1. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 1 STAR's Measurement of Long-range Forward- backward Multiplicity Correlations as the Signature of “Dense Partonic Matter” in Heavy Ion Collisions at Brijesh K Srivastava Purdue University

2. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 2 INTRODUCTION Correlations have always been expected to reflect the features of multiparticle production and the possibility of phase transitions. Simplistic, multipurpose picture of multiparticle production: first formation of sources, then coherent decay of the sources into particles. Correlations in rapidity characterize, in principle, the process of formation and decay of such clusters: how many of them, which size i. e. how many particles do they produce? Particle emission

3. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 3 Multiparticle production at high energies can be described in terms of color strings stretched between the projectile and target. These strings hadronize to produce the observed particles. The no. of strings grow with energy and the no. of participating nucleons. Interaction between the strings must be considered. This problem acquires even more importance considering that, at very high energies, collisions of heavy nuclei at RHIC may produce high density matter. A collective interaction between strings may be required to evolve the system towards a Quark-Gluon Plasma (QGP) state. Ref: M. A. Braun and C. Pajares, Nucl. Phys. B390, 542(1993). M. A. Braun et al., Inter. Jour. Mod. Phys. A14,2689, (1999). H. Satz, Rep. Prog. Phys., 63, 1511(2000). N. Armesto et al., Phys. Lett. B527, 92(2002). INTRODUCTION

4. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 4 FB Correlation Predicted in context of Dual Parton Model (and Color Glass Condensate/Glasma). Test of multiple elementary [partonic] scattering. Linear expression relating N b and N f (forward (and backward multiplicity), found in hadron-hadron experiments (ex. UA5), “b” is the FB correlation strength. –Function of √s and A. –Coefficient can be expressed as, N = # of hadrons A. Capella et al., Phys. Rep. 236, 225(1994). Y. V. Kovchegov et al., Phys. ReV. C63, 024903(2001).

5. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 5 η η1η1 η2η2 - η 1 - η 2 0 Forward N f Backward N b Rapidity Gap Rapidity interval High Energy Short + Long Range Long Range Low Energy Strings

6. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 6 STAR Detector –Used for centrality determination. 6

7. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 7 Analysis 1.Au+Au and pp MB data. 2.For Au+Au, eight centrality bins as defined by STAR charged particle reference multiplicity:0-10%, 10-20%, …, 70-80%. - |  | 0.5) or 0.5 < |  | < 1.0 (for intervals w/ |  < 0.5 or |  < 0.3 + 0.6 < |  | < 0.8 (for  = 1.0) - p T > 0.15 - |v z | < 30 3.Backward and forward intervals are 0.2 units in . Intervals are separated by an increasing gap about midrapidity from  = 0.2 - 1.8, measured from bin centers.  is measured in an absolute coordinate system, where  = 0 is fixed at the primary collision vertex. 7

8. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 8 Calculating Dispersion… By calculating,,, and as functions of STAR reference multiplicity N ch Tracking efficiency and acceptance corrections applied in each event. Method removes the dependence of the FB correlation strength on size of centrality bin. Obtained on event- wise basis as function of event multiplicity. 8 N ch STAR Preliminary

9. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 9 Results corrected for tracking efficiency & detector acceptance. STAR Preliminary Results Au+Au at 200 GeV The correlation strength is almost flat as a function of Δη for all the three centralities All the errors are stat+sys.

10. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 10 STAR Preliminary Results Au+Au Correlation strength for 30-40% centrality shows tendency to fall with Δη For 40-50% the correlation strength decreases with the increase in Δη pp at 200 GeV has similar behavior as 40-50% centrality in Au+Au. This is identified as short range correlation (SRC) FB correlation strength beyond Δη > 1.0 is termed as long range correlation (LRC).

11. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 11 STAR Preliminary Results FB correlation strength b, is controlled by forward- backward dispersion. Au+Au pp STAR Preliminary

12. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 12 STAR Preliminary Energy and System size Dependence Au+Au For details see poster P-176 by Terence Tarnowsky Comparison of LRC in central Au+Au and Cu+Cu shows little difference. LRC still present in Cu+Cu collisions. LRC decreases with energy STAR Preliminary

13. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 13 STAR Preliminary Energy and System size Dependence Comparison of FB correlation strength from pp and Au+Au and Cu+Cu at 200 GeV. The results for Au+Au and Cu+Cu are for mid peripheral collisions. All of them show behavior consistent with SRC.

14. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 14 STAR Preliminary pp at 62, 200 and 400 GeV ? Comparing correlation as function of 1. 200 and 400 GeV in close agreement: * Larger SRC at 400GeV * Plateau at same value of b at large Δη has 200 GeV 2. 62.4 GeV goes smoothly to b = 0. Not even a small LRC. SRC only

15. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 15 Interpretation of F-B Multiplicity Correlations in DPM DPM assumes short-range correlations confined to individual strings. A gap about midrapidity will eliminate effect of short- range correlations (e.g.from clustering, jets, …) –Long-range correlations due to superposition of fluctuating number of strings. In nucleus-nucleus collisions at high energies: A. Capella et al., Phys. Rep. 236, 225(1994).

16. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 16 Comparison with phenomenological models HIJING has only SRC and predicts SRC with a large value of b near midrapidity in agreement with the data. PSM has LRC built in the form of long strings and and is in qualitative agreement with the data. STAR Preliminary

17. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 17 Partonic Core, Hadronic Corona? If A+A collision can be envisioned as a central core due to multiple partonic interactions, surrounded by a hadronic corona. The LRC is built up during partonic scattering: –In central collisions, larger volume of the core implies larger LRC. –In peripheral A+A, smaller (or no) core region  decrease in correlation strength.

18. QM2008 Jaipur, India Feb.4– Feb. 10, 2008 18 Summary  Strong long range correlations are observed for central Au+Au at * Dual Parton Model has been explored to explain the LRC observed in the data. * Both DPM and CGC considerations argue that the long range correlations are due to multiple parton interactions.  Mid-central and peripheral events show relatively weaker long range correlations.  This indicates that dense matter with multi-parton interactions is formed in mid-central and central collisions at