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The Latest Results from PHOBOS @ RHIC

Explore final-state suppression of high-pT hadrons in Au+Au collisions using d+Au measurements at the Europhysics Conference on High Energy Physics. Study hard-scattered partons, hadronization, and probe modifications in a QCD medium.

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The Latest Results from PHOBOS @ RHIC

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  1. The Latest Results from PHOBOS @ RHIC Evidence of final-state suppression of high-pT hadrons in Au + Au collisions using d + Au measurements Rachid NOUICERUniversity of Illinois at Chicago and Brookhaven National Laboratory for the Collaboration International Europhysics Conference on High Energy Physics July 17, 2003

  2. PHOBOSCollaboration (May 2003) Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts, Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll, Patrick Decowski, Edmundo Garcia, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell, Joshua Hamblen,Adam Harrington,Conor Henderson, David Hofman, Richard Hollis, Roman Holynski, Burt Holzman, Aneta Iordanova,Erik Johnson,Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo,Jang Woo Lee, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Aaron Noell, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Christof Roland, Gunther Roland, Joe Sagerer, Pradeep Sarin, Pawel Sawicki, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Ray Teng, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Bernard Wadsworth, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch, Jinlong Zhang ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER 68 Participants; 8 Institutions; 3 Countries

  3. Hard-scattered Partons are built-in QCD Probes within RHI Collisions A main goal of relativistic heavy ion physics is to investigate high-temperature, high-density QCD, by creating and then studying the highly-excited medium produced in high-energy nuclear collisions. One method of diagnosing a QCD medium is to look for any modifications of the probes due to the medium. t =- few fm/c t = 0 fm/c t = + few fm/c t = + few more fm/c Nucleus Nucleus Central Collision Parton Parton Hard scattering between partons Partons within initial nuclei Scattered partons traverse created hot QCD medium Hadronization “fragmentation” We measure high-pT leading hadrons. The basic thing we look for: If scattered partons lose energy, the number of leading hadrons at high-pT will be depleted (suppressed). This is the main goal of this talk Hadrons “Leading” Hadron Detector

  4. PHOBOS Detector for Au+Au at 200 GeV • 4p Multiplicity Array • Two Mid-rapidity Spectrometers • TOF wall for High-Momentum PID • Triggering: Scintillator Paddles , Zero Degree Calorimeter ZDC TOF 1m Spectrometer RingCounters Octagon ZDC Triggering 135000 Silicon Pad channels

  5. Positive Paddles Negative Paddles NegativeCerenkov PositiveCerenkov Negative ZDC Positive ZDC Au Au PN PP x z Triggering on Collisions & Centrality • Coincidence between Paddle counters at Dt = 0 defines a valid collision • Paddle + ZDC timing reject background Data Data+MC • HIJING +GEANT • Glauber calculation • Model of paddle trigger Peripheral Central

  6. Definition of Ncoll and Npart Peripheral Collision Central Collision

  7. Bulk Tail pT Distribution of Charged ParticlesAu + Au at 200 GeV PHOBOS pT coverage pT = 0.030 – 5 GeV/c AuAu Systematic Errors not shown Can we use this tiny tail to probe the QCD medium ?

  8. Transverse Momentum Distributions vs Centrality Au+Au at 200 GeV Data: PHOBOS, nucl-ex/0302015 Submitted to Phys. Lett. B syst. uncertainties for Ncoll: 10-15 % . Centrality Npart NColl 0-6% 344 ± 12 1050 6-15% 276 ± 9 780 500 15-25% 200 ± 8 300 25-35% 138 ± 6 93 ± 5 175 35-45% 65 ± 4 45-50% 107 Centrality range: <b> from 3 to 10 fm <> from 6 to 3 0.2<yp<1.4

  9. n ~ 3 n ~ 6 Ratio of Au+Au and p+p Spectraat 200 GeV Data: PHOBOS, nucl-ex/0302015 Submitted to Phys. Lett . B Relative to UA1 p+p mid-peripheral Npart = 65 ± 4 • We observe a significant change in the spectral shape between p+p and Au+Au collisions already for mid-peripheral events. • Central Au+Au collisions show a strong violation of (expected) collision scaling at high-pT. central Npart = 344 ± 12 High-pT suppression Is this a signature of the final state “jet quenching“ ?

  10. Is final state “jet quenching“ the only explanation ?Au + Au at 200 GeV Data: PHOBOS, nucl-ex/0302015 Submitted to Phys . Lett. B PHOBOS, nucl-ex/0302015 Expectation for Ncoll-scaling • Particle production scales approximately • with Npart at high-pT • Similar centrality dependence at • pT = 0.5 and 4 GeV/c ! Saturation model : Kharzeev, Levin, McLerran hep -ph/0210332 Expectation for Ncoll-scaling Saturation model prediction Particle Yield/<Npart/2> • Initial state parton saturation works qualitatively too … Rachid Nouicer

  11. Conclusions of part I The observed high-pT suppression suggest two possibilities: • Strong final state suppression at high-pT? • Indication of high density, strongly interacting matter! • Strong initial state suppression persisting to high-pT? • Indication of multipartonic effects in the nuclear wavefunction! We need a simpler system such as d + Au in order to understand a complex system Au + Au RHIC Accelerator response : no problem!

  12. Predictions for d+Au pQCD (final state) Parton Saturation (initial state) Vitev, nucl-th/0302002, Phys.Lett.B in press Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002) Kharzeev, Levin, McLerran, hep-ph/021332 “~30% suppression of high-pT particles” (central vs peripheral) Nuclear Modification Factor RdAu Central Peripheral 16% increase central vs peripheral

  13. T0 T0 This Year: PHOBOS Detector 2003 d+Au at 200 GeV mini-pCal SPECTRIG  DAQ upgrade (x10)  Moved TOF walls back ~ 5 m from interaction point  Installed new spectrometer trigger detector that selects on high pT tracks pCal  Installed new “time-zero” (T0) Cerenkov detectors to provide triggering and on-line vertexing as well as a start time for our TOF walls.  Proton calorimeter on Gold and Deuteron “going” sides for dA run

  14. Centrality Determination in d+Au at 200 GeV HIJING Simulation Centrality cuts (4 bins) 0.2<h<1.4 dN/dh 3.0<|h|<5.4 Counts Multiplicity distribution Pseudorapidity • Glauber Calculation • Hijing 1.383 • Hulthen w.f. • 41mb inelastic NN cross-section • Full GEANT Simulation

  15. Invariant Yields for Charged Hadrons vs pT d+Au at 200 GeV Data: PHOBOS, nucl-ex/0306025 Submitted to Phys. Rev. Lett. • Systematic uncertainties : • tracking efficiency : 5-10 % • malfunctioning channels : 5 % Deuteron direction

  16. Compare to p+p reference… 41mb (same as for Glauber) From Glauber (HIJING 1.383) From UA1, using Pythia to go from |h| < 2.5 to 0.2 < h < 1.4 Nuclear Modification factor (RdAu) vs pT Data: PHOBOS, nucl-ex/0306025 Submitted to Phys. Rev. Lett. central Au+Au • Clear evidence of absence of high-pT suppression in d + Au • The observation and the absence of high-pT suppression in Au+Au and d+Au respectively, can be an indication of creation of highly interacting medium in Au+Au. All syst. uncertainties: 90% C.L.

  17. Nuclear Modification Factor (RdAu) vs Ncoll Data: PHOBOS, nucl-ex/0306025 Submitted to Phys. Rev. Lett. syst. uncertainties: 15-20 %@ 90 C.L. Parton saturation predicts : “ ~30% suppression of high-pT particles” (central vs peripheral) • Data disfavor initial state • interpretation of Au+Au • high-pT suppression

  18. Summary • Clear Evidence ofhigh-pT suppression in more central collisions of Au + Au at 200 GeV • Clear Evidence of absenceofhigh-pT suppression in more central collisions of d + Au at 200 GeV • Slight enhancement at high-pT (pT = 4 GeV/c) of nuclear modification factor of dAu vs Ncoll • The latest news from PHOBOS: • We have compared central Au+Au to central d+Au. • This data strongly disfavors the “initial state” parton saturation interpretation of high-pT hadron suppression. • More to come !

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