star surprises from rhic john g. cramer department of physics university of washington john g....

STAR

Surprises from RHICSurprises from RHIC

John G. Cramer

Department of Physics

University of Washington

John G. Cramer

Department of Physics

University of Washington

ColloquiumUW Physics Department

March 4, 2002

ColloquiumUW Physics Department

March 4, 2002

March 4, 2002 John G. Cramer2STAR

Part 1Part 1

About RHIC

(The Relativistic Heavy Ion Collider)

About RHIC

(The Relativistic Heavy Ion Collider)


Brookhaven/RHIC OverviewBrookhaven/RHIC Overview

Systems:

Au + Au

CM Energies:

130 GeV/A

200 GeV/A

1st Collisions:

06/13/2000

Location:

BrookhavenNationalLaboratory,

Long Island,NY

pp


Booster

Ring

AGS

Switchyard

RHIC

TandemVan de Graaff

The RHIC Accelerator SystemThe RHIC Accelerator System

Blue Ring

Yellow Ring


What does RHIC do?What does RHIC do?

RHIC accelerates gold nuclei in twobeams to about 100 Gev/nucleon each(i.e., to kinetic energies that are over100 times their rest mass-energy)and brings these beams into a200 GeV/nucleon collision. Four experiments, STAR,PHENIX, PHOBOS, andBRAHMS study these collisions. In the year 2000 run, RHICoperated at a collision energyof 130 Gev/nucleon. In 2001-2 it operated at 200 GeV/nucleon.


About the STAR Detector.About the STAR Detector.

ZCl

Barrel EM Calorimeter

Endcap Calorimeter

MagnetCoils

TPC Endcap & MWPC

ZCal

FTPCs

Vertex Position DetectorsCentral Trigger Barrel or TOF

Time Projection Chamber

Silicon Vertex Tracker

RICH

STAR is a large solenoidaldetector based on a time-projection chamber. Ituses a 0.5 tesla magneticfield to momentum-analyzeabout 2,000 charged particlesper collision.


The STAR CollaborationThe STAR Collaboration


Run: 1186017, Event: 32, central

colors ~ momentum: low - - - high

Central Au +Au Collision at sNN = 130 GeVCentral Au +Au Collision at sNN = 130 GeV


Part 2Part 2

RHIC SurprisesRHIC Surprises


In Search of the Quark-Gluon Plasma (QGP)

In Search of the Quark-Gluon Plasma (QGP)

A QGP should have more degrees of freedom than a pion gas.

Entropy should be conserved during the fireball’s evolution.

Hence, look in phase space for evidence of:

Large size, Long lifetime, Extended expansion……


Surprises from RHICSurprises from RHIC

1. Relativistic hydrodynamic calculations work surprisingly well, while cascade string-breaking models have problems. Near-threshold QGP behavior is not observed.The “Hydro Paradox”.

2. There is evidence for strong “quenching” of high momentum pions.QGP Absorption?

3. The ratio of the HBT radii Rout/Rside is ~1, while the closest model predicts 1.2, and most models predict 4 or more.In essence, all models on the market have been falsified.The “HBT Puzzle”

4. The pion phase space density is much larger than that observed at CERN or predicted by simple thermal models.A pion chemical potential ~ 50 MeV is needed to explain it.Stimulated emission of pions?


Surprise 1Surprise 1

Event-by-Event Elliptic Flowand Hydrodynamics

Event-by-Event Elliptic Flowand Hydrodynamics


Elliptic Flow and V2Elliptic Flow and V2

Sensitive to initial/final conditions and equation of state (EOS) ! coordinate-space-anisotropy momentum-space-anisotropy

y

x

py

px

22

22

xy

xy )(tan,2cos 12

x

y

p

pv


Elliptic Flow and HydrodynamicsElliptic Flow and Hydrodynamics


The Hydrodynamic ParadoxThe Hydrodynamic Paradox

The system behaves as if it hasreached thermodynamic equilibrium.

How could there be enough time (in~10 fm/c) for the system to come to thermalequilibrium, as relativistic hydrodynamicsassumes?

Quantum effects? Perhaps the multiparticlewave function collapses into a maximumentropy state => TD equilibrium.

The system behaves as if it hasreached thermodynamic equilibrium.

How could there be enough time (in~10 fm/c) for the system to come to thermalequilibrium, as relativistic hydrodynamicsassumes?

Quantum effects? Perhaps the multiparticlewave function collapses into a maximumentropy state => TD equilibrium.



Pion Spectrum Measurements:

Strong Absorption of 2 to 6 GeV/c Pions

Pion Spectrum Measurements:

Strong Absorption of 2 to 6 GeV/c Pions


Gedankenexperiments: + QGP or HGGedankenexperiments: + QGP or HG

High momentum pion beam Lower momentum pionsQGP

High momentum pion beamHadron

gas

High momentum pions

(Transparent)

(Opaque)

Target


High-Momentum Absorption (1)High-Momentum Absorption (1)

Au+Au

p+p

Preliminary

Scales approximately A2 at high pT.

(h+ + h-)/2

Syst. errors from UA1 extrapolation

MinBias/ UA1


High-Momentum Absorption (2)High-Momentum Absorption (2)

• Suppression factor ~2

• Systematic errors from UA1 extrapolation from 200 to 130 GeV Central/ UA1

Conclusion: Central RHIC Au+Au collisions show strongabsorption of high energy pions that is not observed in Pb+Pbcollisions at the CERN SPS or in less central collisions at RHIC.Smoking gun for QGP?



Source Radii and Emission Duration fromBose-Einstein Interferometry

Source Radii and Emission Duration fromBose-Einstein Interferometry


The Hanbury-Brown-Twiss EffectThe Hanbury-Brown-Twiss Effect

y

X

1

2

Source

Neglects Momentum dependence of source• Quantum mechanics up to x and y Final State Interactions after x and y

Nonetheless C2(q) contains shape information True component-by-component in q

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

For non-interacting identical bosons:

S(x,p)=S(x)S(p)

Coherent interference between incoherent sources!


Bertsch-Pratt Momentum CoordinatesBertsch-Pratt Momentum Coordinates

beam direction

p1 p2

Q T

Q

Q L

beam direction

p2p1

Q T

Q S

Q O

)qqR2qRqRqRexp(1 longout2ol

2long

2long

2side

2side

2out

2out

)q,q,q(C longsideout

)T2

PT1

P(2

1T

K


A Bose-Einstein Correlation “Bump”A Bose-Einstein Correlation “Bump”

This 3D histogramhas been corrected forCoulomb repulsion ofidentical pairs andis a projection slice nearqlong=0 .

The “bump” results fromBose-Einstein statistics ofidentical pions (J=0).


Expectations: Pre-RHIC HBT Predictions

Expectations: Pre-RHIC HBT Predictions

“Naïve” picture (no space-momentum correlations):

Rout2 = Rside

2+(pair)2

One step further: Hydro calculation of Rischke &

Gyulassy expects Rout/Rside ~ 2->4 @ kt = 350 MeV.

Looking for a “soft spot” Small Rout/Rside only for

TQGP=Tf (unphysical)).

Rout

Rside


Reality: STAR/RHIC HBT Measurements

Reality: STAR/RHIC HBT Measurements

• ~10% Central AuAu(PbPb) events

• y ~ 0

• kT 0.17 GeV/c

No significant increase in spatio-temporal size of the emitting source at RHIC.

Note the ~100 GeV gap fromSPS to RHIC and the gapbetween AGS and SPS data.

Ro/Rs ~ 1


Conclusion: Transverse Size ~ Constant vs. Energy

Conclusion: Transverse Size ~ Constant vs. Energy

Rout and Rside are energy independent within error bars.

Smooth energy dependence in Rlong

No immediate indication of very different physics

Fit Rlong to:

AGS: A = 2.19 +/- .05

SPS: A = 2.90 +/- .10

RHIC: A = 3.32 +/- .03

Tm

A

A = 0T in 1st order T/mT calculation

-

M. Lisa et al., PRL 84, 2798 (2000)R. Soltz et al., to be sub PRCC. Adler et al., PRL 87, 082301I.G. Bearden et al., EJP C18, 317 (2000)

0 = average freeze-out timeT = freezeout temperature


RO/RS: STAR and PHENIX Agree, Models Fail.

RO/RS: STAR and PHENIX Agree, Models Fail.

Compiled by S. Johnson

STAR and PHENIX agree

Best hydro model does not reproduce the data


Remedies for RHIC HBT Puzzle?Remedies for RHIC HBT Puzzle?

Problems: Ro/Rs (and implied emission duration) are too small, implying near-instantaneous emission.

Rl is also uncomfortably small, calling into question Bjorken “boost invariance”.

Solutions?: Allow single “avalanche” freezeout: tPT=tCF=tF?

Abandon outside-in freezeout scenario? Assume some mysterious energy-loss process at hottest part of collision fireball?

Abandon boost invariance?



Particle Spectrum Measurements+

Bose-Einstein Interferometry:

Pion Phase Space Density

Particle Spectrum Measurements+

Bose-Einstein Interferometry:

Pion Phase Space Density


2D Fit to Pion Spectrum (only)2D Fit to Pion Spectrum (only)

We can do a global fit of the uncorrectedpion spectrum vs. centrality by:

(1) Assuming that the spectrumhas the form of a Bose-Einsteindistribution:d2N/mTdmTdy=A/[Exp(E/T) –1]and

(2) Assuming that A and T have aquadratic dependence on thenumber of participants :

A(p) = A0+A1+A22

T(p) = T0+T1+T22

Value ErrorA0 31.1292 14.5507A1 21.9724 0.749688A2 -0.019353 0.003116T0 0.199336 0.002373T1 -9.23515E-06 2.4E-05T2 2.10545E-07 6.99E-08

STAR Preliminary


A 3D Correlation HistogramA 3D Correlation Histogram


Pion Phase Space Density at Pion Phase Space Density at MidrapidityMidrapidity

Pion Phase Space Density at Pion Phase Space Density at MidrapidityMidrapidity

The Lorentz scalar phase space density f(mT) is the dimensionless average number of pions per 6-dimensional phase space cell 3. At midrapidity f is given by the expression:

LOS

3

TT

2

πT RRR

πλ

ymmπ2

N

λ

1

E

1)m(

)( cdd

df

Momentum Spectrum HBT “volume”PurityJacobian

Average phasespace density


Momentum VolumeMomentum Volume

LOS

3

RRR

πλv

)( cmom

lsolsomom dqdqdqqqqC 1),,(v

The momentum volume can be determinedin two ways:

(1) Fit the correlation function with a 3DGaussian and use the fit parameters toestimate the momentum volume vmom,

(2) Direct summation of the 3D histogram channels.

Method (1) is traditional, but Method (2) is less model-dependentand gives the best statistical accuracy.


<f> from Direct Histogram Sums<f> from Direct Histogram Sums

0.1 0.2 0.3 0.4pT

0.1

0.2

0.3

0.4

0.5<

f>

STAR Preliminary


Tomasik & Heinz PSD PaperTomasik & Heinz PSD Paper

The longitudinal expansion hasreduced the phase space density andbroken the rule that the PSD goesto a Bose-Einstein distributionwhen t=pt=0 (no flow).

The reduction in the PSD leads toa need for a non-zero chemicalpotential 0 to reach high enoughPSD values to match RHIC/STARobservations.

Notice that there is a “sweet spot”near pT=0.1 GeV/c at which <f>is independent of t.


0 0.1 0.2 0.3 0.4 0.5 0.6mT- mp

1

5

10

50

100

500

Ndm Tmd T

yd

Parameters from bestHm0,h,TLfits to PSD

T&H Fit to Pion SpectraT&H Fit to Pion Spectra

Because the longitudinal expan-sion reduces the phase space density,a non-zero chemical potential isrequired to reproduce the mostcentral data.

Pion phase space density dependson and T in essentially the sameway, changing the PSD strength butnot its shape. However, the spectrumslope has very different dependenceson and T, breaking this ambiguity.

Therefore, fitting PSD and spectratogether constrains the parameters.However, the lowest curves wouldprefer a negative -value toreproduce the spectrum slope whilefitting the PSD.

STAR Preliminary


T&H Fit to STAR Phase Space Density (HBT)

T&H Fit to STAR Phase Space Density (HBT)

0.1 0.2 0.3 0.4 0.5pT

0.2

0.4

0.6

0.8

1

<f>

STAR Preliminary

Phase space density ~ 1Multiparticle and laser-likestimulated emission effects?


SummarySummary

What does it all mean?What does it all mean?


Conclusion (1)Conclusion (1)

The theoretical models ofRHIC physics now on themarket allow the source toexpand for too long, so thatthe theoretical predictions“outrun” the boundaries ofexperimental observation.

Something is seriously wrongwith our understanding of thedynamics of RHIC collisions.


Conclusion (2)Conclusion (2)

The useful theoretical models that has served us so well at the AGSand SPS for heavy ion studies have now been overloaded with a largevolume of puzzlingnew data from RHIC,and things are a bitup in the air.

We need moretheoretical helpand more experi-mental data to meetthe challenge ofunderstanding whatis going on in theRHIC regime.

It’s a very excitingtime for us STARexperimentalists!

star surprises from rhic john g. cramer department of physics university of washington john g....

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