aa r X i v : . [ nu c l - e x ] A p r Latest results from the PHOBOS experiment
Barbara Wosiek for the PHOBOS Collaboration ‡ Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, PolandE-mail: [email protected]
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.Garc´ıa , T.Gburek , N.George , K.Gulbrandsen , C.Halliwell , J.Hamblen ,I.Harnarine , M.Hauer , C.Henderson , D.J.Hofman , R.S.Hollis , R.Ho ly´nski ,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.Wo´zniak , S.Wyngaardt , B.Wys louch Argonne National Laboratory, Argonne, IL 60439-4843, USA Brookhaven National Laboratory, Upton, NY 11973-5000, USA Institute of Nuclear Physics PAN, Krak´ow, Poland Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA National Central University, Chung-Li, Taiwan University of Illinois at Chicago, Chicago, IL 60607-7059, USA University of Maryland, College Park, MD 20742, USA University of Rochester, Rochester, NY 14627, USA
Abstract.
Over the past years PHOBOS has continued to analyze the large datasetsobtained from the first five runs of the Relativistic Heavy Ion Collider (RHIC) atBrookhaven National Laboratory. The two main analysis streams have been pursued.The first one aims to obtain a broad and systematic survey of global properties ofparticle production in heavy ion collisions. The second class includes the study offluctuations and correlations in particle production. Both type of studies have beenperformed for a variety of the collision systems, covering a wide range in collisionenergy and centrality. The uniquely large angular coverage of the PHOBOS detectorand its ability to measure charged particles down to very low transverse momentumis exploited. The latest physics results from PHOBOS, as presented at Quark Matter2008 Conference, are contained in this report. ‡ For the full list of PHOBOS authors and acknowledgments, see appendix “Collaborations” atest results from the PHOBOS experiment PACS numbers: 25.75.-q
1. Introduction
PHOBOS [1] is a heavy ion experiment at the BNL RHIC collider, known for itscapability of measuring charged particles over a broad angular acceptance, by farthe largest of all RHIC experiments. With the PHOBOS multiplicity array chargedparticles are measured in pseudorapidity range | η | < . π in the azimuth.In addition, the unique design of the two-arm spectrometer allows for extending theparticle momentum measurements to the lowest limit reachable at RHIC. With thisdetector we have collected data on p+p, d+Au, Cu+Cu and Au+Au collisions duringthe RHIC 2000-2005 runs, covering a wide range of collision energy and centrality. Thiscomprehensive data set allowed for a systematic studies of the overall features of particleproduction mechanisms in nuclear and elementary collisions, which constituted the mainpart of the baseline PHOBOS physics program. While this part is now nearly completed,our current effort is mainly focused on the study of fluctuations and correlations inparticle production, the study of which can provide deeper insight into different stagesof the system evolution.In this paper we briefly summarize the results on global properties of chargedparticle production, including antiparticle to particle ratios and particle yields atvery low transverse momentum ( p T ). The emphasis is put on our recent resultsobtained from fluctuation and correlation studies. The dynamical fluctuations of theelliptic flow, corrected for the non-flow effects are presented for Au+Au collisions at √ s NN = 200 GeV. The same high-statistics data set is used to investigate structures inthe near- and away-side correlations with respect to high- p T trigger particle over a broadrange in ∆ η . Finally, the results from a systematic study of the two-particle angularcorrelations in p+p, Cu+Cu and Au+Au collisions at √ s NN = 200 GeV are shown.
2. System-size dependence of particle production
Recently PHOBOS has completed a systematic study of the bulk properties of theproduced particles, like total multiplicity, dN ch /dη and dN ch /dp T distributions, particlecomposition and collective flow effects, in Cu+Cu and Au+Au collisions as a function ofthe collision energy and centrality. The results, in comparison to d+Au and elementaryp+p collisions, show that particle production in heavy ion collisions can be describedin terms of simple scaling rules. Number of participant ( N part ) scaling is observed forthe total particle multiplicity [2], net-proton yields [3] and also, as will be shown later,the low- p T particle yields. Energy and centrality dependencies of mid-rapidity yieldsfactorize over an extended range of transverse momenta [4, 5]. Particle yields anddirected ( v ) and elliptic flow ( v ) signals measured over a wide range of high to almostcentral pseudorapidities show energy-independence when viewed in the rest frame of atest results from the PHOBOS experiment part N part N
330 0 50 100 part N A n t i pa r t i c l e t o pa r t i c l e r a t i o s /pp + /K - K + p / - p Cu+Cu 62.4 GeVCu+Cu 200 GeVAu+Au 200 GeVd+Au 200 GeVp+p 200 GeVCu+Cu 62.4 GeVCu+Cu 200 GeVAu+Au 200 GeVd+Au 200 GeVp+p 200 GeV
Figure 1:
Antiparticle to particle ratios for pions, kaons and protons as a function of collisioncentrality for different collision systems, measured at mid-rapidity region of 0 . < η < . It is also observed that N part unifies the measurements of the nuclear modificationfactors in Cu+Cu and Au+Au collisions [7]. A consistent and unified description of theelliptic flow measured in Cu+Cu and Au+Au collisions can be obtained after scaling v with the participant eccentricity, ǫ part [8].The above observations show that the collision geometry has a major impact onthe dynamical evolution of the system. They also provide a tool for extrapolating RHICdata to the LHC energy regime, which is of particular importance since to date no theoryor model can consistently explain the scaling rules controlling the particle productionin heavy ion collisions.
3. Particle production at very low transverse momenta
The PHOBOS spectrometer design provides a unique capability of studying particleproduction at the lowest transverse momenta accessible at RHIC. The new results onlow- p T yields for pions, kaons and protons measured in a high-statistics sample Au+Aucollisions at √ s NN = 200 GeV have been shown at this Conference [9]. Figure 2 showsthe invariant low- p T yields measured for 6% of the most central Au+Au collisions incomparison to the PHENIX data at higher p T [10]. The low- p T yields agree withthe extrapolations of Blast-Wave and Bose-Einstein fits to the PHENIX results. Aflattening of the shape of the p T spectra is observed, stronger for heavier particles,which is consistent with a rapid transverse expansion of the system. Invariant yields,integrated over the low- p T range (0 . − .
053 GeV/c for pions, 0 . − .
128 GeV/cfor kaons and 0 . − .
206 GeV/c for protons and antiprotons) and normalized per atest results from the PHOBOS experiment N part , are shown as a function of N part for Au+Au collisions at 200 and 62.4 GeV [3]in Figure 3. One can see that, within the errors, the invariant low- p T yields scale with N part . For more details see Ref.[9]. [GeV/c] T p -1
10 1 ] / G e V [ c T N / d y dp d - ) T p p ( ) - p + + p ( ) - +K + (K )p(p+ PHOBOSPHENIX p K p
Au+Au 200 GeV, 0-6%
PHOBOS Preliminary
Blast-Wave fitBose-Einstein fit
Figure 2:
Identified particle p T spectranear mid-rapidity in Au+Au collisions at √ s NN = 200 GeV. part N0 100 200 300 400 ] / G e V [ c æ T N / d y dp d - ) T p p ( Æ p a r t N -3 -2 -1 Au+Au 200 GeV
Au+Au 62.4 GeV - p + + p - + K + K p + p - p + + p - + K + K p + p PHOBOS Preliminary
Figure 3:
The integrated low- p T yields forAu+Au collisions at 200 GeV (solid symbols)and 62.4 GeV (open symbols) as a function ofcentrality.
4. Elliptic flow fluctuations
The measured magnitude of the elliptic flow signal, v , should reflect the initial spatialeccentricity of the overlap region of the colliding nuclei. As PHOBOS has shown[8], the participant eccentricity, ǫ part , which takes into account the fluctuations in theparticipant nucleon positions in the overlap region, is the relevant eccentricity whichdrives the observed azimuthal anisotropy. We have used the Monte Carlo Glauber(MCG) approach to study the robustness of the participant eccentricity and also tocalculate the higher order cumulants of ǫ part quantifying the event-by-event fluctuationsof the initial source eccentricity [11]. It was shown that the participant eccentricity isa robust quantity, insensitive to the choice of MCG parameters and model assumptionsand that it strongly fluctuates with σ ( ǫ part ) / h ǫ part i varying from about 35% for peripheralup to 50% for most central Au+Au collisions.Comparison of the v measurements to hydrodynamic model predictions [12]indicate that the matter created in heavy ion collisions at RHIC has propertiesresembling those of a perfect fluid [13]. In hydrodynamic models, the fluctuations inthe shape of the initial collision geometry should be reflected in the fluctuations of v .PHOBOS has measured large (of the order of 40 − σ ( v ) / h v i for Au+Au collisions at √ s NN = 200 GeV [14]. The maindifficulty in this measurement is to disentangle dynamical fluctuations of v and non-flow correlations due, for example, to resonance decays or jet production. PHOBOS hasdeveloped a new method to extract the non-flow component from the data. The method atest results from the PHOBOS experiment η , thanks to the large acceptance of thePHOBOS detector. This coefficient, v ( η , η ), contains contributions from genuine flowcorrelations, v ( η ) × v ( η ), and non-flow correlations, δ ( η , η ). At ∆ η >
2, we assumethat the non-flow component is small. In fact a small non-flow effect at ∆ η > v ( η ) × v ( η ), and then estimate the non-flow contribution bysubtracting flow component from v ( η , η ). The contribution from non-flow correlationsto σ ( v ) is q h δ i / v fluctuations andthe non-flow contribution to these fluctuations. The non-flow contribution extractedfrom the data is large, of the order of 25 − part N R e l a t i ve F l u c t u a t i on s non-flow ¯ flow æ v Æ )/ (v s non-flow æ v Æ )/ (v s PHOBOS Preliminary
Figure 4:
Total measured relative flow fluc-tuations (circles) and the contribution fromnon-flow correlations (squares) for Au+Au col-lisions at √ s NN = 200 GeV. Bands show 90 %C.L. systematic errors. part N R e l a t i ve F l u c t u a t i on s MCG æ part ˛Æ )/ part ˛ ( s CGC æ part ˛Æ / part ) ˛ ( s flow æ v Æ )/ (v s PHOBOS Preliminary
Figure 5:
Relative flow fluctuations, correctedfor the non-flow effects, compared to therelative fluctuations of the initial eccentricitycalculated from MC Glauber and CGCmodels.
5. High- p T triggered two-particle correlations Measurements of correlations with respect to the high- p T trigger particle allow us tostudy the medium response to energetic partons produced in early hard scatteringprocesses and then propagated through the dense medium created in heavy ion collisions.Here we report the first measurements of the ∆ η and ∆ φ correlations between the atest results from the PHOBOS experiment p T > . | η | <
3) and azimuthal angle (∆ φ = 2 π ) of the Octagon allowing forthe study of both short- and long-range correlations in ∆ η . A particular emphasis isput on testing the presence of long-range correlations at near- and away-sides over theuniquely broad acceptance in ∆ η ( − < ∆ η < √ s NN = 200 GeV and compared to the correlated particle production in p+p eventssimulated with PYTHIA. The two dimensional correlated yield, N trig d N ch d ∆ ηd ∆ φ , in centralAu+Au collisions exhibits a much broader away-side peak in ∆ φ as compared to p+pcorrelations. The near-side yield in central Au+Au collisions shows a clear jet-likepeak near ∆ φ ≈ η range(the so called ridge structure). In Figure 6 the correlated near-side yield (integratedover | ∆ φ | <
1) is plotted as a function of ∆ η for 10% of the most central Au+Aucollisions. A prominent ridge effect is clearly visible up to ∆ η = − − < ∆ η < −
2, is shown in Figure 7. One can see that the ridge effectweakens with decreasing collision centrality and almost completely disappears for N part less than about 80. For more details of this analysis see Ref. [19]. hD -4 -2 0 2 hD d c h d N t r i g N Au+Au 0-10%PYTHIA v6.325Momentum Kick Model
PHOBOS preliminary
Figure 6:
The correlated near-side yield for0-10% central Au+Au collisions compared top+p PYTHIA simulations (dashed line) andmomentum kick model predictions (solid line). part N æhD d c h d N t r i g N Æ PHOBOS preliminary <-2 hD |<1 -4< fD | Figure 7:
The near-side yield averaged over − < ∆ η < − N part .
6. Two-particle angular correlations
The study of the correlations between the final-state particles should provide additionalinformation on the underlying mechanism of particle production at freeze-out. PHOBOS atest results from the PHOBOS experiment η, ∆ φ ) inelementary p+p collisions [20] as well as Cu+Cu collisions [21]. Recently these studieshave been also carried out for Au+Au collisions at √ s NN = 200 GeV. The broadcoverage of the PHOBOS Octagon detector is fully utilized in these studies. Themultiplicity-independent two-particle correlation function, R (∆ η, ∆ φ ), is calculatedusing the method detailed in [20]. We have concentrated on studying the short-range correlations by projecting the two-dimensional correlation function into a one-dimensional correlation function R (∆ η ). R (∆ η ) exhibits clear short-range correlationswhich can be described using a simple cluster model. The one-dimensional correlationfunction is fitted to a parameterization derived in an independent cluster model [22]in order to extract the effective cluster size, K eff . Figure 8 shows the effective clustersize as a function of N part for √ s NN = 200 GeV Cu+Cu and Au+Au collisions. Forboth collision systems the cluster size decreases with increasing centrality. Furthermore,for the same N part , clusters in Au+Au collisions are larger than in the Cu+Cu system.Interestingly, this dependence on the size of the colliding nuclei disappears after plottingthe same data as a function of fractional cross-section as shown in Figure 9. Moredifferential studies of the effective size of near- and away-side clusters have also beenperformed. It is observed that away-side clusters are smaller and decrease more rapidlywith increasing centrality than near-side clusters. This observation can be qualitativelyexplained by assuming that near-side clusters are preferentially produced close to thesurface of the emission zone, while away-side clusters seem to propagate through themedium. For more details on the two-particle correlation study see [23]. part N e ff K PHOBOS preliminary
Cu+Cu 200 GeVAu+Au 200 GeV
Figure 8:
Effective cluster size as a functionof N part for Cu+Cu and Au+Au collisions at √ s NN = 200 GeV. Dashed line denotes thevalue of K eff measured in p+p collisions. s / s e ff K PHOBOS preliminary
Cu+Cu 200 GeVAu+Au 200 GeV
Figure 9:
The same as in Figure 8 but plottedagainst the fractional cross-section. atest results from the PHOBOS experiment
7. Summary and outlook
PHOBOS is continuing to provide interesting and unique data on particle production indifferent collision systems in the RHIC energy range. The uniqueness of the PHOBOSdata is owed to the distinct features of the PHOBOS detector: its extensive acceptancefor charged particle measurements and capability of measuring particles at very low p T . Comprehensive results on global features of the particle production are onlybriefly summarized in this report. The measurements of low- p T yields, for the firsttime performed as a function of centrality, are presented for Au+Au collisions at √ s NN = 200 GeV. No anomalous enhancement of pion production is observed at verylow p T . The spectral shapes for heavier particles show flattening effects, consistent witha strong transverse expansion of the system.In this report we have presented a wealth of results on the fluctuations andcorrelation measurements from PHOBOS. New results on event-by-event elliptic flowfluctuations, corrected for the non-flow effects extracted from data, are shown forAu+Au collisions at √ s NN = 200 GeV. Although non-flow correlations contributesignificantly to the measured flow fluctuations, the corrected relative flow fluctuationsare large, with a magnitude in agreement with calculations of fluctuations in theparticipant eccentricity. These results indicate that system thermalizes very rapidlyand the initial-state event-by-event source fluctuations are efficiently converted intofinal-state momentum fluctuations.The PHOBOS measurements of correlations between the trigger high- p T particleand associated particles with no p T cut imposed, allow for studying the ridge structurein the near-side correlations over the longitudinal extent larger than accessible in otherRHIC experiments. The results for Au+Au collisions at √ s NN = 200 GeV show that theridge structure of the near-side correlations persists up to the limit of our acceptance,i.e. ∆ η = 4. With decreasing collision centrality the structure becomes less pronounced,eventually disappearing at N part of about 80. These results provide valuable constraintson models aimed to describe jet propagation through a dense medium.The systematic studies of two-particle angular correlations for p+p, Cu+Cu andAu+Au collisions at √ s NN = 200 GeV show that at freeze-out particles tend to beproduced in clusters with a non-trivial centrality dependence observed in nucleus-nucleus collisions. Furthermore, an unexpected system size dependence is seen. Theseobservations provide a challenge to models describing particle production in heavy ioncollisions.The PHOBOS will continue the analysis of the existing comprehensive dataset.The studies of the low- p T particle production as well as of elliptic flow fluctuationsand triggered correlations will be extended to other systems and energies. AlthoughPHOBOS has accomplished its initial goal, the future studies should still provide resultsimproving our understanding of the physics of heavy ion collisions. This work was partially supported by U.S. DOE grants DE-AC02-98CH10886, DE- atest results from the PHOBOS experiment FG02-93ER40802, DE-FG02-94ER40818, DE-FG02-94ER40865, DE-FG02-99ER41099, andDE-AC02-06CH11357, by U.S. NSF grants 9603486, 0072204, and 0245011, by Polish MNiSWgrant N N202 282234 (2008-2010), by NSC of Taiwan Contract NSC 89-2112-M-008-024, andby Hungarian OTKA grant (F 049823).
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