correlations and fluctuations from the star experiment

Correlations and Fluctuations from the STAR Experiment

Terence J Tarnowsky

for the STAR Collaboration

January 6, 2010

Winter Workshop on Nuclear Dynamics 2010

Ocho Rios, Jamaica

T. Tarnowsky WWND 2010, JamaicaJanuary 6, 2010

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Motivation Behind Correlations and Fluctuations

• Many theoretical predictions that behavior of correlations and fluctuations in a deconfined phase different than that in hadron gas.

• Justification from experimental studies of the thermodynamics of phase transitions.

• Even w/o such guidance, can search for discontinuities in fluctuations and correlations as functions of incident energy and centrality (not an inclusive list):– Particle ratio fluctuations (K/, p/, K/p).– Forward-Backward multiplicity correlations.– Balance Functions– Net Charge Fluctuations– Net-proton kurtosis, <pT> flucuations, correlations, etc.


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STAR Detector

• STAR is a large acceptance detector.– Good and coverage for

measuring fluctuations.

• || < 1.0• PID:

– K, p ID for pT < 1 GeV. • ToF upgrade will enhance PID

capabilities.

STAR Preliminary


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F-B Multiplicity Correlations• Predicted in context of Dual Parton Model [DPM] (and

Color Glass Condensate [CGC]).• 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 correlation strength.– Function of √s and A.– Coefficient can be expressed as,

ffb bNaNN )(

2

2

22ff

bf

ff

bfbf

D

D

NN

NNNNb

N = # of hadrons


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ηη1

η2

- η1

- η2 0

Forward nfBackward nb

Pseudorapidity Gap

Pseudorapidity interval

High Energy

Long Range Long Range Short + Long Range

Low EnergyStrings

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1tanln

Probes longitudinal characteristics of the system.


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F-B Correlations• Central 200 GeV

Au+Au collisions show a strong long-range correlation.

• Most peripheral Au+Au have negligible LRC, as does pp.

• HIJING and Parton String Model (PSM) do not agree w/ Au+Au data.

• Multiparton interaction in central Au+Au collisions.

• See talk by M. Skoby.

Phys.Rev.Lett.103:172301,2009


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Characterize Fluctuations

• NA49 uses the variable dyn

• Measure deviation from Poisson behavior using dyn

• It has been demonstrated (for K/ and p/) that,

dyn,K NK NK 1 NK

2 N N 1 N

2 2NKN

NK N

dyn sign data2 mixed

2 data2 mixed

2

is relative width of K / distribution

dyn2 dyndyndyn 2


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Excitation Function for σdyn,K/π

STAR central Au+Au (0-5%) collisions with SPS central Pb+Pb collisions (0-3.5%).

• Large decrease in fluctuations as function of energy from NA49.• Fluctuations measured by STAR approximately constant as function of energy from 19.6-200 GeV. • : 0.2 < pT < 0.6 GeV/c

•K: 0.2 < pT < 0.6 GeV/c

Phys.Rev.Lett.103:092301,2009


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Scaling w/ dN/d in Au+Au

Phys.Rev.Lett.103:092301,2009

• Charge dependent and independent dyn,K/ was found to scale linearly with dN/d (at small dn/d) in Au+Au at 200 and 62.4 GeV

Excitation Function for σdyn,pπ

• Solid points are data from STAR or NA49.

• Open black points are HSD prediction from Konchakovski, et. al. arXiv:0906.3229.

• Open red points are UrQMD run locally with STAR acceptance.


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dyn,p/, STAR and NA49

• dyn,p/ displays strong system size dependence for small dN/d.

• Fit is to STAR Cu+Cu 22.4 GeV data only.

• Interpretation still under study.


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Excitation Function for σdyn,Kp

• dyn,K/p for central Cu+Cu 22.4 GeV is strongly negative.

-General agreement w/ NA49 at 17.3 GeV.

-Does not show large increase seen in model predictions.

• dyn,K/p for central Au+Au 200 and 62.4 GeV are also negative.

HSD points from arXiv:0906.3229

NA49 data from CPOD 2009 talk by T. Schuster


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Balance Function

• Prediction that delayed hadronization will cause width of longitudinal charge balance function to be narrower in central vs. peripheral collisions.

• Definition:

– ij is number of charged particles pairs in a (pseudo)rapidity range.

– Calculated by taking ith particle and incrementing a histogram of wrt all jth particles in that event.

NNB

2

1


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Balance Function• Width in 200 GeV Au+Au data narrows with increasing collision centrality.

• Shuffled events and Hijing and URQMD show no centrality dependence.

• Width from pp collisions agrees w/ most peripheral Au+Au bin.

• Results consistent with narrowing of the balance function in most central Au+Au collisions.

STAR Preliminary

STAR Preliminary


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Net Charge Fluctuations

• Predictions that net charge fluctuations due to QGP << Hadron Gas.

• Measured using,

NN

NN

N

NN

N

NNdyn 2

1122,


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Net Charge Fluctuations

• Large differences between pp, Cu+Cu, and Au+Au. Slope depends on correlation length in . Larger slope = shorter correlation length. Agrees w/ balance function results.

• Larger slope indicate correlated pairs of -/+ particles emitted closer in rapidity than those in peripheral Au+Au or pp collisions.

• Could be related to narrowing of balance function, or radial flow.

Phys. Rev. C 79 (2009) 24906


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Search for the QCD Critical Point

•Proposal for future running at RHIC to consist of an “energy scan” to search for predicted QCD critical point.

•Fluctuations and correlations (particle ratios, multiplicity, pT, etc.) and behavior of flow (directed and elliptic) in vicinity of the critical point are expected to be primary signatures.

•STAR has the capability to measure correlations and fluctuations at all energies.


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Search for the QCD Critical Point II

• In a phase transition near a critical point, an increase in non-statistical fluctuations is expected.

• Finite system-size effects may influence fluctuation measurements.

• Finite-size scaling of fluctuations may indicate existence of critical point.

• E.g. Change in behavior of quark susceptibilities.

Aoki, Endrodi, Fodor, Katz, and Szabó Nature 443, 675-678 (2006)

• These may manifest in final-state measurements.


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Critical Fluctuations

• An example of critical fluctuations:

• Mixture of cyclohexane (C6H12) and aniline (C6H7N).


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2008 Test at √sNN = 9.2 GeV


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STAR

• The STAR detector is in a prime configuration for measuring fluctuations and correlations:– Full ToF is installed. Will provide enhanced PID

capabilities.

– Low material budget between beam pipe and TPC.

– DAQ 1000 installed as of Run 9. Not rate limited by DAQ at higher energies of BES.


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Summary• The STAR experiment has results on fluctuations and correlations for several colliding

systems and energies that have provided new insights into particle production.– K/ fluctuations approximately constant at RHIC energies.– First measurement of long-range forward-backward multiplicity correlations provides information on

multiparton interactions.– Narrowing of balance function coincides with shorter correlation lengths as measured from net charge

fluctuations.

• The RHIC Beam Energy Scan (BES) program will probe new areas of the QCD phase diagram while revisiting energies studied at fixed target experiments using a mature collider and well understood detector setup.

– Provide a comprehensive picture of the T-B phase space at the same facility.

• The STAR experiment is fully prepared to capitalize on collisions at all energies during the RHIC BES.

• STAR will continue to produce new results in the future from higher statistics data sets w/ the Time-of-Flight upgrade.

correlations and fluctuations from the star experiment

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