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Recent results from PHOBOS at RHIC

Abstract

PHOBOS is one of the four heavy ion experiments currently running at the Relativistic Heavy Ion Collider (RHIC). In this paper we will present some of the current results from PHOBOS: the study of charged multiplicity distribution as function of pseudorapidity and centrality at <IMG SRC="/img/revistas/bjp/v34n3a/a32img01.gif" > or = 19.6, 130 and 200 GeV, a discussion of multiplicity scaling, and a measurement of transverse momentum spectra (pT) of charged hadrons produced in d+Au and Au+Au interactions at <IMG SRC="/img/revistas/bjp/v34n3a/a32img01.gif" > or = 200 GeV. The dependence of the Au+Au yields as function of centrality is found to be independent of pT. A strong suppression in the Au+Au hadron spectra relative to p<IMG SRC="/img/revistas/bjp/v34n3a/a32img05.gif"> at high pT is observed. The spectral shape for d+Au collisions is significantly different from the observations in Au+Au, the yield suppression is not observed in d+Au.


Recent results from PHOBOS at RHIC

B.B.BackI; M.D.BakerII; M.BallintijnIV; D.S.BartonII; B.BeckerII; R.R.BettsVI; A.A.BickleyVII; R.BindelVII; A.BudzanowskiIII; W.BuszaIV; A.CarrollII; M.P.DecowskiIV; E.GarcíaVI; T.GburekIII; N.GeorgeI, II; K.GulbrandsenIV; S.GushueII; C.HalliwellVI; J.HamblenVIII; A.S.HarringtonVIII; G.A.HeintzelmanII; C.HendersonIV; D.J.HofmanVI; R.S.HollisVI; R.HoynskiIII; B.HolzmanII; A.IordanovaVI; E.JohnsonVIII; J.L.KaneIV; J.KatzyIV, VI; N.KhanVIII; W.KucewiczVI; P.KulinichIV; C.M.KuoV; J.W.LeeIV; W.T.LinV; S.ManlyVIII; D.McLeodVI; A.C.MignereyVII; R.NouicerII, VI; A.OlszewskiIII; R.PakII; I.C.ParkVIII; H.PerneggerIV; C.ReedIV; L.P.RemsbergII; M.ReuterVI; C.RolandIV; G.RolandIV; L.RosenbergIV; J.SagererVI; P.SarinIV; P.SawickiIII; I.SedykhII; W.SkulskiVIII; C.E.SmithVI; P.SteinbergII; G.S.F.StephansIV; A.SukhanovII; J.-L.TangV; M.B.TonjesVII; A.TrzupekIII; C.ValeIV; G.J.van NieuwenhuizenIV; R.VerdierIV; G.I.VeresIV; F.L.H.WolfsVIII; B.WosiekIII; K.WozniakIII; A.H.WuosmaaI; B.WysouchIV; J.ZhangIV

IPhysics Division, Argonne National Laboratory, Argonne, IL 60439-4843, USA

IIChemistry and C-A Departments, Brookhaven National Laboratory, Upton, NY 11973-5000, USA

IIIInstitute of Nuclear Physics, Kraków,Poland

IVLaboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA

VDepartment of Physics, National Central University,Chung-Li, Taiwan

VIDepartment of Physics, University of Illinois at Chicago, Chicago, IL 60607-7059, USA

VIIDepartment of Chemistry, University of Maryland,College Park, MD 20742, USA

VIIIDepartment of Physics and Astronomy, University of Rochester, Rochester,NY 14627, USA

ABSTRACT

PHOBOS is one of the four heavy ion experiments currently running at the Relativistic Heavy Ion Collider (RHIC). In this paper we will present some of the current results from PHOBOS: the study of charged multiplicity distribution as function of pseudorapidity and centrality at = 19.6, 130 and 200 GeV, a discussion of multiplicity scaling, and a measurement of transverse momentum spectra (pT) of charged hadrons produced in d+Au and Au+Au interactions at = 200 GeV. The dependence of the Au+Au yields as function of centrality is found to be independent of pT. A strong suppression in the Au+Au hadron spectra relative to p at high pT is observed. The spectral shape for d+Au collisions is significantly different from the observations in Au+Au, the yield suppression is not observed in d+Au.

The fundamental theory of strong interactions predicts that a Quark-Gluon Plasma (QGP) will be formed in hadronic systems at sufficiently high energy density [1]. The QGP is the state of matter that is believed to have existed about 10 micro-seconds after the Big Bang, and is a conglomerate of free quarks and gluons. The goal of the relativistic heavy ion physics program over the past two decades has been to find signatures of the formation of the QGP. From data collected at the AGS, the SPS and so far from RHIC, it is clear that the search for the QGP requires a systematic effort of the entire heavy ion physics community. In this paper we present PHOBOS' contribution to this task. The data described in this paper were taken during the RHIC 2000 run, Au+Au at = 19.6 and 130 GeV, the 2001 Au + Au run at = 200 GeV, and the 2003 d + Au run at = 200 GeV. A full description of the PHOBOS detector used during these runs as well a description of the event selection process can be found at [2,3].

One of the important contributions of PHOBOS to the field is the measurement of charged particle multiplicity as over a large pseudorapidity (h) coverage. Fig. 1 summarizes all the Au+Au multiplicity measurements taken by PHOBOS. Details of the analysis procedures can be found in Ref. [4]. The centrality of the collisions is characterized by the average number of participants (Npart). For the 130 and 200 GeV data sets this was estimated from the data using two sets of 16 plastic scintillator counters (the ''paddle'' counters), with acceptance of 3 < |h| < 4.5. It was found that the truncated mean of the ADC signals from these counters is proportional to the charged multiplicity in their acceptance. Npart is then extracted for a specified fraction of the cross-section from the truncated mean, assuming that this signal increases monotonically with centrality. The monotonic relation was verified using the correlation between the paddle counters and the zero degree calorimeters. The calculation of Npart and the associated systematic errors is outlined in Ref [5]. For the = 19.6 GeV data set, the lower beam rapidity precludes use of this method; instead a different quantity was constructed "EOCT". This quantity is proportional to the path-length corrected sum of the energy deposited in the octagon detector. As for the paddle counters, it is assumed only that EOCT varies monotonically with Npart. Finally, for the d+Au data set, the silicon ring detectors were use in a similar way to determine centrality bins.


An empirical observation derived from the multiplicity measurements is the scaling of the multiplicity distributions with Npart. Fig. 2, shows the total charged multiplicity per participant pair as a function of the number of participants for Au + Au at three different energies. Also shown in the figure are results for p and e+e- systems at the same value of . The total multiplicity in Au + Au scales approximately with the number of participants and is the same as the production for e+e- at the same energy. On the other hand, production in p is reduced, in comparison to Au + Au. A detailed discussion on this scaling behavior is found in Ref. [6].


In Fig. 3, we present the invariant cross-section of charged hadrons, obtained by averaging the yields of positive and negative hadrons as a function of transverse momentum for = 200 GeV Au+Au collisions. Data are shown for 6 centrality bins, ranging from áNpartñ = 58 to áNpartñ = 344. The spectra exhibit a smooth fall with increasing pT, with a slightly concave shape. Details of the analysis and a broader discussion of the pT spectra are in Ref. [2].


In order to study the evolution of the spectra with centrality in detail Fig. 4 shows the multiplicity yield per participant pair for six pT values, divided by a fit to the most peripheral bin plotted as a function of the number of participants. For reference p data are included in the plot. The dashed and solid lines represent the expectation for áNcoll ñ and áNpart ñ scaling. The data seem to scale closer to áNpart ñ than to áNcollñ for all pT bins and centralities. We also observe surprisingly that the spectra do not depend on centrality.


Figure 5 shows the yield per participant pair divided by a fit to the invariant cross section for proton-antiproton collisions (200 GeV UA1) plotted as a function of pT, for the most peripheral and most central bin. The dashed and solid lines show the expectation of áNcollñ and áNpart ñ scaling. The brackets show the systematic uncertainty. We can see, again, that the general shape of the curves is only weakly dependent on the centrality and that, for pT values up to 2 GeV/c there is an increase of the yield in Au+Au collisions compared to p. After 2 GeV/c the relative yield decreases, for all centrality bins.


Two possible explanations have been suggested for the this yield ''quenching''. The first (''initial state'') model suggest that the effect is due to a modification of the wave functions of the nucleons in the colliding ions. This produces an effective increase of the interaction length in such way that the nucleons in an ion will interact coherently with all the nucleons in the other ion in the longitudinal dimension [7]. This model predicts not only the yield quenching at high pT, but also the scaling of the yield with the number of participants. The second (final statef) model suggests that the suppression comes from an increase of the energy loss of the partons traveling through the hot dense medium formed in the Au+Au collisions [8]. One way to discriminate between these models is to reduce the size of the hot and dense medium, by colliding d+Au instead of Au+Au. RHIC's 2003 run was dedicated mainly to d+Au collisions at = 200 Gev. Fig. 6 shows the measured invariant yield of charged hadrons for d+Au collisions as function of pT. Details of the analysis and a broader discussion of pT spectra are given in Ref. [3].


In Fig. 7 we present the nuclear modification factor RdAu as function of pT for four centrality bins defined as the ratio of the d+Au invariant cross section divided by the p UA1 yield, scaled by the proton-antiproton invariant cross section (41 mb) and áNcollñ . For all the centrality bins we observe a rapid rise of RdAu from low pT reaching a maximum at 2 GeV/c. For comparison this factor is also plotted for the most central bin from central Au+Au collisions at = 200 GeV. In striking contrast to the behavior of RdAu, RAuAu also increases initialy as function of pT but after 2 GeV/c decreases sharply at larger pT values.


In Fig. 8 the nuclear modification factor as a function of áNcollñ in four pT bins is presented. The ''initial state'' parton saturation model predicts a decrease of 25 - 30 % for RdAu for áNcollñ > 6. At pT = 4 GeV/c, RdAu(Ncoll = 14.6)/RdAu(Ncoll = 5.4) = 1.08± 0.06. The data therefore disfavor the initial state interpretation of high-pT hadron suppression. Thus, the yield quenching of high pT in Au+Au collisions can not be accounted by initial state interactions that should be also present in d+Au. The suppression of the inclusive yield observed in central Au+Au is consistent with final-state interactions with a dense medium generated in such collisions.


This work was partially supported by U.S. DOE grants DE-AC02-98CH10886, DE-FG02-93ER40802, DE-FC02-94ER40818, DE-FG02-94ER40865, DE-FG02-99ER41099, and W-31-109-ENG-38, US NSF grants 9603486, 9722606 and 0072204, Polish KBN grant 2-P03B-10323, and NSC of Taiwan contract NSC 89-2112-M-008-024.

Received on 26 September, 2003

  • [1] J. P. Blaizot, Nucl.Phys. A661, 3 (1999).
  • [2] B. B. Back et al., hep-ex/0210006.
  • [3] B. B. Back et al., hep-ex/0306025.
  • [4] B. B. Back et al., Phys. Rev. Lett. 87, 102303 (2001).
  • [5] B. B. Back et al., Phys. Rev. C 65, 061901R (2002).
  • [6] B. B. Back et al., hep-ex/0301017.
  • [7] D. Kharzeev, E. Levin, and L. McLerran, hep-ex/0210332.
  • [8] M. Gyulassy, I. Vitev, X. Wang, and B. Zhang, hep-ex/0302077.

Publication Dates

  • Publication in this collection
    26 Oct 2004
  • Date of issue
    Sept 2004

History

  • Received
    26 Sept 2003
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