The baryon production mechanism at the intermediate $p_T$ (2 - 5 GeV/$c$) at
RHIC is still not well understood. The beam energy scan data in Cu+Cu and Au+Au
systems at RHIC may provide us a further insight on the origin of the baryon
anomaly and its evolution as a function of $\sqrt{s_{NN}}$. In 2005 RHIC
physics program, the PHENIX experiment accumulated the first intensive low beam
energy data in Cu+Cu collisions. We present the preliminary results of
identified charged hadron spectra in Cu+Cu at $\sqrt{s_{NN}}$ = 22.5 and 62.4
GeV using the PHENIX detector. The centrality and beam energy dependences of
(anti)proton to pion ratios and the nuclear modification factors for charged
pions and (anti)protons are presented.Comment: 5 pages, 9 figures, proceedings for Hot Quarks 2006 workshop,
  Villasimius, Sardinia, Italy, May 15 - 20, 2006. Proceedings of the
  conference will be published in The European Physical Journal 

B. Alper

C. Alt

D. Antreasyan

J. Cronin

R.C. Hwa

R.J. Fries

S.S. Adler

T. Chujo

T. Hirano

V. Greco

English

arXiv

The baryon production mechanism at the intermediate $p_T$ (2 - 5 GeV/$c$) at RHIC is still not well understood. The beam energy scan data in Cu+Cu and Au+Au systems at RHIC may provide us a further insight on the origin of the baryon anomaly and its evolution as a function of $\sqrt{s_{NN}}$. In 2005 RHIC physics program, the PHENIX experiment accumulated the first intensive low beam energy data in Cu+Cu collisions. We present the preliminary results of identified charged hadron spectra in Cu+Cu at $\sqrt{s_{NN}}$ = 22.5 and 62.4 GeV using the PHENIX detector. The centrality and beam energy dependences of (anti)proton to pion ratios and the nuclear modification factors for charged pions and (anti)protons are presented

Chujo, T

CERN Document Server

arXiv:nucl-ex/0608020 v1   13 Aug 2006EPJ manuscript No.(will be inserted by the editor)Search for the onset of baryon anomaly at RHIC-PHENIXTatsuya Chujo for the PHENIX CollaborationInstitute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, JapanReceived: date / Revised version: dateAbstract. The baryon production mechanism at the intermediate pT (2 - 5 GeV/c) at RHIC is still notwell understood. The beam energy scan data in Cu+Cu and Au+Au systems at RHIC may provide usa further insight on the origin of the baryon anomaly and its evolution as a function of√sNN . In 2005RHIC physics program, the PHENIX experiment accumulated the first intensive low beam energy data inCu+Cu collisions. We present the preliminary results of identified charged hadron spectra in Cu+Cu at√sNN = 22.5 and 62.4 GeV using the PHENIX detector. The centrality and beam energy dependences of(anti)proton to pion ratios and the nuclear modification factors for charged pions and (anti)protons arepresented.PACS. 25.75.Dw1 IntroductionOne of the most surprising observations at RHIC is a par-ticle type dependence of hadron yield suppression at theintermediate transverse momentum pT (2 - 5 GeV/c) [1,2].In Au+Au central collisions at√sNN = 200 GeV, yieldsfor mesons are largely suppressed [3] with respect to theyields in proton-proton collisions in the intermediate pT ,while those for baryons are not suppressed, and show abinary nucleon-nucleon collision (Ncoll) scaling.In d+Au experiment at RHIC, there is also a parti-cle type dependence in the nuclear modification factorRAA [4]. The RAA for protons in d+Au is larger than thatfor pions and kaons. This observations can be understoodby Cronin effect [5,6]. It is found, however, that the par-ticle species dependence in d+Au is not large enough toaccount for the absence of suppression for protons as seenin Au+Au central collisions at RHIC. This phenomena inAu+Au central collisions is called ”Baryon Anomaly atRHIC”. To explain the baryon anomaly, many theoreticalmodels has been proposed, such as quark recombinationmodels [7], and a hybrid model of hydrodynamics with acolor glass condensate (CGC) and jet quenching [8]. All ofthese models are able to reproduce the experimental dataqualitatively for both pions and protons.On the elliptic flow (v2) measurements, there is a alsoclear particle species dependence [9]. The number of con-stituent quark scaling of v2 shows a clear universal curveregardless of the particle species, and these scaling prop-erties support the quark recombination picture for thehadron production at RHIC.In order to test the quark recombination picture, φme-son is one of the ideal test particles, because the mass isSend offprint requests to: chujo@sakura.cc.tsukuba.ac.jpsimilar to protons’, but it’s a meson particle. If the hydro-dynamical collective flow is a dominant source of baryonanomaly, φ meson’s RAA should follow the data for pro-tons (no suppression) due to its heavy mass. Thanks tothe high statistics data in Au+Au collisions at√sNN =200 GeV taken in 2004 RHIC run, it is confirmed thatthe RAA for φ mesons behaves like a meson [10], and itsv2 follows the same universal quark number scaling curveof all other particle species [11]. Giving the fact that φmesons behave like a ”meson”, it is now widely believedthat the quark recombination is one of the main mecha-nisms for the hadronization process at the intermediatepT in central Au+Au collisions at RHIC.Now the key questions are; where the onset of thebaryon anomaly is, and how they evolve as a function ofbeam energy. In order to answer these questions, the lowerenergy beam data in Cu+Cu collisions were taken duringthe 2005 RHIC run by the PHENIX experiment. In thispaper, we present the preliminary results of single particlepT spectra for identified charged particles in Cu+Cu colli-sions at√sNN = 22.5 GeV and 62.4 GeV measured by thePHENIX experiment. The beam energy and centrality de-pendences of the particle ratios and nuclear modificationfactors are presented and discussed.2 PID charged particle spectra analysisThe low beam energy data sets in Cu+Cu have been takenduring the 2005 RHIC run. We analyzed 5.2 M minimumbias events for 22.5 GeV Cu+Cu, and 66.5 M minimumbias events for 62.4 GeV Cu+Cu data set. We constructedthe tracks within a collision vertex ± 30 cm from the cen-ter of the spectrometer.2 Tatsuya Chujo for the PHENIX Collaboration: Search for the onset of baryon anomaly at RHIC-PHENIX [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5 4 4.5]2 dy [(c / GeV)TN /dp2) d T ppi1/(2 -510-410-310-210-110110210310410 = 22.5 GeVNNs p+X @  →Cu+Cu 0-10% (x20)10-30% (x10)30-60% (x5)60-100% (x10)Min. Bias (x300)PHENIX PreliminaryFig. 1. Centrality dependence of pT spectra for protons inCu+Cu collisions at√sNN = 22.5 GeV. No feeddown correc-tion is applied.Charged particle tracks are reconstructed by the DriftChamber (DC) based on a combinatorial Hough trans-form. The first layer of Pad Chamber (PC1) is used tomeasure the position of the hit in the longitudinal direc-tion (along the beam axis). When combined with the loca-tion of the collision vertex along the beam axis (from theBeam-Beam Counter(BBC)), the PC1 hit gives the po-lar angle of the track. Only tracks with valid informationfrom both the DC and PC1 are used in the analysis. Inorder to associate a track with a hit on the high resolutionTime-of-Flight detector (TOF), the track is projected toits expected hit location on the TOF. Tracks are requiredto have a hit on the TOF within ±2σ of the expected hitlocation in both the azimuthal and beam directions. Thecharged particle identification (PID) is performed by us-ing the combination of three measurements: time-of-flightfrom the BBC and the TOF, momentum from the DC,and flight path length from the collision vertex point tothe hit position on the TOF.The geometrical acceptance and the in-flight decay arecorrected by the GEANT based Monte Carlo simulation.No occupancy correction (detector inefficiency correctiondue to the high track densities) is applied. The expectedoccupancy correction is below 4% in Cu+Cu 62.4 GeV atthe most central events and less for peripheral collisions,the correction is negligible. No weak decay feeddown cor-rection is applied for proton and antiproton spectra in thisanalysis. [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5 4 4.5]2 dy [(c / GeV)TN /dp2) d T ppi1/(2 -510-410-310-210-110110210310410 = 62.4 GeVNNs p+X @  →Cu+Cu 0-10% (x20)10-30% (x10)30-60% (x5)60-88% (x10)Min. Bias (x300)PHENIX PreliminaryFig. 2. Centrality dependence of pT spectra for protons inCu+Cu collisions at√sNN = 62.4 GeV. No feeddown correc-tion is applied.The centrality determination in Cu+Cu 22.5 GeV ispurely based on the PC1 hit multiplicity in order to avoidan effect of the spectator nucleons. Since at√sNN =22 GeV the beam rapidity gap is quite narrower thanthose at the higher beam energy, the BBC (located in3.0 < |η| < 3.9) is able to see the spectator nucleons. ThePC1 multiplicity distribution with the BBC z vertex cor-rection is subdivided by four centrality bins. For Cu+Cu62.4 GeV data, we use the charge distribution measuredby the BBC. We obtain the number of binary collisions(Ncoll) and the number of participant nucleons (Npart)for each data set and each centrality bin by the GlauberMonte Carlo calculation in PHENIX.3 ResultsWe present the following preliminary results in Cu+Cucollisions at√sNN = 22.5 GeV and 62.4 GeV, measuredby the PHENIX experiment. (1) the identified chargedparticle pT spectra, (2) particle ratios (p/pi+, p/pi−) as afunction of pT for each centrality class, and (3) the nuclearmodification factor RAA for pions and (anti)protons as afunction of pT .3.1 pT spectraFig. 1 and Fig. 2 are the centrality dependences of pT spec-tra for protons in Cu+Cu collisions at√sNN = 22.5 GeVTatsuya Chujo for the PHENIX Collaboration: Search for the onset of baryon anomaly at RHIC-PHENIX 3 [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5 4 4.5]2 dy [(c / GeV)TN /dp2) d T ppi1/(2 -510-410-310-210-110110210310410 = 22.5 GeVNNs+X @  p →Cu+Cu 0-10% (x20)10-30% (x10)30-60% (x5)60-100% (x10)Min. Bias (x300)PHENIX PreliminaryFig. 3. Centrality dependence of pT spectra for p in Cu+Cucollisions at√sNN = 22.5 GeV. No feeddown correction isapplied.and 62.4 GeV, respectively. The pT spectra for antiprotonsare shown in Fig. 3 and 4. No weak decay feeddown correc-tion is applied for both protons and antiprotons spectraand both beam energies. We measured the spectra in fourcentrality bins (0-10%, 10-30%, 30-60%, 60-88 or 60-100%)and in the minimum bias events. The pT spectra for eachcentrality bin for pi± and K± are also measured at bothbeam energies (see [12]).3.2 Particle ratios (p/pi+ and p/pi−)Fig. 5 shows the centrality dependence of p/pi+ and p/pi−ratios as a function of pT in Cu+Cu collisions at√sNN =22.5 GeV. The similar plots for Cu+Cu at 62.4 GeV isshown in Fig. 6.In 22.5 GeV Cu+Cu data, p/pi+ ratio is significantlylarger than those in the higher beam energy in Au+Au [1],and it is increasing as a function of collision centrality.This large p/pi+ ratios can be understood by the influ-ence of the incoming nucleons from the beams. For p/pi−ratios, (almost) no centrality dependence is observed, andthe ratios are constant at the value of 0.3 - 0.4 at pT = 2.0GeV/c, which is close to the expected values in the frag-mentation and the data in p+p collisions.In 62.4 GeV Cu+Cu data, the influence of incomingproton are smaller than those in Cu+Cu 22.5 GeV, asseen in p/pi+ ratios in Fig. 6. The centrality dependenceof p/pi− is seen. In the most central collisions, p/pi− ra-tio is about 0.6 at pT = 2 GeV/c, and in the peripheral [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5 4 4.5]2 dy [(c / GeV)TN /dp2) d T ppi1/(2 -510-410-310-210-110110210310410 = 62.4 GeVNNs+X @  p →Cu+Cu 0-10% (x20)10-30% (x10)30-60% (x5)60-88% (x10)Min. Bias (x300)PHENIX PreliminaryFig. 4. Centrality dependence of pT spectra for p in Cu+Cucollisions at√sNN = 62.4 GeV. No feeddown correction isapplied.collisions, the value becomes consistent with that for p+pcollisions.Fig. 7 shows the beam energy dependence of p/pi+ andp/pi− ratios in Cu+Cu collisions from√sNN = 22.5 GeVto 200 GeV [13]. There is a clear beam energy dependencein Cu+Cu from 22.5 GeV to 200 GeV, i.e. p/pi+ (p/pi−)decreases (increases) as a function of√sNN , respectively.For the study of the baryon anomaly at the lower beamenergies at RHIC, p/pi− ratios would be a good measure,because antiprotons are “produced particles”, but mea-sured protons are the mixture of produced particles andthe incoming protons from the beams. The absence of cen-trality dependence in p/pi− ratio in Cu+Cu 22.5 GeV maysuggest that there is no baryon anomaly at this beam en-ergy. To conclude this observation, the high statistics datawith the heavier collisions system like Au+Au at around22.5 GeV is necessary in the future RHIC run.3.3 Nuclear modification factor: RAATo obtain the nuclear modification factors at the lowerenergy, we used the pT spectra at mid-rapidity in p+pcollisions at√s = 23 GeV and 63 GeV for kaons and(anti)protons as the reference spectra, measured by theBritish-Scandinavian Collaboration [14]. Those data arefitted by the empirical functional form (see [12] in de-tail). For pions, we use the parameterization as describedin [15]. The nuclear modification factor RAA is defined asthe following equation;4 Tatsuya Chujo for the PHENIX Collaboration: Search for the onset of baryon anomaly at RHIC-PHENIX [GeV/c]Tp0 0.5 1 1.5 2 2.5 3+pip / 0.20.40.60.811.21.41.61.822.2PHENIX Preliminary [GeV/c]Tp0 0.5 1 1.5 2 2.5 3-pi / p0.20.40.60.811.21.41.61.822.2Cu+Cu 22.5 GeV (0-10%)Cu+Cu 62.4 GeV (0-10%)Cu+Cu 200  GeV (0-10%)=53 GeV, ISRsp+p Fig. 7. Beam energy dependence of p/pi+ (left) and p/pi− (right) ratios as a function of pT in Cu+Cu collisions from√sNN =22.5 GeV to 200 GeV. The data for 200 GeV Cu+Cu is taken from the PHENIX preliminary data [13]. [GeV/c]Tp0 0.5 1 1.5 2 2.5 3Ratio0.20.40.60.811.21.41.61.822.2 Cu+Cu 22.5 GeV+pip/PHENIX Preliminary [GeV/c]Tp0 0.5 1 1.5 2 2.5 3Ratio0.20.40.60.811.21.41.61.822.2 Cu+Cu 22.5 GeV-pi/p0-10%10-30%30-60%60-100%Min. BiasPHENIX PreliminaryFig. 5. Centrality dependence of p/pi+ (left) and p/pi− (right)ratios as a function of pT in Cu+Cu collisions at√sNN = 22.5GeV. No feeddown correction is applied. [GeV/c]Tp0 0.5 1 1.5 2 2.5 3Ratio00.20.40.60.811.21.41.61.822.2 Cu+Cu 62.4 GeV+pip/PHENIX Preliminary [GeV/c]Tp0 0.5 1 1.5 2 2.5 3Ratio00.20.40.60.811.21.41.61.822.2 Cu+Cu 62.4 GeV-pi/p0-10%10-30%30-60%60-88%Min. BiasPHENIX PreliminaryFig. 6. Centrality dependence of p/pi+ (left) and p/pi− (right)ratios as a function of pT in Cu+Cu collisions at√sNN = 62.4GeV. No feeddown correction is applied.RAA(pT ) =(1/NevtAA) d2NAA/dpTdy〈Ncoll〉/σinelpp × d2σpp/dpTdy, (1)where the 〈Ncoll〉/σinelpp is the average Glauber nuclearoverlap function, 〈TAuAu〉.The RAA for pions and (anti)protons are shown inFig. 8 for 22.5 GeV Cu+Cu, and in Fig. 9 for 62.4 GeVCu+Cu. The centrality selection for RAA is 0-10% cen-tral collisions for both beam energies. The valules of theaverage number of collisions used here are 〈Ncoll〉 =140.7(σinel = 32.2 mb) for 22.5 GeV Cu+Cu, and 〈Ncoll〉 =152.3(σinel = 35.6 mb) for 62.4 GeV Cu+Cu. The systematicerror on RAA consists of; (1) < Ncoll > uncertainty (one σerror as shown in dotted-lines above and below RAA = 1),(2) combined systematic errors from p+p reference dataand Cu+Cu spectra (not shown in the figures). The sta-tistical errors are shown in the error bars on each datapoint.The data shows that there is no suppression on chargedpions for both 22.5 and 62.4 GeV Cu+Cu central colli-sions. For proton’s RAA, a clear enhancement is seen andit can be attribute to the effect from the incoming beamnucleons. The value of proton’sRAA in 62.4 GeV is slightlysmaller than that in 22.5 GeV. The RAA for antiprotonsis almost unity at the intermediate pT for both 22.5 and62.4 GeV, and they are very similar to those for pions.In 2006, the PHENIX experiment has successfully col-lected the new reference p+p data at√sNN = 62.4 GeV.The precision of RAA measurements at 62.4 GeV for bothCu+Cu and Au+Au are expected to be improved by usingthis new p+p data set.4 ConclusionsIn summary, we have measured pT spectra for pi±, K±, p,p in Cu+Cu collisions at√sNN = 22.5 and 62.4 GeV.Tatsuya Chujo for the PHENIX Collaboration: Search for the onset of baryon anomaly at RHIC-PHENIX 5 [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5AAR1234567 = 22.5 GeV (0-10%)NNsCu+Cu p+piPHENIX Preliminary [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5AAR1234567 = 22.5 GeV (0-10%)NNsCu+Cu p-piPHENIX PreliminaryFig. 8. Nuclear modification factor RAA (0-10%) for chargedpions and (anti)protons in Cu+Cu collisions at√sNN = 22.5GeV. The positively charged particles are shown in the leftpanel, and the negatively charged particles are shown in theright panel. [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5AAR1234567 = 62.4 GeV (0-10%)NNsCu+Cu p+piPHENIX Preliminary [GeV/c]Tp0 0.5 1 1.5 2 2.5 3 3.5AAR1234567 = 62.4 GeV (0-10%)NNsCu+Cu p-piPHENIX PreliminaryFig. 9. Nuclear modification factor RAA (0-10%) for chargedpions and (anti)protons in Cu+Cu collisions at√sNN = 62.4GeV. The positively charged particles are shown in the leftpanel, and the negatively charged particles are shown in theright panel.In 22.5 GeV Cu+Cu data, we observed a larger p/pi+ratio compared to those at the higher beam energy inAu+Au, and it’s increasing as a function of collision cen-trality. For p/pi− ratio, almost no centrality dependence isseen, and the ratio is about 0.3 - 0.4 at pT = 2.0 GeV/c,which is close to the value in p+p collisions. In 62.4 GeVCu+Cu data, p/pi+ ratios are reduced, compared to thosein Cu+Cu 22.5 GeV, and the centrality dependence ofp/pi− is seen. There is a clear beam energy dependenceon those ratios in Cu+Cu from 22.5 GeV to 200 GeV, i.e.p/pi+ (p/pi−) decreases (increases) as a function of√sNN ,respectively. The observed p/pi− ratio may suggest thereis no baryon anomaly at 22.5 GeV Cu+Cu collisions. Tomake this point clearer, a further data set (a high statis-tics low beam energy data in heavy collision system) isnecessary in the future RHIC run.For the RAA, no suppression is observed for chargedpions for both 22.5 and 62.4 GeV in Cu+Cu central col-lisions. For protons RAA, there is a clear enhancement,which can be attribute to the incoming beam nucleons.The RAA in 62.4 GeV is slightly smaller than that in 22GeV. The antiprotons RAA is almost unity at the inter-mediate pT region for both beam energies.References1. S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. C69, (2004) 034909.2. S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. Lett.91, (2003) 172301.3. S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. Lett.91, (2003) 072301.4. S. S. Adler et al. (PHENIX Collaboration),nucl-ex/0603010.5. J. Cronin et al., Phys. Rev. D 11, (1975) 3105.6. D. Antreasyan et al., Phys. Rev. D 19, (1979) 764.7. R. C. Hwa and C. B. Yang, Phys. Rev. C 67, (2003) 034902;R. J. Fries, B. Mu¨ller, C. Nonaka and S. A. Bass, Phys. Rev.Lett. 90, (2003) 202303; V. Greco, C. M. Ko and P. Le´vai,Phys. Rev. Lett. 90, (2003) 202302.8. T. Hirano, Y. Nara, Phys. Rev. C 69, (2004) 034908, T. Hi-rano, Y. Nara, Nucl. Phys. A743, (2004) 305.9. S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. Lett.91, (2003) 182301.10. D.Pal (PHENIX Collaboration),hep-ex/0510020.11. M. Issah, A. Taranenko (PHENIX Collaboration),nucl-ex/0604011.12. HQ 2006 presentation file can be found in;http://www.phenix.bnl.gov/WWW/talk/newtalk.php13. M. Konno (PHENIX Collaboration), nucl-ex/0510022.14. B. Alper et al., Nucl. 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Search for the onset of baryon anomaly at RHIC-PHENIX

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