Terence Tarnowsky Long-Range Multiplicity Correlations in Au+Au at Terence J Tarnowsky Purdue...

Terence Tarnowsky

Long-Range Multiplicity Correlations in Au+Au at

Terence J Tarnowsky Purdue University

for the STAR Collaboration

22nd Winter Workshop on Nuclear DynamicsSan Diego, CA

March 12-18, 2006

GeVsNN 200

WWND, 3/12/06, La Jolla, CA 2Terence Tarnowsky

Outline

• I. Motivation

• II. Color Strings/Soft Particle Production Models

• III. F-B Multiplicity Correlations

• IV. Results

• V. Summary

3Terence Tarnowsky

Study of correlations among particles produced in different rapidity regions helps to understand the mechanisms of particle production.

Many experiments show strong positive short-range correlations, indicating clustering of particles over a region of ~ 1 unit in rapidity.

Production of particles in the central rapidity region dominated at all energies by these short-range correlations.

Longer range correlations are observed in h-h interactions only at high energies.

It has been suggested that the long-range correlations might be much stronger in h-A and A-A interactions, compared to h-h scattering at the same energy.

Strong, long-range correlations are an indication of multiple inelastic collisions [1,2].

Motivation

1. Dual Parton Model (DPM): A. Capella et al., Phys. Rep. 236, 225 (1994).2. A. Capella and A. Krzywicki , Phys. Rev. D184,120(1978).


Color Strings

• At low energies, valence quarks of nucleons form strings that then hadronize wounded nucleon model.

• At high energies, contribution of sea quarks and gluons becomes dominant.– Additional color strings formed.– Multiple inelastic parton scattering.


Color Strings

• Created in nuclear collisions.

• Considered effective sources w/ fixed

transverse area, rT ≈ 0.2 fm

• At large string density:• Strings overlap.• Clusters formed.

• Study of cluster dynamics allows calculation of physical observables.

A.Capella, et al. Phys. Report. 236,225(1994)

M.A.Braun and C.Pajares.

Nucl. Phys. B390,542(1993)


Dual Parton Model (DPM)

• Model describing soft hadronic particle processes.

• Particle production proceeds via a Schwinger-type mechanism,– Strings hadronize to produce quark-antiquark

pairs.

• DPM assumes that all strings hadronize independently.

A. Capella et al., Phys. Rep. 236, 225 (1994).

7Terence Tarnowsky

The independence of string fragmentation in models such as DPMis considered a strong assumption.

Since the average number of strings increases with energy: • Simple mechanism of multiparticle production becomes invalid. • Overlapping strings could merge into “ropes” or fuse, leading to F-B correlations very different from the ones predicted in independent string models.

When strings fuse w/o producing long-range correlations, a decrease of F-B correlations compared to independent string picture is expected [1,2,3].

1. N. S. Amelin, N. Armesto, M.A.Braun , E. G. Ferrerio and C. Pajares Phys. Rev. Lett. 73, 2813(1994).2. N. Armesto, M.A.Braun , E. G. Ferrerio and C. Pajares Z. Phys. C67, 489 (1995). 3. M. A. Braun, C. Pajares and J. Ranft Int. J. Mod. Phys. A14, 2689(1999).

Beyond DPM

Can interpret string fusion as an intermediate stage towards QGP formation.


Parton String Model (PSM)

• At high string densities, color fields overlap and fusion can occur.

• Monte Carlo model, based on DPM, but includes string interaction:– Fusion of soft string pairs.

• Fusion leads to:– Reduction in particle multiplicity (≈

30%) compared to Nbin scaling from pp.– <pT> enhancement.

• Conservation of energy-momentum.• Pion <pT> 0.35 GeV/c in pp, 0.44 GeV/c in

Au+Au.

– Both effects seen in RHIC data.

Similar to QGSM, A.B.Kaidalov, Phys. Lett. B116 (1982) 459


F-B Multiplicity Correlations• Predicted in context of DPM.• Test of multiple [partonic] scattering.• Linear expression relating Nb, Nf found in hadron-

hadron experiments (ex. UA5),

• “b” is correlation strength.– Function of √s and A.– Coefficient can be expressed as,

ffb bNaNN )(

2

2

22ff

bf

ff

bfbf

D

D

NN

NNNNb

22 nnLRC exists only if:


Measurement of Long-Range Multiplicity Correlations

• A gap about midrapidity will eliminate effect of short-range correlations (e.g .from clustering, jets, …)– DPM assumes short-range correlations

confined to individual strings.– Long-range correlations due to superposition of

fluctuating number of strings. bfbfbfbf NNnnNNNND 00

222

Fluctuation in # of inelastic collisions


STAR Detector


ηη1

η2

- η1

- η2 0

Forward nfBackward nb

Rapidity Gap

Rapidity interval

High Energy

Long Range Long Range Short + Long Range

Low Energy

Strings


Analysis1. Au+Au at 200 GeV.

2. For Au+Au, eight centrality bins as defined by STAR charged particle reference multiplicity:0-10%, 10-20%, …, 70-80%.

- Primary tracks- || < 0.5- TPC fit points >= 10 - dca < 3 cm

3. Eliminate short-range correlation by considering backward and forward intervals (0.2 units) separated by at least 1.5 pseudorapidity units.

4. The forward region is 0.8 < < 1.0, while the backward region is -1.0 < < - 0.8. Intervals are symmetric with a gap of =1.6.

6. 0.1 < pT < 1.2 GeV, |vz| < 30 cm.


Calculating Dispersion• Calculate <nf>, <nb>, <nf>2, and <nf nb> as functions of STAR

reference multiplicity.

Nch<n f>

<n b>

Nch

STAR Preliminary STAR Preliminary

<n f*

n f>

Nch Nch<n f*

n b>



Calculating Dispersion

• Previous quantities now expressed as function of Nch, eg.

• Use to calculate respective dispersions as function of Nch,

.

),(

,)(

etc

Nnn

Nn

chbf

chf

)()()(

)()()(2

222

chbfchbfchbf

chfchfchff

NnnNnnND

NnNnND


Uncorrected Dispersion Results• Results *before* corrections for TPC

tracking efficiency/acceptance.


• Non-zero long-range correlation!


Binned Results

2

2 2 2

f b f b bf

f f ff

n n n n Db

n n D

STAR Preliminary

STAR Preliminary

STAR Preliminary

• Can now be binned according to STAR minbias centrality cuts .

Results corrected for tracking efficiency & detector acceptance.


Comparison to Independent PSM

• Independent string picture shows:

- Good agreement in peripheral collisions.

- Large discrepancy in central collisions

STAR Preliminary

STAR Preliminary


STAR Preliminary

STAR Preliminary

Comparison to Collective PSM

• PSM w/ string fusion describes RHIC, 200 GeV Au+Au data:

• Multiplicity

• <pt> enhancement.

• Particle ratios.

• Strangeness production.

PSM w/ 2 String Fusion:

• Introduction of collective effect via fusion of strings.

• PSM minbias multiplicity distribution matched corrected data.

Armesto, et. al. Phys.Lett. B527 (2002) 92-98


Further Study

• Determine energy and system size dependence of correlations.– Studies of d+Au and pp done.– 62.4 GeV Au+Au.– 200, 62.4, 22.4 GeV Cu+Cu.

• Rapidity dependence over entire STAR acceptance.– Midrapidity (TPC, -1.0<<1.0)– Forward rapidity (FTPC, 2.8<||<3.8)

• Possible threshold effect?


Summary

• Measurement of long-range correlations in high energy, heavy ion collisions.

• Suppression of correlation strength in Au+Au, compared to the independent string picture, indicates a dynamical reduction in the number of particle sources.

• Overlap and interaction of color strings has been considered to explain this additional collectivity.

• Can interpret long-range correlation results and string fusion as a precursor of high energy density, quark-gluon matter produced in central Au+Au collisions.

Terence Tarnowsky Long-Range Multiplicity Correlations in Au+Au at Terence J Tarnowsky Purdue...

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