2. Outline Background Global Observables in Heavy Ion Collisions
Quark-Gluon Plasma and Heavy Ions
Global Observables in Heavy Ion Collisions
What are they? What do we learn from them?
The STAR Experiment
Analysis of Charged Particles in STAR
Results
Charged hadrons: h-
Identified particles
Conclusions
January 7, 2002

3. Phase Diagram
e ~ 1-3 GeV/fm3
F. Wilczek
hep-ph/
Heavy Ions: How does nuclear matter look at high temperature?
January 7, 2002

4. QGP in the Laboratory Space-time evolution of HI collision
January 7, 2002

5. Global observables First tool to probe the collision environment
Multiplicity distribution
Inclusive single particle spectra at low p^
Represent system at Kinetic Freeze-out
Thermalization, Expansion
Boost invariance?
Initial conditions & Evolution of the system
Essential reference for systematic studies of probes of deconfinement
January 7, 2002

6. Glauber Model: TAA, Npart, Ncoll
Collision Geometry
Use Woods-Saxon density profile: from e-A
Overlap Integral: s:
Binary Collisions:
Participants:
January 7, 2002

7. Charged particle production
How many particles?
Multiplicity distribution
Geometry of the collision
Correlated with impact parameter
January 7, 2002

8. Momentum distributions
Difference between SPS and RHIC
At high energy
larger contribution from jets, mini-jets
h- p^ distribution closer to power law than exponential
y distribution  plateau at mid-rapidity
spectra peaked at low energy (“stopping”),
boost invariance at RHIC?
Factorize?
January 7, 2002

9. Predictions (s = 200 GeV) Different models  different predictions
pQCD parton models : Hijing dNch/dh =
EKS dNch/dh =
Parton hard scattering : Nexus dNch/dy ~ 1100
Microscopic Transport : UrQMD dNch/dy ~ 900
Different models  different predictions
Within any model, data needed as initial conditions
January 7, 2002

10. The STAR Detector at RHIC
January 7, 2002

11. Time evolution of STAR
1st year detectors (2000) 2nd year detectors 3rd year detectors
Magnet
Time Projection Chamber
Coils
Barrel EM Calorimeter
Endcap Calorimeter
Silicon Strip Detector
Photon Multiplicity Detector
Silicon Vertex Tracker
Forward Time Projection Chambers
Vertex Position Detectors
+ TOF patch
TPC Endcap & MWPC
Zero Degree Calorimeter
Central Trigger Barrel
RICH
January 7, 2002

12. Au+Au in STAR TPC High Multiplicity Event  End view of detector
Side View 
January 7, 2002

13. Physics Run 2000 zvertex Multiplicity distribution
Event selection, triggers, vertex finding efficiency
Inclusive single particle spectra
Track finding
Corrections: acceptance, efficiency, etc. as a function of
momentum space cell
h-pt, h-
y-pt, p-
zvertex
multiplicity
particle species
This analysis:
Tracking: TPC
Trigger ZDC + CTB
PID: de/dx in TPC
TPC:
|| < 1.8
0 < f < 2p
P > 75 MeV/c
Bfield:
0.25 T (1/2 nominal)
Trigger:
ZDC at  18 m
CTB || < 1
January 7, 2002

14. Event Selection and Triggers
Zero Deg. Cal.
Main Minimum Bias Trigger
99% Efficient even at high multiplicity
Trigger
ZDC Coincidence (East and West) OR
High CTB Signal Au Au
ZDC East
ZDC West
Central Trigger Barrel (CTB)
5% Central
January 7, 2002

15. Primary h- Multiplicity
6% sys. error shown on 3 points only
January 7, 2002

16. h- & h+ dN/dh, Central Events
Jacobian
dN/dh, |h|<0.1
h- : 280 1 20
h+ : 287 1 20
Approaching boost invariance
at RHIC
January 7, 2002

17. h- p Distribution, Central
Power Law Fit
(p^ = GeV/c)
A (1+p  /p0) - n
p0 =3.0 ± 0.3 GeV/c
n= 14.8 ± 1.2
STAR
<p>=0.508 ± GeV/c
NA49
<p>=0.414 ± GeV/c
UA1
<p>=0.392 ±0.003 GeV/c
January 7, 2002

18. p Spectra in pp Collisions
Data available for s = , but not at 130 GeV
Power law: E d3/d3p = A (1+p^/p0) –n
interpolate A, p0, n to 130 GeV
Extrapolation used in STAR
January 7, 2002

19. Comparison topp Low pt  Wounded Nucleon applies
Compare to UA1
Problem UA1 s = 200
R(130/200)
From power law scaling
R = 0.92 at 0.2 GeV/c
R = 0.70 at 2 GeV/c
“Hard” Scaling
Nuclear Overlap Integral
TAA = 26 mb-1
for 5% most central
NAA/Npp= Nbin coll=1050
“Soft” Scaling
NAA / Npp=( 344/ 2 )
Low pt  Wounded Nucleon applies
Rising pt  Approaching hard scaling limit?
January 7, 2002

20. High p Analysis
Preliminary High-pt analysis shows turnover Quenching?
January 7, 2002

21. h- Centrality Classes
Central collisions: cut on ZDC only (CTB veto on low multiplicity)
Peripheral collisions: Cut in ZDC+CTB space
STAR
January 7, 2002

22. h- , Centrality dependence
Power Law behavior is observed in all centrality classes, although curvature varies.
STAR
Shape of dN/dh at mid-rapidity does not change with centrality.
Power Law A (1+p  /p0) - n
January 7, 2002

23. h- <p>, Centrality Dependence
15% Increase in <p> from 80% sample to 5% central
Increase with respect to pp
Collective effects
Systematic uncertainty shown for STAR data
January 7, 2002

24. - m Spectra (centrality, y)
Fits to Bose-Einstein
Including low-pt
No additional low-pt enhancement
January 7, 2002

25. K- and p, m Distribution
K- Slope: moderate centrality dependence
Stronger for p
January 7, 2002

26. m Inverse Slopes Slope: stronger centrality dependence
with increasing
particle mass
Radial flow?
January 7, 2002

27. - rapidity distribution
Yield fairly flat
decreasing slightly
with increasing y
|y| < 0.1
dN/dy = 286 ± 10
y= ± 0.8
dN/dy = 271 ± 13
January 7, 2002

28. Teff vs y Teff shows more pronounced y dependence
Boost invariance does not yet hold at RHIC
Flow? Additional baryons with increasing y?
January 7, 2002

29. Conclusions Collisions dominated by geometry
Negative hadron distributions (Central collisions)
Increased particle production relative to SPS and UA1
<p> = GeV/c (NA49 = 0.414, UA1 = 0.392)
low p: ‘Wounded nucleon’ scaling; rising p: ‘binary collisions’ scaling ?
Negative hadrons, centrality dependence
<p> shows weak dependence
 dependence ~flat, small dependence on centrality
Identified
 slope parameter T, weak centrality dependence
Increase is most dramatic in anti-protons (Radial flow)
visible y dependence: boost invariance not yet reached
Collision picture including other observables is beginning to emerge
January 7, 2002

30. Identified Spectra: dE/dx
Use calibrated curves:
Z variable
zp = ln(Imeas/Ip) p K- - e-
January 7, 2002

31. - p^ distribution, centrality
Fits to Bose-Einstein
Including low-pt
No additional low-pt enhancement
0.05 < p^ < 0.75
Measure >80% of total yield
Teff almost constant with centrality
January 7, 2002

32. - Teff, centrality dependence
Slight increase in
Teff with centrality
Mainly in peripheral
From 3d bin, consistent
with no centrality dependence
Even with radial flow, p are only slightly affected, small m
January 7, 2002

33. - p^ distribution, rapidity
5% Most Central
Mid-rapidity region
range |y|<0.8 
Fits to Bose-Einstein
No low-p^ enhancement
Each y bin
scaled by factors of 2
January 7, 2002

34. PID & dE/dx Resolution p - e- K- dN/dZp Resolution ~9%
0.4 < p <0.45 GeV/c
|y| < 0.1
Resolution ~9%
Approaching design value of 8 %
January 7, 2002