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Brijesh K Srivastava Purdue University

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. INTRODUCTION Correlations have always been expected to reflect the features of

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Brijesh K Srivastava Purdue University

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  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. 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. Particle emission 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?

  3. INTRODUCTION 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 thesystem 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).

  4. FB Correlation Predicted in context of Dual Parton Model (and ColorGlassCondensate/Glasma). Test of multiple elementary [partonic] scattering. Linear expression relating Nb and Nf (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. Strings Low Energy Long Range Long Range Short + Long Range High Energy Backward Nb Forward Nf η - η1 η1 η2 - η2 0 Rapidity interval Rapidity Gap

  6. STAR Detector Used for centrality determination. 6

  7. Analysis • Au+Au and pp MB data. • For Au+Au, eight centrality bins as defined • by STAR charged particle reference • multiplicity: 0-10%, 10-20%, …, 70-80%. • - |h| < 0.5 (for intervals w/ |h| > 0.5) or 0.5 < |h| < 1.0 (for intervals w/ |h| < 0.5 or • |h| < 0.3 + 0.6 < |h| < 0.8 (for Dh = 1.0) • - pT > 0.15 • - |vz| < 30 • Backward and forward intervals are 0.2 units in h. Intervals are separated by an increasing gap about midrapidity from Dh = 0.2 - 1.8, measured from bin centers. • Dh is measured in an absolute coordinate system, where h = 0 is fixed at the primary collision vertex. 7

  8. Calculating Dispersion… By calculating <Nf>, <Nb>, <Nf2>, and <Nf Nb> as functions of STAR reference multiplicity Nch • Obtained on event-wise basis as function of event multiplicity. STAR Preliminary • Tracking efficiency and acceptance corrections applied in each event. • Method removes the dependence of the FB correlation strength on size of centrality bin. <Nf> Nch 8

  9. Results Au+Au at 200 GeV Results corrected for tracking efficiency & detector acceptance. STAR Preliminary The correlation strength is almost flat as a function of Δηfor all the three centralities All the errors are stat+sys.

  10. Results 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) STAR Preliminary Au+Au Au+Au FB correlation strength beyond Δη > 1.0 is termed as long range correlation (LRC).

  11. Results Au+Au STAR Preliminary STAR Preliminary pp FB correlation strength b, is controlled by forward-backward dispersion .

  12. Energy and System size Dependence Au+Au • Comparison of LRC in central Au+Au and Cu+Cu shows little difference. • LRC still present in Cu+Cu collisions. STAR Preliminary STAR Preliminary LRC decreases with energy For details see poster P-176 by Terence Tarnowsky

  13. 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. STAR Preliminary

  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. 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. A. Capella et al., Phys. Rep. 236, 225(1994). In nucleus-nucleus collisions at high energies:

  16. Comparison with phenomenological models PSM has LRC built in the form of long strings and and is in qualitative agreement with the data. HIJING has only SRC and predicts SRC with a large value of b near midrapidity in agreement with the data. STAR Preliminary

  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. 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

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