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)
March 4, 2002 John G. Cramer3STAR
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
March 4, 2002 John G. Cramer4STAR
Booster
Ring
AGS
Switchyard
RHIC
TandemVan de Graaff
The RHIC Accelerator SystemThe RHIC Accelerator System
Blue Ring
Yellow Ring
March 4, 2002 John G. Cramer5STAR
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.
March 4, 2002 John G. Cramer6STAR
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.
March 4, 2002 John G. Cramer7STAR
The STAR CollaborationThe STAR Collaboration
March 4, 2002 John G. Cramer8STAR
Run: 1186017, Event: 32, central
colors ~ momentum: low - - - high
Central Au +Au Collision at sNN = 130 GeVCentral Au +Au Collision at sNN = 130 GeV
March 4, 2002 John G. Cramer9STAR
Part 2Part 2
RHIC SurprisesRHIC Surprises
March 4, 2002 John G. Cramer10STAR
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……
March 4, 2002 John G. Cramer11STAR
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?
March 4, 2002 John G. Cramer12STAR
Surprise 1Surprise 1
Event-by-Event Elliptic Flowand Hydrodynamics
Event-by-Event Elliptic Flowand Hydrodynamics
March 4, 2002 John G. Cramer13STAR
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
March 4, 2002 John G. Cramer14STAR
Elliptic Flow and HydrodynamicsElliptic Flow and Hydrodynamics
March 4, 2002 John G. Cramer15STAR
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.
March 4, 2002 John G. Cramer16STAR
Surprise 2Surprise 2
Pion Spectrum Measurements:
Strong Absorption of 2 to 6 GeV/c Pions
Pion Spectrum Measurements:
Strong Absorption of 2 to 6 GeV/c Pions
March 4, 2002 John G. Cramer17STAR
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
March 4, 2002 John G. Cramer18STAR
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
March 4, 2002 John G. Cramer19STAR
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?
March 4, 2002 John G. Cramer20STAR
Surprise 3Surprise 3
Source Radii and Emission Duration fromBose-Einstein Interferometry
Source Radii and Emission Duration fromBose-Einstein Interferometry
March 4, 2002 John G. Cramer21STAR
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!
March 4, 2002 John G. Cramer22STAR
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
March 4, 2002 John G. Cramer23STAR
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).
March 4, 2002 John G. Cramer24STAR
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
March 4, 2002 John G. Cramer25STAR
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
March 4, 2002 John G. Cramer26STAR
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
March 4, 2002 John G. Cramer27STAR
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
March 4, 2002 John G. Cramer28STAR
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?
March 4, 2002 John G. Cramer29STAR
Surprise 4Surprise 4
Particle Spectrum Measurements+
Bose-Einstein Interferometry:
Pion Phase Space Density
Particle Spectrum Measurements+
Bose-Einstein Interferometry:
Pion Phase Space Density
March 4, 2002 John G. Cramer30STAR
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
March 4, 2002 John G. Cramer31STAR
A 3D Correlation HistogramA 3D Correlation Histogram
March 4, 2002 John G. Cramer32STAR
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
March 4, 2002 John G. Cramer33STAR
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.
March 4, 2002 John G. Cramer34STAR
<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
March 4, 2002 John G. Cramer35STAR
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.
March 4, 2002 John G. Cramer36STAR
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
March 4, 2002 John G. Cramer37STAR
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?
March 4, 2002 John G. Cramer38STAR
SummarySummary
What does it all mean?What does it all mean?
March 4, 2002 John G. Cramer39STAR
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.
March 4, 2002 John G. Cramer40STAR
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!