Zimanyi2010noAni Mitchell

8/8/2019 Zimanyi2010noAni Mitchell

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First Results from the RHICBeam Energy Scan Program

Jeffery T. MitchellBrookhaven National Laboratory


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Outline

The QCD Phase Diagram

The RHIC Beam Energy Scan Program

The Matter Created at the top RHIC Energy

Experimental Challenges at Low Energies

Recent Results from the Beam Energy Scan

Jeffery T. Mitchell - Zimnyi Winter School / Ortvay Colloquium - 12/2/10 2


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Phases of Nuclear Matter

3

The original goal of therelativistic heavy ion programwas to create matter at highenough temperature andpressure that nucleons and

mesons would decouple intoquarks and gluons. Effectively,we strive to recreate theconditions immediately after theBig Bang.

Jeffery T. Mitchell - Zimnyi Winter School / Ortvay Colloquium - 12/2/10


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The Phase Diagram of H2OThere are many similarities between the phase diagramsof H2O and QCD

4

First order phasetransition

Critical Point

Crossover



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A Schematic Phase Diagram of QCD

SB(T) 2

30(Nbosons 7/8 Nfermions)T

4

5

There are many similarities between the phase diagramsof H2O and QCD

First orderphasetransition

Critical Point

Crossover



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QCD Phase Transition

6

QuantumChromodynamics (QCD)predicts a strong increasein the energy density ata critical temperature of

Tc~170 MeV.

There is a phasetransition from hadronicto partonic matter(quarks, gluons) at a

critical energy density of0~1 GeV/fm3

Z. Fodor et al., PLB 568 (2002) 73.



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But, there is much room forexperimental input

M. Stephanov hep-lat/0701002

Large sensitivityto model inputs(such as quarkmasses), latticesizes, and otherassumptions

7

Each black andgreen pointrepresents thecritical pointlocation from

differentcalculations.



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Search for the QCD Critical Point:Experimental Strategy

By systematicallyvarying the RHIC beam

energy, heavy ioncollisions will be ableto probe differentregions of the QCDphase diagram.

8Jeffery T. Mitchell - Zimnyi Winter School / Ortvay Colloquium - 12/2/10


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Simulated Collision Trajectories

9

Shown are locations

on the phasediagram forcollisions atdifferent energiestaken at varioustimes after the

collision occurs.

The simulation isfrom Ultra-Relativistic QuantumMolecular Dynamics(UrQMD).



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How big is the target?

10

M.Asakawa et al.,PRL 101,122302(2008)

From a hydrodynamicscalculation.

For a given chemicalfreeze-out point, 3isentropic trajectories(s/nB=constant) areshown.

The presence of thecritical point can deformthe trajectories describingthe evolution of theexpanding fireball in the(T,B) phase diagram.

A large region can beaffected, so we do notneed to hit the criticalpoint precisely.



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Energy Scan Results from the SPS

11

Discontinuities in the average K/ ratio (the horn) and the kaon spectraslope (the step) are observed.



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Enter the Beam Energy Scan Program atthe Relativistic Heavy Ion Collider



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The Relativistic Heavy Ion Collider (RHIC)


f


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RHIC Specifications


A llid i ll f b


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A collider is an excellent for a beamenergy scan

The detector occupancy in acollider is much less dependenton beam energy than in a fixedtarget accelerator. The acceptanceis independent of beam energy.

Fixed targetCollider


Th RHIC B E S P


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The RHIC Beam Energy Scan Program:Overview

Species: Gold + Gold

Collision Energies [sqrt(sNN)]:200 GeV, 130 GeV, 62.4 GeV, 39 GeV, 19.6 GeV,18 GeV (2011), 11 GeV (STAR only)9.2 GeV (short test run), 7.7 GeV

Species: Copper + Copper

Collision Energies [sqrt(sNN)]:200 GeV, 62.4 GeV, 22 GeV

Species: Deuteron + Gold

Collision Energies [sqrt(sNN)]:200 GeV

Species: Proton + ProtonCollision Energies [sqrt(sNN)]:

500 GeV, 200 GeV, 62.4 GeV

Coming Soon:Uranium + Uranium



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200 GeV Au+Au Event Displays

A head-on collision produces hundreds of particle tracks inthe detector.


Wh i h i i i l ?


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What is the initial temperature?

Hot matter emits thermal radiation, so thetemperature can be measured from theemission spectrum of thermal photons.



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S f Ph


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Sources of Photons

quark gluon

g

pQCD direct photons

from initial hardscatteringof quarks andgluons

g

r

Thermal photonsfromhadron gasafter

hadronization

g

g

Decay Photonsfrom hadrons(0, h, etc)

background

Thermal photons

from hot quark gluonplasma


Di t Ph t S t (PHENIX)


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Direct Photon Spectra (PHENIX)

pQCD consistent with p+pdata down to pT=1GeV/c

Au+Au data lie above Ncoll-scaled p+p data for pT < 2.5GeV/c

Fitting the excess data yields:

TAuAu = 221 19stat 19syst MeV Lattice QCD predicts a phase

transition to quark gluonplasma at Tc~ 170 MeV

The initial temperature isabove the predicted criticaltemperature.

exp + TAA scaled pp

NLO pQCD (W. Vogelsang)Fit to pp

A. Adare et al.,PRL accepted


Elli ti Fl


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Elliptic Flow

v2>0: in-plane emission of particlesv2


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Elliptic Flow: Excitation Function

There is atransition fromsqueeze-out

flow to in-plane flow atAGS energies



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Elliptic Flow Indicates that thematter behaves like a perfect fluid!

STAR 200 GeV Au+Audata (black dots) withvarioushydrodynamicalcalculations overlayed.

The system cannot bedescribed or simulatedunless the followingcondition is set:


S li f Elli ti Fl


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Scaling of Elliptic Flow

Phys. Rev. Lett.98, 162301

(2007)

Mesons

Baryons

Quark-Like Degrees of Freedomare Evident


H d


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HadronSuppression

26

The medium isextremely opaque

near away

RAA(pT) d2

NAA

/ dpTdhNbinary d

2Npp / dpTdh



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Summary of RHIC 200 GeVAu+Au Collision Results

The matter is very hot with a temperature well abovethe expected critical temperature for a phasetransition to a Quark-Gluon Plasma

The matter behaves like a strongly interactingperfect fluid

The matter is described by quark degrees of freedom

The matter is very opaque


Triggering at Low Energy


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Triggering at Low EnergyThe problem:

The placement of thetrigger detectors (BBCs)are not optimized for lowenergy running.They have a reducedacceptance, especially

below RHIC energies of ~20 GeV.

Fermi motion to therescue!

At low energies, Fermimotion is enough tobring nucleons back intothe BBC acceptance.


Beam Quality at Low Energy


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Beam Quality at Low Energy

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The problem:The quality of the

RHIC beam tuningdegrades at lowerenergies. The beamposition is difficult tomonitor.

Each point representsthe vertex coordinateof an eventreconstructed fromthe tracks in theSTAR TPC.

Due to the lowerquality beam tune,some backgroundexists.



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STAR 7 7 GeV Au+Au Event Displays


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STAR 7.7 GeV Au+Au Event Displays

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Beam + Beam pipe collision

Downstream collision


PHENIX 7 7 GeV Au+Au Event Displays


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PHENIX 7.7 GeV Au+Au Event Displays


STAR Particle Identification:


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STAR Particle Identification:7.7 GeV Au+Au

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Excellent Particle Identification Demonstrated


STAR 7 7 G V P i l Id ifi i


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STAR 7.7 GeV Particle Identification


PHENIX 0 Yields


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PHENIX 0 Yields

Enhanced pT reach

(Run-10)

Previous pT reach

(Run-4)

62 GeV

39 GeV


PHENIX Dilepton Expectations at 39 GeV


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PHENIX Dilepton Expectations at 39 GeV

mee

1/Nevtd

N/dmee

36

How does the dilepton excess and modification at SPSevolve into the large low-mass excess at RHIC?

200M simulated events in 20cm vertex

If excess is the same at 39 GeVas 200, expect a 6 result

Black: simulation withsame enhancement asat 200 GeVBlue: no enhancement



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Experimental Approaches to theBeam Energy Scan

1. Search for the onset of deconfinement. Breakdown of the constituent quark number scaling of

elliptic flow Disappearance of hadron suppression in central

collisions

Local parity violation

2. Search for direct signals of the critical point and/or phasetransition.

Fluctuation measurements Measurements of higher moments (kurtosis) Excitation functions



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Searching for the Onset of

Deconfinement


Charged Particle Multiplicity


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Charged Particle Multiplicity

The dN/dh perparticipant pair

at mid-rapidityin central heavyion collisionsincreases withln s from AGSto RHIC

energies

The sdependence isdifferent for pp

and AAcollisions


l l d ( )


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Multiplicity at 7.7 and 39 GeV (Raw)


Identified Particle Yields


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Identified Particle Yields

Identified particle yields areconsistent with measurementsat the SPS.

41

Pions

Protons

Kaons


Identified Particle Ratios


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Identified Particle Ratios

pbar/p


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Exploring the Horn

RHIC results so far are consistent with the SPS results


Directed Flow (v1)


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Directed Flow (v1)Directed flowdescribescollective

sideways motion.

Directed flow issensitive to theEquation of State.Expect non-linear

behavior nearmid-rapidity neara 1st-order phasetransition..

ybeam at 200 GeV = 5.4



The 9.2 GeV data show a different trendcompared to the 200 and 62 GeV data. Results at7 GeV coming soon.


Elliptic Flow vs Beam Energy


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Elliptic Flow vs. Beam Energyand System Size

45

The scaling of elliptic flow persistsin 62.4 GeV Au+Au collisions. Dataat lower energies coming soon.Jeffery T. Mitchell - Zimnyi Winter School / Ortvay Colloquium - 12/2/10

Elliptic Flow at 39 GeV


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Elliptic Flow at 39 GeV

46

No surprises at these energies.


Elliptic Flow at 9 2 GeV


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Elliptic Flow at 9.2 GeV


Pion Interferometry


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Pion Interferometry

48

STAR 9.2 GeV: Phys. Rev. C81(2010) 024911.

x

adapted from

Annu. Rev. Nucl. Part. Sci. 200555:357-402Detector


Freeze-out Volume from Pion Interferometry


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Freeze out Volume from Pion InterferometryA freeze-out volumecan be extracted fromthe interferometryresults:

Vfo = R2sideRlong

A minimum in thisquantity exists at ~7GeV

9 GeV results areconsistent with SPSresults.

The particle emission lifetime is

related to:

t Rout/RsideAn increase is expected nearthe critical point. This is notobserved.



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Local Parity Violation


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Local Parity Violation

Under a strong magnetic field,

when the system isdeconfined and chiralsymmetry is restored, localfluctuations may lead to parityviolation.

Experimentally, look forseparation of the charges inhigh energy nuclear collisions.

Searching for thedisappearance of this effect

can signal the location of thephase boundary.

D.E. Kharzeev et al., NPA 803 (2008) 227.K. Fukushima et al., PRD 78 (2008) 074033.


Local Parity Violation Results


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Local Parity Violation Results

52

The signal is consistent with local parity violation in 200 GeV and62.4 GeV Au+Au collisions. Lower energy results are coming soon.


The Mach cone feat re


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The Mach cone feature

STAR : Phys. Rev. Lett. 102, 052302 (2009)

QM09 : Teiji Kunihiro

From a model withrelativistic dissipativehydrodynamics couplingdensity fluctuations tothermal energy. Thermallyinduced density fluctuationsand sound modes getsuppressed at the criticalpoint. The disappearance ofthe mach cone featurecould be a signature of thecritical point.

53

PHENIX arXiv:0801.4545


The Ridge Feature


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The Ridge Feature

There is a ridge feature that

extends in pseudorapidity. It persistsat 62 GeV.



55/95

Searching for Signals of the

Critical Point and PhaseTransition


Antiproton/Proton Ratio vs. pT


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Antiproton/Proton Ratio vs. pT

Fitting Procedure :y = a (mT - m) + b

Observable proposed as a signature offocusing near the critical point.No large drop in ratio observedfor intermediate p

T

range

Phys. Rev. C73, 044910 (2006)

Phys.Rev. C78, 034918 (2008)

STAR : PLB 655, 104 (2007)PRL 97, 152301 (2006) STAR


Divergent Quantities at the Critical Point


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57

Divergent Quantities at the Critical PointNear the critical point, several properties of a system diverge. The rateof the divergence can be described by a set of critical exponents. For

systems in the same universality class, all critical exponent valuesshould be identical.

The critical exponent for compressibility, g:g )(

C

cT

T

TTk

The critical exponent for heat capacity, a:g )(0

C

c

T

T

TTT

kk a )(

C

cV

TTTC

The critical exponent for correlation functions, h: )2()(h dRRC

(d=3)

g

)(0

C

c

T

T

T

TT

k

k n

)( C

c

T

TT The critical exponent for correlation length, n:


Susceptibilities at the Critical Point


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58

pConsider quarksusceptibility, cq at thecritical point.

cq = = n(T,m)/m

This is related to theisothermal compressibility:

kT = cq(T,m)/n2(T,m)

In a continuous phasetransition, kT diverges at

the critical point

B.-J. Schaefer and J. Wambach, Phys. Rev. D75

(2007) 085015.

g )(C

cT

T

TTk

TBNBD

N kV

Tkk

mm

m 1

2

Grand Canonical Ensemble


Multiplicity Distributions (PHENIX)


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59

Multiplicity Distributions (PHENIX)200 GeV Au+Au

62.4 GeV Au+Au

200 GeV Cu+Cu 62.4 GeV Cu+Cu

22.5 GeV Cu+CuRed lines

represent the NBDfits. The

distributions havebeen normalizedto the mean and

scaled forvisualization.Distributionsmeasured for

0.2


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60

Multiplicity Fluctuation ResultsNear the critical point, the multiplicity fluctuations should exceed thesuperposition model expectation No significant evidence for criticalbehavior is observed. Low energy results coming soon.


Fluctuations Excitation Function


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pT

a

)(C

c

V

T

TTC

Fluctuations in mean pT arerelated to the criticalexponent a:

No increase in fluctuations have beenobserved. Results at 7 and 39 GeV are coming

soon.

SpT = (event-by-event pTvariance) [(inclusive pT

variance)/(meanmultiplicity per event)],normalized by theinclusive mean pT.Random = 0.0.

SpT is the mean of thecovariance of all particlepairs in an eventnormalized by theinclusive mean pT.

SpT can be related to theinverse of the heatcapacity.


Like-Sign Pair Azimuthal Correlations


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62

g

62 GeVAu+Au,0-5%Central

200 GeVAu+Au,0-5%Central

PHENIXPreliminary PHENIXPreliminary

0.2 < pT,1 < 0.4 GeV/c, 0.2 < pT,2 < 0.4 GeV/c, |Dh|


63/95

Searching for the Critical Point with


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64

gHBT Qinv Correlations

T. Csorgo,S. Hegyi,T. Novk,W.A.Zajc,

Acta Phys. Pol. B36 (2005) 329-337

a = Levy index of stability = ha = 2 for Gaussian sourcesa = 1 for Lorentzian sources

This technique proposes to searchfor variations in the exponent h. The exponent h can be extracted byfitting HBT Qinv correlations with a

Levy function:C(Qinv) = l exp( -|Rq/hc|a)

Measure a as a function of collisionenergy and look for a change fromGaussian-like sources to a sourcecorresponding to the expectationfrom the universality class of QCD.


Fluctuations of Particle Ratios


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65

K/

p/

arXiv: 0901.1795

Results consistent with previous NA49measurements.

The event-by-event fluctuations of thekaon/pion ratio are expected toincrease at the critical point.


Measuring skewness or kurtosis


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g

Skewness describes the asymmetry of the distribution Kurtosis describes the peakness of the distribution

For a Gaussian distribution, skewness and kurtosis are both zero. These measures are ideal for measuring non-Gaussian fluctuations.


Kurtosis of proton/antiproton ratios


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67

STAR: PRL 105(2010) 022302,aXiv:1004.4959

No large non-Gaussian fluctuations seen.Lower energy results coming soon.


Summary and Outlook


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y At the top RHIC energies, the created matter is a hot, opaque, stronglyinteracting perfect fluid described by quark degrees of freedom.

The location of the phase transition and QCD critical point remains anopen question.

RHIC has embarked on an beam energy scan program to search for theQCD critical point.

In 2010, RHIC executed a very successful run covering beam energies of62.4, 39, 11, and 7.7 GeV.

First RHIC low energy results are consistent with measurements atsimilar energies from the SPS. So far, no clear signs of the critical point.

RHIC plans to run at 18 GeV in the upcoming run that will start inJanuary 2011.

Results have only just started to appear. We are looking forward to manynew results in the coming months!



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Auxiliary Slides


Statistical Models: Estimating the Freeze-out


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Temperature

AuAu - s=130 GeV

Minimum ofc2 for: T=1665 MeV mB=3811 MeV

A. Andronic et al., Nucl. Phys. A772 (2006) 167.

Basic assumption: System is described by a grand canonical ensemble ofnon-interacting fermions and bosons in thermal and chemical equilbrium


Statistical Model Fits


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neutron star

Baryonic Potential mB [MeV]

early universe

ChemicalTemperatureTch

[MeV]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

AGS

SIS

SPS

RHIC

quark-gluon plasma

hadrongas

deconfinementchiral restauration

LatticeQCD

atomic

nuclei

For s > 10 GeV , chemical freeze-out very close to phase boundary

Extracted T & B values



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Statistical Model Results

Results from differentbeam energiesAnalysis of particle yieldswith statistical models

Freeze-out points reach QGPphase boundary at top SPSenergies


Direct Photons: Comparisons to TheoryH d d i l d l


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Hydrodynamical models arecompared with the data

D.dEnterria &D.Peressounko

T=590MeV, t0=0.15fm/cS. Rasanen et al.

T=580MeV, t0=0.17fm/c

D. K. Srivastava

T=450-600MeV, t0

=0.2fm/c

S. Turbide et al.

T=370MeV, t0=0.33fm/c

J. Alam et al.

T=300MeV, t0=0.5fm/c

F.M. Liu et al.

T=370MeV, t0=0.6 fm/c

Hydrodynamical modelsagree with the data within afactor of ~2

A.Adare et al.arXiv:0912.0244


Temperature fit summary


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p y

Significant yield of the exponential component (excess over the scaledp+p)

The inverse slope TAuAu = 2211919 MeV (>Tc ~ 170 MeV) p+p fit funciton: App(1+pt

2/b)-n

If power-law fit is used for the p+p spectrum, TAuAu = 24021 MeV

Lattice QCD predicts a phase transition to quark gluon plasma atTc ~ 170 MeV


Direct Photons: Initial temperature


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p

From data: T in i > TAuAu ~ 220 MeVFrom models: T in i = 300 to 600 MeV for t0 = 0.15 to 0.6 fm/cLattice QCD predicts a phase transition to quark gluon plasma at Tc ~170 MeV

TC from Lattice QCD ~ 170 MeVTAuAu(fit) ~ 220 MeV

A.Adare et al.arXiv:0912.0244


PHENIX DetectorCentral Arm Tracking


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Central Arm Tracking

Drift Chamber

Pad Chambers

Time Expansion Chamb.

Muon Arm Tracking

Muon Tracker

Calorimetry

PbGl

PbSc

MPC

Particle Id

Muon IdentifierRICH, HBD

TOF E & W

Aerogel

TEC

Global Detectors

BBC

ZDC/SMD Local Polarim.

Forward Hadron Calo.

RXNP

DAQ and Trigger System

Online Calib. & Production

VTXReplaces HBDMuon Trigger:

mTr FEERPC station 3


Elliptic Flow: Excitation Function


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p

77

There is atransition fromsqueeze-out

flow to in-plane flowbetween AGSand SPSenergies



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PHENIX 39 GeV Au+Au Event Displays


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STAR 9.2 GeV Au+Au Event Displays


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Multiplicity Measurements!


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central

central

peripheralperipheral

energy s81Jeffery T. Mitchell - Zimnyi Winter School / Ortvay Colloquium - 12/2/10

STAR 39 GeV Particle Identification


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J/: analyzed 25% of 62 GeV


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J/ ystatistics

per semi cen

83 Recombination

(e.g. Rapp et al.)J/ yield at 200 GeV isdominantly fromrecombination

Predict suppression

greater at 62 GeVJ/ yield down by 1/3Recombination down

1/10

600 M min. bias events 500 J/ measure J/ suppression

Key test of recombination!


CLAN ModelA Giovannini et al Z Phys C30 (1986) 391


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84

The CLAN model was developed toattempt to explain the reason thatp+p multiplicities are described by

NBD rather than Poissondistributions.

Hadron production is modeled asindependent emission of a numberof hadron clusters, Nc, each with amean number of hadrons, nc.

These parameters can be relatedto the NBD parameters:

Nc = kNBD log(1 + ch/kNBD) and = (ch/kNBD)/log(1 +ch/kNBD).

A+A collsions exhibit weakclustering characteristics,independent of collision energy.

A. Giovannini et al., Z. Phys. C30 (1986) 391.


Enhancement of low pT particles?


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Velocity of Sound in the Medium


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Quark Number Susceptibilities


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Q p


STAR fluctuations


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Jeffery T. Mitchell - Zimnyi 2010 Winter School - 12/2/10 89


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Fluctuationsa

)(C

c

V

T

TTC


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Jeffery T. Mitchell - Zimnyi 2010 Winter School - 12/2/10 91

Above Npart~30, the data can be described by a power law in Npart,independent of the pT range down to 0.2


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PHENIX PRELIMINARY



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Mach Cone at the SPS


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Mach Cone at the SPS

f = -900

Wave Energy at Circle BoundaryPb-Au 17.3 GeV 0-5%

CERESPreliminary

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Zimanyi2010noAni Mitchell