Baryon-Strangeness correlations in Parton/Hadron transport model for Au + Au collisions at \sqrt{s_{NN}} = 200 GeV
F. Jin, Y. G. Ma, G. L. Ma, J. H. Chen, S. Zhang, X. Z. Cai, H. Z. Huang, J. Tian, C. Zhong, J. X. Zuo
aa r X i v : . [ nu c l - t h ] D ec Baryon-Strangeness correlations in Parton/Hadrontransport model for Au + Au collisions at √ s N N =200 GeV
F. Jin , , Y. G. Ma ∗ , G. L. Ma , J. H. Chen , , S. Zhang , , X.Z. Cai , H. Z. Huang , J. Tian , , C. Zhong , J. X. Zuo , Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O.Box800-204, Shanghai 201800, China Graduate School of the Chinese Academy of Sciences, Beijing 100080, China Deptartment of Physics and Astronomy, University of California, Los Angeles, CA90095, USA
Abstract.
Baryon-strangeness correlation (C BS ) has been investigated with a multi-phasetransport model (AMPT) in Au +
Au collisions at √ s NN = 200 GeV. Thecentrality dependence of C BS is presented within the model, from partonic phase tohadronic matter. We find that the system still reserve partial predicted signatures ofC BS after parton coalescence. But after hadronic rescattering, the predicted signatureswill be obliterated completely. So it seems that both coalescence hadronization processand hadronic rescattering are responsible for the disappearance of the C BS signatures. Submitted to:
J. Phys. G: Nucl. Phys.
1. Introduction
Ultra-relativistic heavy ion collision may provide sufficient conditions for the formationof a deconfined plasma of quarks and gluons [1]. Experimental results from RHICindicate that a strongly-interacting partonic matter (termed sQGP) has been created inthe early stage of central Au + Au collisions at √ s NN = 200 GeV at RHIC [2]. In orderto uncover the nature of this matter, probes based on fluctuations have been proposedthroughout the last decade [3, 4, 5]. Recently a novel event-by-event observable hasbeen introduced by Koch et al. [6], i.e. the baryon-strangeness correlation coefficientC BS .The correlation coefficient C BS is defined as C BS = − σ BS σ S = − h BS i − h B ih S ih S i − h S i (1) ∗ Corresponding author. Email: [email protected] aryon-Strangeness correlations in Parton/Hadron transport model B is the baryon charge and S is the strangeness in a given event.This correlation is proposed as a tool to specify the nature of the highly compressedand heated matter created in heavy-ion collisions [7]. The idea is from that the relationbetween baryon number and strangeness will be different when the phase of system isdifferent. On the one hand, if the basic degrees of freedom are weakly interacting quarksand gluons, the strangeness is carried exclusively by the s and ¯s quarks, B S =- S S .Thus the correlation coefficient C BS =1. On the other hand, if the degrees of freedomare hadronic matter, the case is different because the baryon-strangeness correlationcoefficient strictly depends on the hadronic environments. For example, in a systemcomposed of kaons the coefficient C BS ≈
0, but C BS ≈
2. Results
Because the default AMPT is based on string mechanisms it provides an estimate of C BS value in the case where no partonic matter is created. And the string melting AMPTis based on strong parton cascade, therefore it provides an estimate of C BS value asthe partonic matter is created. So we can compare the values of C BS in the two modelversions to learn information about partonic matter at RHIC.Firstly, we study the partonic phase with the string melting AMPT. we will chooseappropriate pseudorapidity windows and study the effect of parton cascade. C BS = − σ BS σ S = 1 + σ us σ S + σ ds σ S (2) aryon-Strangeness correlations in Parton/Hadron transport model σ us ≈ σ ds ≈
0, we get C BS ≈
1. The resultsfrom the lattice QCD also predicted C BS ≈ T C . So models of the deconfinedmatter should obey such constraints. -1 0 1 2 3 4 5 6 7 8 90.70.80.911.11.21.31.4 |<0.1 h |Au+Au: 200GeVString Melting AMPTbefore hadronization centrality:0-10%|<0.5 h | |<1.0 h | (a) BS C lifetime(fm/c) |<0.1 h | Au+Au: 200GeVbefore hadronizationlifetime=500000fm/cstring melting AMPT (b) |<0.5 h | |<1.0 h | BS C part N Figure 1. (a) The time evolution of baryon-strangeness correlation coefficient C BS forpartonic matter at η max =0.1, 0.5 and 1.0; (b) The dependence of C BS on the numberof participant particles in different pseudorapidity windows, namely η max = 0.1, 0.5and 1.0 for an infinite lifetime of partonic matter. Fig. 1 (a) depicts the time evolution of C BS of partonic matter. We find C BS ≈ C BS . But Fig. 1 shows thatwhen η max = 1.0, the conditions that σ us =0 and σ ds =0 are not perfectly satisfied afterlong parton cascade period. Therefore, we will focus on the correlations only in the twopseudo-rapidity windows, namely η max = 0.1 or 0.5. In the following work, we presenthadronic C BS including all hadrons with masses up to that of Ω − .In AMPT model, hadronization is described with a coalescence model, and thepseudo-rapidity distribution will change during this process. After hadronization, thestrangeness will be enhanced, so the C BS will drop. In the following work, we can seethat.In Fig. 2(a) C BS is depicted as a function of the number of participant particles( N part ). For small acceptance windows around mid-pseudo-rapidity, C BS stays roughlyconstant. The value of C BS may be estimated as simply the ratio of probability toobserve a strange baryon to that of strange meson [18]; 0.5 for the string melting AMPTand 0.35 for the default AMPT. In this case, it is concluded that if the deconfined phaseexists, the ratio of multiplicity of strange baryon to that of strange meson will beenhanced before hadron rescattering. For η max =0.1, the pseudo-rapidity windows maybe too small to include essential correlation information. So in the following study, wewill fix the pseudo-rapidity window at 0.5. Fig. 2(b) shows that for a larger acceptancewindow η max =0.5, C BS increases from peripheral toward central collisions for the string aryon-Strangeness correlations in Parton/Hadron transport model default AMPT Au+Au: 200GeV|<0.1 h |without hadron rescattering (a) string melting AMPT BS C part N default AMPT Au+Au: 200GeV|<0.5 h |without hadron rescattering (b) string melting AMPT BS C part N Figure 2. C BS as a function of N part at the η max =0.1 (a) and η max =0.5 (b)in the default AMPT and the string melting AMPT without hadronic rescattering,respectively. melting AMPT because of higher baryon density. For central collisions C BS goes upto 0.6 and becomes flat. But for the default AMPT, C BS still stays roughly constantbecause of fragmentation mechanism. The results are consistent with [7]. From Fig. 2(b)we can say there exists an enhanced C BS for central collisions if there is a partonic phasebefore the hadronic rescattering. without hadron rescattering Au+Au: 200GeV|<0.5 h |default AMPT (a) with hadron rescattering BS C part N without hadron rescattering Au+Au: 200GeV|<0.5 h |string melting AMPT (b) with hadron rescattering BS C part N Figure 3. C BS as a function of N part at the η max =0.5 in the default AMPT (a)and the string melting AMPT (b) without hadronic rescattering and with hadronicrescattering. We also investigate the effect of hadronic rescattering which is shown in Fig. 3.We find the hadronic rescattering has a larger effect on the C BS for the string meltingAMPT simulation than that of the default AMPT from Fig. 3(b). For the default case,the hadronic effect on C BS is trivial as shown in Fig. 3(a), this is consistent with theresults from UrQMD [7]. In central collisions, C BS ∼ . aryon-Strangeness correlations in Parton/Hadron transport model default AMPT Au+Au: 200GeV|<0.5 h |with hadron rescatteringstring AMPT BS C part N Figure 4. C BS as a function of N part at η max = 0.5 in the default AMPT and thestring melting AMPT with hadronic rescattering. Finally, we try to choose a particle subset to see how much hadronic rescatteringeffect on the C BS . Here, the subset only includes Kaons and Protons. We find thevalue of C BS goes down to 0.2 from Fig. 5. But after hadronic rescattering we getthe similar results that hadronic rescattering finally obliterates the signals of partonicmatter completely. default AMPT Au+Au: 200GeV|<0.5 h |without hadron rescattering (a) string melting AMPT BS C part N default AMPT Au+Au: 200GeV|<0.5 h |with hadron rescattering (b) string melting AMPT BS C part N Figure 5. C BS of Kaons and Protons combination as a function of N part at the η max = 0.5 in the default AMPT and the string melting AMPT without(a) or with hadronicrescattering(b). aryon-Strangeness correlations in Parton/Hadron transport model
3. Summary
We have studied the dependence of the C BS as a function of the N part with a multi-phasetransport model. At η max = 0.5, we find the hadronazation makes the C BS value drop,but we still obtain the residual signal. However, after the hadronic rescattering, theresidual signal will be washed out. In order to analyze the effect of hadronic rescattering,we choose a special particle group which only includes kaons and protons, but the resultdoes not help. Up till now, there are no powerful fluctuation probes of the deconfinedmatter in the experiment, perhaps both hadronization and hadronic rescattering effectsmay be responsible for the disappearance of the signals.Authors appreciate the organizers of SQM2007. This work was supported in partby the National Natural Science Foundation of China under Grant No. 10610285 and10705044, the Knowledge Innovation Project of the Chinese Academy of Sciences underGrant No. KJCX2-YW-A14 and and KJXC3-SYW-N2, the Shanghai DevelopmentFoundation for Science and Technology under Grant No. 05XD14021. And we thankInformation Center of Shanghai Institute of Applied Physics of Chinese Academy ofSciences for using PC-farm. References [1] F. Karsch,
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