# Presentation: 149088

## 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

## Simplistic, multipurpose picture of multiparticle production: first formation of sources, then coherent decay of the sources into particles.

**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

## INTRODUCTION

**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**

## 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).**

**Notes:**
Studied at ISR, SPS experiments...

## η

**η**

**η****1**

**η****2**

**- ****η****1**

**- ****η****2**

**0**

**Forward N****f**

**Backward N****b**

**Rapidity Gap **

**Rapidity interval**

## STAR Detector

- Used for centrality determination.

## Analysis

**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%.       **
**	****	****- |********| < 0.5 (for intervals w/ |******** > 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****     **
**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.**

## Calculating Dispersion...

- By calculating <Nf>, <Nb>, <Nf2>, and <Nf Nb> as functions of STAR reference multiplicity Nch

**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.**

**N****ch**

**<N****f****>**

**STAR Preliminary**

## Results 

- 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. **

## Results 

**STAR Preliminary**

**Results **

**Au+Au**

**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). **

## Results

**STAR Preliminary**

**Results**

- FB correlation strength b, is controlled by forward-backward dispersion      .

**Au+Au **

**pp**

**STAR Preliminary**

## Energy and System size Dependence 

**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**

## Energy and System size Dependence

**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.

## pp at 62, 200 and 400 GeV

**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**

## 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).**

**Notes:**
SRC due to Jets, resonances, HBT corr, cluster formation

## 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**

## 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.

## Summary

**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