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Recent Results from PHOBOS experiment at RHIC

This paper discusses the recent results from the PHOBOS experiment at RHIC, including matter phases in heavy ion reactions, experimental characterization of collisions, global observables, scaling behaviors, and limiting fragmentation.

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Recent Results from PHOBOS experiment at RHIC

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  1. Recent Results from PHOBOS experiment at RHIC VI Simposio Latinoamericano de Física Nuclear Iguazú, Argentina Octubre 2005 Edmundo García, UIC

  2. PHOBOS Collaboration Burak Alver, Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

  3. Outline • Matter phases in Heavy Ion Reactions • Experimental characterization of RHIC Collisions • Global observables • Scaling behaviors • Limiting fragmentation

  4. Description of a heavy-ion collision M. Gyulassy, L. McLerran Nucl. Phys. A 750 (2005)30

  5. Color Glass Condensate (CGC) ZEUS data for gluon distribution In partons Gluon density grows McLerran, hep-ph/0311028 low x -> as<<1 At a given Q(x,A) -> gluon density saturation -> saturation models for heavy ions

  6. centrality Saturation Models Au+Au PHOBOS dN/dhch / (<Npart>/2) PHOBOS 130 GeV <Npart> • Saturation Model (KLN) Phys.Lett.B523 79 (2001)

  7. Saturation Models d + Au • Saturation Model (KLN) hep-ph/0212316 PHOBOS PHOBOS CGC not valid in the Au fragmentation region Assume dN/dh=NpartAu dNpp/dh in the Au fragmentation region

  8. Quark Gluon Matter/Plasma QGM energy/density particles Thermalization

  9. Estimation of the energy density reached at RHIC Bearden et al., BRAHMS Collaboration, Nucl. Phys. A757 (2005) 1-27 Nucleon transparency at RHIC larger than for AGS and SPS. However the energy density achieved ~ 5 GeV/fm at mid rapidity: high energy density system formed during collisions

  10. Baryon free medium created at RHIC

  11. Evidence of thermalization STAR, Nucl. Phys. A 757 (2005) 102 BRAHMS, Nucl. Phys. A757 (2005) 1-27 Evidence of successful description of data by hydrodynamics, high energy density system is to certain degree thermalized. From PHOBOS particle rations T~120 MeV, mB~ 27.2 MeV

  12. Elliptic Flow: v2 Another characteristic of the high energy density matter formed at RHIC: Flow Initial spatial anisotropy z z Reaction plane (YR) y f x y y x x (defines YR) dN/d(f -YR ) = N0 (1 + 2v1cos (f-YR) + 2v2cos (2(f-YR)) + ... ) Final momentum anisotropy py px

  13. Strongly interactive system I PHOBOS PHOBOS centrality Evidence of large elliptic component of flow found, the high density system interacts more like a fluid than like a gas.

  14. Strongly interacting system II PHOBOS dAu 200 GeV 41mb (same as for Glauber) PHOBOS data From Glauber (HIJING 1.383) From UA1, using Pythia to go from |h| < 2.5 to 0.2 < h < 1.4 Suppression of high-pT particles: Medium is highly interactive and parton density is high. PRL 91, 072302 (2003) AuAu 200 GeV

  15. Strongly interacting system II cont. Suppression of high pt particles

  16. preliminary pT(trig) pT(assoc) > 2 GeV/c New on dijet supression: Emergence of dijets with increasing pT(trig) Au+Au, 0-5% •  correlations (not background subtracted) • Hint of narrow back-to-back peak for higher pT(trig) • Jets do exist but they are highly suppressed

  17. RHIC • High energy density medium • Baryon Free medium • Evidence of Thermalization • Medium created at RHIC strongly interacts with high pt partons. What other global observables define RHIC collisions so far as measured by PHOBOS? • Scaling behaviors • Limiting fragmentation

  18. L~N1/3 Wang,Gulassy Phys. Rev. Lett 86(2001) Kharzeev,Nardi Phys. Lett. B 501(2001) “Participant” Scaling Npart/2 = # of participating pairs of nucleons: = 1 Npart/2 = # of participating nucleons: N “Collision” Scaling Ncoll = # of NN collisions: = 1 Ncoll= # of NN collisions: ~N4/3

  19. Au+Au centrality dependence allows only about 10% Ncoll scaling at mid rapidity 20 dN/dh/(Npart/2) 200 GeV - |h|<1 Binary Collision(Ncoll) Scaling 10 PHOBOS p+p Au+Au Participant (Npart) Scaling 0 400 200 0 <Npart> peripheral central

  20. All RHIC energies show a similar Npart dependence p + p Data is normalized by p+p value for each energy. Binary collision scaling 200 GeV 130 GeV 19.6 GeV preliminary Au+Au PHOBOS Participant (Npart)scaling peripheral central

  21. Npart scaling for asymmetric collisions: arXiv:nucl-ex/0403033

  22. Scaling Laws dN/d in Cu+Cu vs. Au+Au PHOBOS 62.4 GeV 200 GeV Cu+Cu Preliminary 3-6%, Npart = 96 Cu+Cu Preliminary 3-6%, Npart = 100 Au+Au 35-40%,Npart = 98 Au+Au 35-40%, Npart = 99 Cu+Cu Preliminary 15-25%, Npart = 61 Cu+Cu Preliminary 15-25%, Npart = 60 Au+Au 45-55%, Npart = 56 Au+Au 45-50%,Npart = 62

  23. Cu+Cu preliminary Au+Au Scaling Laws Yields vs. Npart, 200 GeV PHOBOS Au+Au: PRL 94, 082304 (2005), PLB 578, 297 (2004)

  24. Limiting Fragmentation PHOBOS Au + Au 0 – 6 % central UA5, Z.Phys.C33, 1 (1986) dN/dh¢ selected systematic errors 35 – 40 % central PRL 91, 052303 (2003)

  25. Rest frame of A Rest frame of p or d

  26. Elliptic flow: PHOBOS PRL 91, 052303 (2003)

  27. Final Notes RHIC collisions • Produce a high energy density medium • Baryon Free medium • Evidence of thermalization • Medium created at RHIC strongly interacts with high pt partons. Global observables (not clearly understood yet) that define RHIC so far: • <Npart> scaling behaviors • Limiting fragmentation … • LHC

  28. Backup Transparencies

  29. PHOBOS Experiment

  30. 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

  31. Energy per Unit Volume Detail Number of Particles Produced at y=0 <E> ~ 0.7 GeV dNch/dh Therefore total energy released in |h| < 1 is ~2000GeV Energy of Collision “Relevant” Initial Volume ~ R2 ( 1 fm)  2 Initially released Energy per Unit Volume  5 GeV/fm3 Note: Energy Density inside Proton ≈ 0.5 GeV/fm3 Data from: PRL 85, 3100 (2000); PRL 88, 22302 (2002); PRL 91, 052303 (2003); arXiv:nucl-ex/0405027

  32. mT = pT2+mh2 Strongly interacting system III  T= 229 MeV for (++-) 293 MeV for (K++ K-) 392 MeV for (p + pbar) Au+Au at sNN = 200 GeV No enhancement in low-pT yields for pions is observed flattening of (p+pbar) spectra down to very low pT, consistent with strong radial flow of the systm:

  33. Baryon puzzle at RHIC

  34. PHOBOS 200 GeV Statistical errors only 0-40% centrality PHOBOS 200 GeV h± PHOBOS 200 GeV h± Statistical errors only preliminary Au+Au Cu+Cu preliminary Scaling Laws Elliptic Flow in Cu+Cu vs Au+Au

  35. Total charged multiplicity scaling with Npart PHOBOS: nucl-ex/0301017 Open symbols are UA5 data at 200 GeV and results from an interpolation at lower energies Shaded band is uncertainty on extrapolation procedure Errors include contributions from Nch and Npart scaling

  36. Standard Eccentricity Participant Eccentricity x Nucleus 1 Nucleus 1 y Nucleus 2 y Nucleus 2 b b x Participant Region Participant Region Au+Au Au+Au Cu+Cu Scaling Laws Eccentricity Calculation Au+Au Au+Au Cu+Cu

  37. Standard Eccentricity Participant Eccentricity PHOBOS 200 GeV PHOBOS 200 GeV Cu+Cu preliminary Cu+Cu preliminary Au+Au Au+Au Npart scalling of Flow “Participant Eccentricity” allows v2 scaling from Cu+Cu to Au+Au

  38. Participant Eccentricity Au+Au Cu+Cu <dN/dy> / <S> scaling Standard Eccentricity Cu+Cu Au+Au <dN/dy> / <S> scaling: STAR, PRC 66 034904 (2002) Voloshin, Poskanzer, PLB 474 27 (2000) Heiselberg, Levy, PRC 59 2716, (1999)

  39. 8 < pT(trig) < 15 GeV/c Emergence of dijets w/ increasing pT(assoc) •  correlations (not background subtracted) pT(assoc) > 2 GeV/c pT(assoc) > 3 GeV/c pT(assoc) > 4 GeV/c pT(assoc) > 5 GeV/c pT(assoc) > 6 GeV/c pT(assoc) > 7 GeV/c pT(assoc) > 8 GeV/c • Narrow peak emerges cleanly above vanishing background

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