HIC03

  31. Dh < 0.5 Dh > 0.5 Near-side correlations II • Azimuthal correlation function: • Trigger particle pT trig> 4 GeV/c • Associate tracks 2 < pT < pTtrig Phys. Rev. Lett. 90, 032301 (2003) Central Au+Au N.B. Away-side jet contribution subtracted by construction, needs different method… Near-side correlation shows jet-like signal in central Au+Au HIC03

  32. ||<0.5 - ||>0.5 (scaled) Au+Au 0<||<1.4 p+p Reality check: charge-sign dependence • Compare same-sign (++, --) and opposite-sign (+-) pairs • Known jet physics: charge ordering in fragmentation DELPHI, PL B407, 174 (1997) STAR preliminary Opposite/same correlation strength similar in Au+Au, p+p, JETSET  pT~3-4 GeV are jet fragments HIC03

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