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Forward-backward multiplicity correlations in pp, p¯p and Au+Au collisions at RHIC energy
Nuclear Physics A 834 (2010) 320c–322c www.elsevier.com/locate/nuclphysa
Forward-backward multiplicity correlations in pp, p¯p and Au+Au collisions at RHIC energy Yu-Liang Yana , Bao-Guo Donga , Dai-Mei Zhoub , Xiao-Mei Lia , Hai-Liang Maa , Ben-Hao Saa a
China Institute of Atomic Energy, P. O. Box 275(18), Beijing 102413, China
b
Institute of Particle Physics, Huazhong Normal University, Wuhan 430079, China
The strength √ of charged particle forward-backward multiplicity correlation in p¯p and pp collisions at s=200 GeV is studied by the PYTHIA model and compared with the UA5 and STAR data correspondingly. It turns out that a factor of 3-4 apparent discrepancy between UA5 and STAR data can be attributed to the differences in detector acceptance and observing bin width. We have also studied the centrality bin size dependence of the √ correlation strength in 0-10 to 0-5 and to 5% most central Au+Au collisions at sNN =200 GeV with the PACIAE model. It turns out that the correlation strength decreases with decreasing centrality bin size monotonously. 1. Introduction The study of fluctuations and correlations has been suggested as a useful means for revealing the mechanism of particle production and Quark-Gluon-Plasma formation in relativistic heavy ion collisions [1,2]. Recently STAR collaboration measured the charged par√ ticle forward-backward multiplicity correlation in pp and Au+Au collisions at sN N =200 GeV [3]. The STAR pp data are about 3-4 times smaller than the UA5 p¯p data measured at the same energy [4], and the correlation strength is approximately flat in Au+Au collisions. The charged particle forward-backward multiplicity correlation strength b is defined as[3,4] b=
nf nb − nf nb , n2f − nf 2
(1)
where nf and nb are, respectively, the number of charged particles in forward and backward pseudorapidity bins. nf and nb refer to the mean value of nf and nb , respectively. In this paper, the PACIAE/PYTHIA models are employed to analyze the discrepancy between UA5 and STAR data as well as the centrality bin size dependence of the correlation strength in central Au+Au collisions. 2. The PACIAE model The parton and hadron cascade model, PACIAE, is composed of four stages: parton initialization, parton evolution, hadronization, and hadron evolution. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.009
Y.-L. Yan et al. / Nuclear Physics A 834 (2010) 320c–322c
321c
1) PARTON INITIALIZATION: A nucleus-nucleus collision is decomposed into nucleonnucleon collisions based on the collision geometry. A nucleon-nucleon collision is described with the PYTHIA model[5] while the hadronization is switched off temporarily. So the initial partonic state in a nucleus-nucleus collision is composed of quarks, anti-quarks, and gluons. 2) PARTON EVOLUTION: The next stage is the parton evolution (parton rescattering). Here the 2 → 2 LO-pQCD differential cross sections [6] and Monte Carlo simulation are employed. 3) HADRONIZATION: The parton evolution stage is followed by the hadronization at the moment of partonic freeze-out. In the PACIAE model, the phenomenological fragmentation model or coalescence model is applied for hadronization. We refer to [7] for the details of the hadronization stage. 4) HADRON EVOLUTION: After hadronization the rescattering among produced hadrons is dealt with the usual two-body collision model. The details of hadronic rescattering can be seen in [8]. 3. Calculations and results The PYTHIA results calculated for correlation strength in p¯p and pp collisions are compared with the corresponding experimental data in Fig. 1. One sees in this figure that the theoretical results are not so far apart from the experimental data for both collisions. The theoretical correlation strength in p¯p collision is also a factor of 3-4 times larger than the one in pp collision, especially. The Fig. 1(c) gives the results calculated
Figure 1. The forward-backward multiplicity correlation strength b in (a)¯ pp collisions with detector acceptance and η bin width same as in UA5 experiment, (b)pp collisions as in STAR, (c)¯ pp and pp collisions as in STAR. The Δη denotes the distance between √ the forward and backward η bins, and reaction energies are all sN N =200 GeV. The experimental data of p¯p and pp collisions are taken from [4] and [3], respectively. for pp and p¯p collisions both with STAR’s detector acceptance and η bin width. One sees here the discrepancy of correlation strength in p¯p and pp collisions are about 10 percent and the factor of 3-4 disappears. So the discrepancy between p¯p and pp collisions can be attributed to the differences in detector acceptance and observing bin width. In Fig. 2(a) we compare the PACIAE calculated correlation strength b with the STAR √ data[3] in 0-10% most central Au+Au collisions at sN N =200 GeV. The STAR data
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Y.-L. Yan et al. / Nuclear Physics A 834 (2010) 320c–322c
Figure 2. (a)Forward-backward multiplicity correlation strength b in 0-10, 0-5, and 5% √ most central Au+Au collisions at sNN =200 GeV. (b)The calculated total and statistical correlation strengths in 0-10, 0-5, and 5% most central Au+Au collisions at same energy. The experimental data are taken from [3]. feature of correlation strength b is approximately flat across a wide range in Δη are well reproduced. For comparison we also give the correlation strength in 0-5 and 5% most central collisions in Fig. 2(a). One sees that the correlation strength decreases with decreasing centrality bin size monotonously. We also use the mixed event method[9] to study the statistical correlation in 0-10, 0-5, and 5% most central Au+Au collisions at √ sNN =200 GeV, the calculated results are shown in Fig. 2(b). One can see that the behavior of correlation strength decreases with decreasing centrality bin size, which not only exist in total correlation but also in statistical one. That is because the charged particle multiplicity fluctuation, which decides the strength of statistical correlation, decreases from 0-10 to 0-5 and to 5% central collisions. If the discrepancy between total and statistical correlations is identified as dynamical correlation, one then sees that the dynamical correlation may just be a few percent of the total correlation and is not dependant on the centrality bin size. Finally, the financial support from NSFC (10635020, 10605040, and 10705012) in China is acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
R. C. Hwa, arXiv:nucl-th/0701053v1. T. K. Nayak, J. of Phys. G 32, S187 (2006); arXiv:nucl-ex/0608021v1. B. K. Srivastavs, STAR Collaboration, Int. J. Mod. Phys. E 16, 3363 (2008). R. E. Ansorge et al., UA5 Collaboration, Z. Phys. C 37, 191 (1988). T. S¨ojstrand, S. Mrenna, and P. Skands, J. High Energy Phys. JHEP05, 026 (2006); arXiv:hep-ph/0603175v1. B. L. Combridge, J. Kripfgang, and J. Ranft, Phys. Lett. B 70, 234 (1977). Yu-Liang Yan, Dai-Mei Zhou, Bao-Guo Dong, Xiao-Mei Li, Hai-Liang Ma, and BenHao Sa, Phys. Rev. C 79, 054902 (2009); arXiv:0903.0915v2[nucl-th]. Ben-Hao Sa and Tai An, Comput. Phys. Commun. 90, 121 (1995); Tai An and BenHao Sa, Comput. Phys. Commun. 116, 353 (1999). Yu-Liang Yan, Bao-Guo Dong, Dai-Mei Zhou, Xiao-Mei Li, and Ben-Hao Sa, Phys. Lett. B 660, 478 (2008); arXiv:0710.2187v2[nucl-th].
Forward-backward multiplicity correlations in pp, p¯p and Au+Au collisions at RHIC energy Yu-Liang Yana , Bao-Guo Donga , Dai-Mei Zhoub , Xiao-Mei Lia , Hai-Liang Maa , Ben-Hao Saa a
China Institute of Atomic Energy, P. O. Box 275(18), Beijing 102413, China
b
Institute of Particle Physics, Huazhong Normal University, Wuhan 430079, China
The strength √ of charged particle forward-backward multiplicity correlation in p¯p and pp collisions at s=200 GeV is studied by the PYTHIA model and compared with the UA5 and STAR data correspondingly. It turns out that a factor of 3-4 apparent discrepancy between UA5 and STAR data can be attributed to the differences in detector acceptance and observing bin width. We have also studied the centrality bin size dependence of the √ correlation strength in 0-10 to 0-5 and to 5% most central Au+Au collisions at sNN =200 GeV with the PACIAE model. It turns out that the correlation strength decreases with decreasing centrality bin size monotonously. 1. Introduction The study of fluctuations and correlations has been suggested as a useful means for revealing the mechanism of particle production and Quark-Gluon-Plasma formation in relativistic heavy ion collisions [1,2]. Recently STAR collaboration measured the charged par√ ticle forward-backward multiplicity correlation in pp and Au+Au collisions at sN N =200 GeV [3]. The STAR pp data are about 3-4 times smaller than the UA5 p¯p data measured at the same energy [4], and the correlation strength is approximately flat in Au+Au collisions. The charged particle forward-backward multiplicity correlation strength b is defined as[3,4] b=
nf nb − nf nb , n2f − nf 2
(1)
where nf and nb are, respectively, the number of charged particles in forward and backward pseudorapidity bins. nf and nb refer to the mean value of nf and nb , respectively. In this paper, the PACIAE/PYTHIA models are employed to analyze the discrepancy between UA5 and STAR data as well as the centrality bin size dependence of the correlation strength in central Au+Au collisions. 2. The PACIAE model The parton and hadron cascade model, PACIAE, is composed of four stages: parton initialization, parton evolution, hadronization, and hadron evolution. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.009
Y.-L. Yan et al. / Nuclear Physics A 834 (2010) 320c–322c
321c
1) PARTON INITIALIZATION: A nucleus-nucleus collision is decomposed into nucleonnucleon collisions based on the collision geometry. A nucleon-nucleon collision is described with the PYTHIA model[5] while the hadronization is switched off temporarily. So the initial partonic state in a nucleus-nucleus collision is composed of quarks, anti-quarks, and gluons. 2) PARTON EVOLUTION: The next stage is the parton evolution (parton rescattering). Here the 2 → 2 LO-pQCD differential cross sections [6] and Monte Carlo simulation are employed. 3) HADRONIZATION: The parton evolution stage is followed by the hadronization at the moment of partonic freeze-out. In the PACIAE model, the phenomenological fragmentation model or coalescence model is applied for hadronization. We refer to [7] for the details of the hadronization stage. 4) HADRON EVOLUTION: After hadronization the rescattering among produced hadrons is dealt with the usual two-body collision model. The details of hadronic rescattering can be seen in [8]. 3. Calculations and results The PYTHIA results calculated for correlation strength in p¯p and pp collisions are compared with the corresponding experimental data in Fig. 1. One sees in this figure that the theoretical results are not so far apart from the experimental data for both collisions. The theoretical correlation strength in p¯p collision is also a factor of 3-4 times larger than the one in pp collision, especially. The Fig. 1(c) gives the results calculated
Figure 1. The forward-backward multiplicity correlation strength b in (a)¯ pp collisions with detector acceptance and η bin width same as in UA5 experiment, (b)pp collisions as in STAR, (c)¯ pp and pp collisions as in STAR. The Δη denotes the distance between √ the forward and backward η bins, and reaction energies are all sN N =200 GeV. The experimental data of p¯p and pp collisions are taken from [4] and [3], respectively. for pp and p¯p collisions both with STAR’s detector acceptance and η bin width. One sees here the discrepancy of correlation strength in p¯p and pp collisions are about 10 percent and the factor of 3-4 disappears. So the discrepancy between p¯p and pp collisions can be attributed to the differences in detector acceptance and observing bin width. In Fig. 2(a) we compare the PACIAE calculated correlation strength b with the STAR √ data[3] in 0-10% most central Au+Au collisions at sN N =200 GeV. The STAR data
322c
Y.-L. Yan et al. / Nuclear Physics A 834 (2010) 320c–322c
Figure 2. (a)Forward-backward multiplicity correlation strength b in 0-10, 0-5, and 5% √ most central Au+Au collisions at sNN =200 GeV. (b)The calculated total and statistical correlation strengths in 0-10, 0-5, and 5% most central Au+Au collisions at same energy. The experimental data are taken from [3]. feature of correlation strength b is approximately flat across a wide range in Δη are well reproduced. For comparison we also give the correlation strength in 0-5 and 5% most central collisions in Fig. 2(a). One sees that the correlation strength decreases with decreasing centrality bin size monotonously. We also use the mixed event method[9] to study the statistical correlation in 0-10, 0-5, and 5% most central Au+Au collisions at √ sNN =200 GeV, the calculated results are shown in Fig. 2(b). One can see that the behavior of correlation strength decreases with decreasing centrality bin size, which not only exist in total correlation but also in statistical one. That is because the charged particle multiplicity fluctuation, which decides the strength of statistical correlation, decreases from 0-10 to 0-5 and to 5% central collisions. If the discrepancy between total and statistical correlations is identified as dynamical correlation, one then sees that the dynamical correlation may just be a few percent of the total correlation and is not dependant on the centrality bin size. Finally, the financial support from NSFC (10635020, 10605040, and 10705012) in China is acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
R. C. Hwa, arXiv:nucl-th/0701053v1. T. K. Nayak, J. of Phys. G 32, S187 (2006); arXiv:nucl-ex/0608021v1. B. K. Srivastavs, STAR Collaboration, Int. J. Mod. Phys. E 16, 3363 (2008). R. E. Ansorge et al., UA5 Collaboration, Z. Phys. C 37, 191 (1988). T. S¨ojstrand, S. Mrenna, and P. Skands, J. High Energy Phys. JHEP05, 026 (2006); arXiv:hep-ph/0603175v1. B. L. Combridge, J. Kripfgang, and J. Ranft, Phys. Lett. B 70, 234 (1977). Yu-Liang Yan, Dai-Mei Zhou, Bao-Guo Dong, Xiao-Mei Li, Hai-Liang Ma, and BenHao Sa, Phys. Rev. C 79, 054902 (2009); arXiv:0903.0915v2[nucl-th]. Ben-Hao Sa and Tai An, Comput. Phys. Commun. 90, 121 (1995); Tai An and BenHao Sa, Comput. Phys. Commun. 116, 353 (1999). Yu-Liang Yan, Bao-Guo Dong, Dai-Mei Zhou, Xiao-Mei Li, and Ben-Hao Sa, Phys. Lett. B 660, 478 (2008); arXiv:0710.2187v2[nucl-th].