A bstract. O verthepastyearsPHO BO S hascontinued to analyzethelargedatasets obtained from the (cid:12)rst (cid:12)ve runs of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven NationalLaboratory.The two m ain analysisstream shave been pursued. The (cid:12)rst one aim s to obtain a broad and system atic survey ofglobalproperties of particle production in heavy ion collisions. The second class includes the study of (cid:13)uctuations and correlations in particle production. Both type ofstudies have been perform ed for a variety of the collision system s,

for the PHOBOS Collaboration: B.Alver4, B.B.Back1, M.D.Baker2, M.Ballintijn4, D.S.Barton2, R.R.Betts 6, A.A.Bickley7, R.Bindel 7, W.Busza 4, A.Carroll2, Z.Chai2, V.Chetluru6, M.P.Decowski 4, E.García6, T.Gburek3, N.George2, K.Gulbrandsen 4, C.Halliwell6, J.Hamblen8, I.Harnarine6, M.Hauer2, C.Henderson 4, D.J.Hofman6, R.S.Hollis6, R.Hołyński 3, B.Holzman2, A.Iordanova6, E.Johnson 8, J.L.Kane4, N.Khan8, P.Kulinich4, C.M.Kuo5, W.Li4, W.T.Lin5, C.Loizides4, S.Manly8, A.C.Mignerey7, R.Nouicer 2, A.Olszewski 3, R.Pak2, C.Reed4, E.Richardson 7, C.Roland4, G.Roland4, J.Sagerer 6, H.Seals2, I.Sedykh2, C.E.Smith6, M.A.Stankiewicz 2, P.Steinberg 2, G.S.F.Stephans 4, A.Sukhanov 2, A.Szostak 2, M.B.Tonjes7, A.Trzupek 3, C.Vale4, G.J.van Nieuwenhuizen 4, S.S.Vaurynovich 4, R.Verdier 4, G.I.Veres4, P.Walters8, E.Wenger 4, D.Willhelm7, F.L.H.Wolfs8, B.Wosiek 3, K.Woźniak3, S.Wyngaardt 2, B.Wysłouch 4

This Letter presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v(2) in Au+Au collisions at square root(s(NN))=200 GeV as a function of collision centrality. The relative nonstatistical fluctuations of the v(2) parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (nonflow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.

United States. Department of Energy (Grants DE-AC02-98CH10886, DE-FG02-93ER40802, DE-FG02- 94ER40818, DE-FG02-94ER40865, DE-FG02-99ER41099, and DE-AC02-06CH11357)

United States Department of Energy (grants DE-AC02-98CH10886, DE-FG02-93ER40802, DEFG02- 94ER40818, DE-FG02-94ER40865, DE-FG02- 99ER41099, and DE-AC02-06CH11357)

B. Alver, B. B. Back, M.D. Baker, M. Ballintijn, D. S. Barton, R. R. Betts, A.A. Bickley, R. Bindel, W. Busza, A. Carroll, Z. Chai, V. Chetluru, M. P. Decowski, E. Garcı́a, T. Gburek, N. George, K. Gulbrandsen, C. Halliwell, J. Hamblen, M. Hauer, C. Henderson, D. J. Hofman, R. S. Hollis, R. Hołyński, B. Holzman, A. Iordanova, E. Johnson, J. L. Kane, N. Khan, P. Kulinich, C.M. Kuo, W. Li, W. T. Lin, C. Loizides, S. Manly, A. C. Mignerey, R. Nouicer, A. Olszewski, R. Pak, C. Reed, C. Roland, G. Roland, J. Sagerer, H. Seals, I. Sedykh, C. E. Smith, M.A. Stankiewicz, P. Steinberg, G. S. F. Stephans, A. Sukhanov, M.B. Tonjes, A. Trzupek, C. Vale, G. J. van Nieuwenhuizen, S. S. Vaurynovich, R. Verdier, G. I. Veres, P. Walters, E. Wenger, F. L. H. Wolfs, B. Wosiek, K. Woźniak, and B. Wysłouch Argonne National Laboratory, Argonne, Illinois 60439-4843, USA Brookhaven National Laboratory, Upton, New York 11973-5000, USA Institute of Nuclear Physics PAN, Kraków, Poland Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA National Central University, Chung-Li, Taiwan University of Illinois at Chicago, Chicago, Illinois 60607-7059, USA University of Maryland, College Park, Maryland 20742, USA University of Rochester, Rochester, New York 14627, USA (Received 16 March 2009; revised manuscript received 24 December 2009; published 11 February 2010)

This paper presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200Gev, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (Delta eta < 2). Assuming that non-flow correlations are of the order that is observed in p+p collisions for long range correlations (Delta eta > 2), relative elliptic flow fluctuations of approximately 30-40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long range non-flow correlations in Au+Au collisions would have to be more than an order of magnitude stronger compared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.

A measurement of two-particle correlations with a high transverse momentum trigger particle (p(T)(trig) > 2.5 GeV/c) is presented for Au+Au collisions at square root(s(NN)) = 200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < Delta eta < 2). A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all Delta eta. As in p+p collisions, the near side is characterized by a peak of correlated partners at small angle relative to the trigger particle. However, in central Au+Au collisions an additional correlation extended in Delta eta and known as the "ridge" is found to reach at least |Delta eta| approximately = 4. The ridge yield is largely independent of Delta eta over the measured range, and it decreases towards more peripheral collisions. For the chosen (p(T)(trig) cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

A measurement of two-particle correlations with a high transverse momentum trigger particle (p trig T > 2.5 GeV/c) is presented for Au+Au collisions at √ s N N =200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < ∆η < 2).A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all ∆η.As in p+p collisions, the near-side is characterized by a peak of correlated partners at small angle relative to the trigger particle.However, in central Au+Au collisions an additional correlation extended in ∆η and known as the 'ridge' is found to reach at least |∆η| ≈ 4. The ridge yield is largely independent of ∆η over the measured range, and it decreases towards more peripheral collisions.For the chosen p trig T cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

Recently PHOBOS has focused on the study of fluctuations and correlations in particle production in heavy-ion collisions at the highest energies delivered by the Relativistic Heavy Ion Collider (RHIC). In this report, we present results on event-by-event elliptic flow fluctuations in Au + Au collisions at root s(NN) = 200 GeV. A data-driven method was used to estimate the dominant contribution from non-flow correlations. Over the broad range of collision centralities, the observed large elliptic flow fluctuations are in agreement with the fluctuations in the initial source eccentricity.

A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (p T >2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at \(\sqrt{s_{NN}}=200\) GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral.

We present first results on event-by-event elliptic flow fluctuations in Au+Au collisions at √ sNN = 200 GeV obtained with the PHOBOS detector. Over the measured range in centrality, large relative fluctuations of 40–50% are found. The elliptic flow fluctuations are well described as being proportional to fluctuations in the shape of the initial collision region, as estimated event-by-event with the participant eccentricity using Glauber Monte Carlo. PACS numbers: 25.75.-q Submitted to: J. Phys. G: Nucl. Phys.

high-pT trigger particle, elliptic flow fluctuations and two particle correlations. ∗the PHOBOS Collaboration: B.Alver4, B.B.Back1, M.D.Baker2, M.Ballintijn4, D.S.Barton2, R.R.Betts6, A.A.Bickley7, R.Bindel7, W.Busza4, A.Carroll2, Z.Chai2, V.Chetluru6, M.P.Decowski4, E.Garcia6, T.Gburek3, N.George2, K.Gulbrandsen4, C.Halliwell6, J.Hamblen8, I.Harnarine6, M.Hauer2, C.Henderson4, D.J.Hofman6, R.S.Hollis6, R.HoAlynski3, B.Holzman2, A.Iordanova6, E.Johnson8, J.L.Kane4, N.Khan8, P.Kulinich4, C.M.Kuo5, W.Li4, W.T.Lin5, C.Loizides4, S.Manly8, A.C.Mignerey7, R.Nouicer2, A.Olszewski3, R.Pak2, C.Reed4, E.Richardson7, C.Roland4, G.Roland4, J.Sagerer6, H.Seals2, I.Sedykh2, C.E.Smith6, M.A.Stankiewicz2, P.Steinberg2, G.S.F.Stephans4, A.Sukhanov2, A.Szostak2, M.B.Tonjes7, A.Trzupek3, C.Vale4, G.J.van Nieuwenhuizen4, S.S.Vaurynovich4, R.Verdier4, G.I.Veres4, P.Walters8, E.Wenger4, D.Willhelm7, F.L.H.Wolfs8, B.Wosiek3, K.Woźniak3, S.Wyngaardt2, B.WysAlouch4 1 Argonne National Laboratory, Argonne, IL 60439-4843, USA 2 Brookhaven National Laboratory, Upton, NY 11973-5000, USA 3 Institute of Nuclear Physics PAN, Krakow, Poland 4 Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA 5 National Central University, Chung-Li, Taiwan 6 University of Illinois at Chicago, Chicago, IL 60607-7059, USA 7 University of Maryland, College Park, MD 20742, USA 8 University of Rochester, Rochester, NY 14627, USA

We introduce an analysis method to measure elliptic flow (v 2 ) fluctuations using the PHOBOS detector for Au+Au collisions at √ s NN = 200 GeV.In this method, v 2 is determined event-by-event by a maximum likelihood fit.The non-statistical fluctuations are determined by unfolding the contribution of statistical fluctuations and detector effects using Monte Carlo simulations(MC).Application of this method to measure dynamical fluctuations embedded in special MC are presented.It is shown that the input fluctuations are reconstructed successfully for v 2 ≥ 0.03.

Transverse momentum spectra of pions, kaons, protons, and antiprotons from Au+Au collisions at root s(NN) = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon spectrometer, and time-of-flight measurement. These methods cover the transverse momentum ranges 0.03-0.2, 0.2-1.0, and 0.5-3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The p(T) region in which the proton to pion ratio reaches unity in central Au+Au collisions at root s(NN) = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse mass, the spectra of various species exhibit a significant deviation from transverse mass scaling. The observed particle yields at very low p(T) are comparable to extrapolations from higher p(T) for kaons, protons and antiprotons. By comparing our results to Au+Au collisions at root s(NN) = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.

for the PHOBOS Collaboration B.Alver, B.B.Back, M.D.Baker, M.Ballintijn, D.S.Barton, R.R.Betts, A.A.Bickley, R.Bindel, W.Busza, A.Carroll, Z.Chai, V.Chetluru, M.P.Decowski, E.Garćıa, N.George, T.Gburek, K.Gulbrandsen, C.Halliwell, J.Hamblen, I.Harnarine, M.Hauer, C.Henderson, D.J.Hofman, R.S.Hollis, R.Ho lyński, B.Holzman, A.Iordanova, E.Johnson, J.L.Kane, N.Khan, P.Kulinich, C.M.Kuo, W.Li, W.T.Lin, C.Loizides, S.Manly, A.C.Mignerey, R.Nouicer, A.Olszewski, R.Pak, C.Reed, E.Richardson, C.Roland, G.Roland, J.Sagerer, H.Seals, I.Sedykh, C.E.Smith, M.A.Stankiewicz, P.Steinberg, G.S.F.Stephans, A.Sukhanov, A.Szostak, M.B.Tonjes, A.Trzupek, C.Vale, G.J.van Nieuwenhuizen, S.S.Vaurynovich, R.Verdier, G.I.Veres, P.Walters, E.Wenger, D.Willhelm, F.L.H.Wolfs, B.Wosiek, K.Woźniak, S.Wyngaardt, B.Wys louch 1 Argonne National Laboratory, Argonne, IL 60439-4843, USA 2 Brookhaven National Laboratory, Upton, NY 11973-5000, USA 3 Institute of Nuclear Physics PAN, Kraków, Poland 4 Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA 5 National Central University, Chung-Li, Taiwan 6 University of Illinois at Chicago, Chicago, IL 60607-7059, USA 7 University of Maryland, College Park, MD 20742, USA 8 University of Rochester, Rochester, NY 14627, USA

A brief overview of the current results and conclusions from the PHOBOS experiment at the Relativistic Heavy Ion Collider (RHIC) is given. No evidence is found for non-monotonic behavior of observables measured by PHOBOS in the RHIC energy region. Convincing evidence is found that we have created a state of matter with high energy-density, that is nearly net-baryon free and is strongly interacting. The data are found to exhibit "simple" scaling behaviors, which include extended longitudinal scaling and scaling with the number of participating nucleons. The Au+Au collision charged particle data also exhibit a remarkable factorization of collision energy and geometry.

The PHOBOS collaboration has carried out a systematic study of charged particle multiplicities in Cu+Cu and Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory. A unique feature of the PHOBOS detector is its ability to measure charged particles over a very wide angular range from 0.5 to 179.5 deg. corresponding to |eta|<5.4. The general features of the charged particle multiplicity distributions as a function of pseudo-rapidity, collision energy and centrality, as well as system size, are discussed.

Forward-backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity (eta) are studied in order to gain insight into the underlying correlation structure of particle production in Au+Au collisions. The PHOBOS detector is used to measure integrated multiplicities in bins defined within eta<3, centered at eta and covering an interval Delta-eta. The variance (sigma^2_C) of a suitably defined forward-backward asymmetry variable is calculated as a function of eta, Delta-eta, and centrality. It is found to be sensitive to short range correlations, and the concept of ``clustering'' is used to interpret comparisons to phenomenological models.