Surprises from RHIC STAR John G. Cramer Department of Physics University of Washington Colloquium UW Physics Department March 4, 2002
Download ReportTranscript Surprises from RHIC STAR John G. Cramer Department of Physics University of Washington Colloquium UW Physics Department March 4, 2002
STAR
Surprises from RHIC John G. Cramer
Department of Physics University of Washington Colloquium UW Physics Department March 4, 2002
Part 1
About RHIC (The Relativistic Heavy Ion Collider)
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March 4, 2002 2 John G. Cramer
Brookhaven/RHIC Overview
Systems:
Au + Au
p
p
CM Energies: 130 GeV/A 200 GeV/A
1 st Collisions:
06/13/2000
Location:
Brookhaven National Laboratory, Long Island, NY
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The RHIC Accelerator System
AGS Booster Ring Tandem Van de Graaff Switchyard
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March 4, 2002 4 Yellow Ring RHIC Blue Ring John G. Cramer
What does RHIC do?
RHIC accelerates
gold
nuclei in two beams to about 100 Gev/nucleon each (i.e., to kinetic energies that are over 100 times their rest mass-energy) and brings these beams into a 200 GeV/nucleon collision.
Four experiments,
STAR
, PHENIX, PHOBOS, and BRAHMS study these collisions.
In the year 2000 run, RHIC operated at a collision energy of 130 Gev/nucleon.
In 2001-2 it operated at 200 GeV/nucleon.
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STAR
About the STAR Detector.
March 4, 2002
Magn et Coils ZC al TPC Endcap & MWPC Endcap Calorime ter Barrel EM Calorim eter
6
STAR is a large solenoidal detector based on a time projection chamber. It uses a 0.5 tesla magnetic field to momentum-analyze about 2,000 charged particles per collision.
John G. Cramer
Time Projecti on Chamb er Tracker FTPCs ZCl Vertex Positio n Dete ors RI CH c t
The STAR Collaboration
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Central Au +Au Collision at
s NN = 130 GeV
Run: 1186017, Event: 32, central colors ~ momentum: low
-
high
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Part 2
RHIC Surprises
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March 4, 2002 9 John G. Cramer
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……
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March 4, 2002 10 John G. Cramer
Surprises 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 R out /R side 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 ?
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Surprise 1
Event-by-Event Elliptic Flow and Hydrodynamics
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Elliptic Flow and V 2 Sensitive to initial/final conditions and equation of state (EOS) !
coordinate-space-anisotropy
momentum-space-anisotropy
y x
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y
2
y
2
x
2
x
2
March 4, 2002
v
2
cos 2
,
tan
1 (
p y p x
)
13 John G. Cramer
Elliptic Flow and Hydrodynamics
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March 4, 2002 14 John G. Cramer
The Hydrodynamic Paradox
The system behaves as if it has reached thermodynamic equilibrium.
How could there be enough time (in ~10 fm/c) for the system to come to thermal equilibrium, as relativistic hydrodynamics assumes?
Quantum effects?
Perhaps the multiparticle wave function collapses into a maximum entropy state => TD equilibrium.
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Surprise 2 Pion Spectrum Measurements:
Strong Absorption of 2 to 6 GeV/c Pions
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Gedankenexperiments:
p
+ QGP or HG Target High momentum pion beam Hadron gas High momentum pions (Transparent)
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High momentum pion beam QGP Lower momentum pions
March 4, 2002
(Opaque)
17 John G. Cramer
High-Momentum
p
Absorption (1)
Au+Au Syst. errors from UA1 extrapolation Preliminary (h + )/2 + h p+p MinBias/ UA1
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March 4, 2002 18 Scales approximately A 2 at high p T .
John G. Cramer
High-Momentum
p
Absorption (2)
• Suppression factor ~2 • Systematic errors from UA1 extrapolation from 200 to 130 GeV Central/ UA1
Conclusion:
Central RHIC Au+Au collisions show strong absorption of high energy pions that is not observed in Pb+Pb collisions at the CERN SPS or in less central collisions at RHIC.
Smoking gun for QGP?
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Surprise 3 Source Radii and Emission Duration from Bose-Einstein Interferometry
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The Hanbury-Brown-Twiss Effect Coherent interference between incoherent sources!
For non-interacting identical bosons: 1 X Source y 2 S(x,p)=S(x)S(p) 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
2 1 Width ~ 1/R
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March 4, 2002 21 0.05
0.10
Qinv (GeV/c) John G. Cramer
Bertsch-Pratt Momentum Coordinates
C ( q out , q side , q long ) 1 exp( R 2 out q 2 out R 2 side q 2 side R 2 long q 2 long 2 R 2 ol q out q long ) Q K T 1 2 ( P T1 P T2 ) Q T p 1 p 2 Q L Q S Q T Q O beam direction p 1 p 2 beam direction
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March 4, 2002 22 John G. Cramer
A Bose-Einstein Correlation “Bump”
This 3D histogram has been corrected for Coulomb repulsion of identical q long =0 .
pairs and is a projection slice near The “bump” results from Bose-Einstein statistics of identical pions (J =0 ).
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March 4, 2002 23 John G. Cramer
Expectations:
Pre-RHIC HBT Predictions
R side “Naïve” picture (no space-momentum correlations): R out 2 = R side 2 +( b
pair
t ) 2 R out
One step further:
Hydro calculation of Rischke & Gyulassy expects R out /R side ~ 2 >4 @ k t = 350 MeV.
Looking for a “soft spot” Small R out /R side T QGP =T f
only
for (unphysical)).
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March 4, 2002 24 John G. Cramer
STAR
Reality:
STAR/RHIC HBT Measurements
• ~10% Central AuAu(PbPb) events • y ~ 0 • k T 0.17 GeV/c No significant increase in spatio-temporal size of the p emitting source at RHIC.
Note the ~100 GeV gap from SPS to RHIC and the gap between AGS and SPS data.
Ro/Rs ~ 1
March 4, 2002 25 John G. Cramer
Conclusion:
Transverse Size ~ Constant vs. Energy
R out and R side are energy independent within error bars.
Smooth energy dependence in R long No immediate indication of very different physics Fit R long to:
A m T
M. Lisa et al., PRL 84, 2798 (2000) R. Soltz et al., to be sub PRC C. Adler et al., PRL 87, 082301 I.G. Bearden et al., EJP C18, 317 (2000) p AGS: A = 2.19 +/- .05
SPS: A = 2.90 +/- .10
RHIC: A = 3.32 +/- .03
A = t 0 T in 1 st order T/m T calculation t
0 = average freeze-out time T = freezeout temperature
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R O /R S : STAR and PHENIX Agree, Models Fail.
Compiled by S. Johnson STAR and PHENIX agree Best hydro model does not reproduce the data
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Remedies for RHIC HBT Puzzle?
Problems:
Ro/Rs (and implied emission duration) are too small, implying near-instantaneous emission.
R l is also uncomfortably small, calling into question Bjorken “boost invariance”.
Solutions?:
Allow single “avalanche” freezeout: t PT =t CF =t F ?
Abandon outside-in freezeout scenario? Assume some mysterious energy-loss process at hottest part of collision fireball?
Abandon boost invariance?
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Surprise 4 Particle Spectrum Measurements + Bose-Einstein Interferometry: Pion Phase Space Density
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2D Fit to Pion Spectrum (only)
We can do a global fit of the
uncorrected
pion spectrum vs. centrality by: (1) Assuming that the spectrum has the form of a Bose-Einstein distribution: d 2 N/m T dm T dy=A/[Exp(E/T) –1] and (2) Assuming that A and T have a quadratic dependence on the number of participants : A(p) = A 0 +A 1 n +A 2 n 2 T(p) = T 0 +T 1 n +T 2 n 2
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A0 A1 A2 T0 T1 T2 Value
31.1292
Error
14.5507
21.9724 0.749688
-0.019353 0.003116
0.199336 0.002373
-9.23515E-06 2.4E-05 2.10545E-07 6.99E-08 March 4, 2002 30 John G. Cramer
A 3D Correlation Histogram
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Pion Phase Space Density at Midrapidity
f(m T
The Lorentz scalar
)
phase space density is the dimensionless average number of pions per 6-dimensional phase space cell At midrapidity
f
is given by the expression: .
Average phase space density
f
( m T ) 1 E π
Jacobian
1 λ
Purity
2 π
d
2 N m T
d
m T
d
y
Momentum Spectrum
λ ( R
c
S R O π R ) L
HBT “volume”
3
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March 4, 2002 32 John G. Cramer
Momentum Volume
The momentum volume can be determined in two ways: (1) Fit the correlation function with a 3D Gaussian and use the fit parameters to estimate the momentum volume v mom ,
v
mom
λ
(
R
c
S
R
O
π R
) L 3 (2) Direct summation of the 3D histogram channels.
v
mom
C
(
q o
,
q s
,
q l
) 1
dq o dq s dq l
Method (1) is traditional, but Method (2) is less model-dependent and gives the best statistical accuracy.
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STAR
0.5
0.3
0.2
0.1
0.1
March 4, 2002 34
0.2
p T 0.3
0.4
John G. Cramer
Tomasik & Heinz PSD Paper
The longitudinal expansion has reduced the phase space density and broken the rule that the PSD goes to a Bose-Einstein distribution when t =p t =0 (no flow).
The reduction in the PSD leads to a need for a non-zero chemical potential 0 to reach high enough PSD values to match RHIC/STAR observations.
Notice that there is a “sweet spot” near p T =0.1 GeV/c at which
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March 4, 2002 35 John G. Cramer
T&H Fit to Pion Spectra Parameters from best
H m
0 ,
h
,T
L
fits to PSD
Because the longitudinal expan sion reduces the phase space density, a non-zero chemical potential 0 is required to reproduce the most central data.
500
Pion phase space density depends on 0 and T in essentially the same way, changing the PSD strength but not its shape. However, the spectrum on m 0 and T, breaking this ambiguity.
Therefore, fitting PSD and spectra together constrains the parameters.
However, the lowest curves would prefer a negative fitting the PSD. 0 -value to reproduce the spectrum slope while
100 50 10 5 1 0 0.1
0.2
0.3
m T
-
m
p
0.4
0.5
0.6
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March 4, 2002 36 John G. Cramer
0.8
1 T&H Fit to STAR Phase Space Density (HBT)
Phase space density ~ 1 Multiparticle and laser-like stimulated emission effects?
0.6
0.4
0.2
STAR
0.1
March 4, 2002
0.2
37
p T 0.3
0.4
0.5
John G. Cramer
Summary
What does it all mean?
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Conclusion (1) The theoretical models of RHIC physics now on the market allow the source to expand for too long, so that the theoretical predictions “outrun” the boundaries of experimental observation.
Something is seriously wrong with our understanding of the dynamics of RHIC collisions.
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March 4, 2002 39 John G. Cramer
Conclusion (2)
The useful theoretical models that has served us so well at the AGS and SPS for heavy ion studies have now been overloaded with a large volume of puzzling new data from RHIC, and things are a bit up in the air.
We need more theoretical help and more experi mental data to meet the challenge of understanding what is going on in the RHIC regime.
It’s a very exciting time for us STAR experimentalists!
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March 4, 2002 40 John G. Cramer