1. A Strange Perspective – Preliminary Results from the STAR Detector at RHIC
Science is a wonderful thing if one does not have to earn one's living at it
– Einstein (1879—1955)

2. The STAR Collaboration
Brazil: Universidade de Sao Paolo
China: IHEP - Beijing, IPP - Wuhan
England: University of Birmingham
France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes
Germany: Max Planck Institute – Munich University of Frankfurt
Poland: Warsaw University, Warsaw University of Technology
Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino
U.S. Labs: Argonne, Berkeley, Brookhaven National Labs
U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale
Spokesperson: John Harris
Institutions:
Collaborators: 415
Students: ~50

3. QCD The Quark-Gluon Plasma!
Quarks confined within hadrons via strong force
v(r) = a/r + s*r
At large r -second term dominates
At small r -Coulomb-like part dominates
However a function of q( mtm transfer) and a -> 0 faster
than q (or 1/r) -> infinity (called asymptotic freedom)
This concept of asymptotic freedom among closely packed
coloured objects (q and g) has led to one of the most exciting
predictions of QCD !!
The formation of a new phase of matter where the colour degrees of freedom are liberated. Quarks and gluons are no longer confined within colour singlets.
The Quark-Gluon Plasma!

4. Don’t Panic!!! - New Scientist
most dangerous event in human history: - ABC News –Sept ‘99
"Big Bang machine could destroy Earth" -The Sunday Times – July ‘99
No… the experiment will not tear our region of space to subatomic shreds.
- Washington Post – Sept ‘99
the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99
Apocalypse2 – ABC News – Sept ‘99
Will Brookhaven Destroy the Universe? –
NY Times – Aug ‘99

5. The Phase Space Diagram
TWO different phase transitions at work!
– Particles roam freely over a large volume
– Masses change
Calculations show that these occur at approximately the same point
Two sets of conditions:
High Temperature
High Baryon Density
Lattice QCD calc. Predict:
Deconfinement transition
Chiral transition
Tc ~ MeV
ec ~ GeV/fm

6. Why are we interested in Strangeness?
Tfireball < Tc(170MeV) Hadron gas
Hard to make S  0 particles
p + N  L + K
(Ethresh  530MeV)
p + K L + N
(Ethresh  1420MeV)
Mtm phase space suppressed
Need to create 3 qq pairs
(initially there are no q) with
similar momenta in a region
already containing many quarks.
Tfireball >Tc(170MeV) QGP
Easy to make s quarks
E=2ms ( 300MeV)
Free gluons
g-g fusion - dominate ss creation faster reaction
time than qq
Pauli blocking may aid creation of ss quarks
( probably not true at high T, too many states). _ _ _ _ _ _ _

7. Introduction Chemical content – Yields
When is Strangeness Produced – Resonances
Flow – How much and when does it start?
Thermal Freeze-out – Radii and Inverse slopes
Chemical Freeze-out - Ratios

8. Previous Strangeness Highlights
Enhancement W > X > L > h
SPS s=17GeV
WA97
|s|
Evidence of strangeness enhancement between pA and AA collisions at the SPS – Not reproducible by models

9. Strangeness Highlights (2)
Multi-Strange Particles appear to
freeze out at a cooler temperature/ earlier or have less flow
SPS
AGS
AGS and SPS > 1
Need to consider p absorption _

10. The CERN announcement
Strangeness was one of the corner stones of the CERN announcement.
Have numerous pointers that there is evidence of a new state of matter even at SPS energies so why RHIC?
Still a large baryon number so need models to understand what’s going on.
Those models can probably be tuned to reproduce the experimental data but would require more “knobs”
So want to go to “cleaner” system
Less baryon number (only look at produced particles)
Closer to the region where QCD predictions work – Definite theory not models
Higher energies – further across phase transition boundary – In new regime for longer and more frequently

11. The STAR Detector (Year-by-Year)
Magnet
Time Projection Chamber
Coils
FTPCs
Silicon Vertex Tracker *
Vertex Position Detectors
year 2001,
+ TOF patch
TPC Endcap & MWPC
ZCal
ZCal
installation in 2003
Endcap Calorimeter
Central Trigger Barrel
Barrel EM Calorimeter
year-by-year until 2003,
RICH
* yr.1 SVT ladder
Year 2000,

12. STAR Pertinent Facts L3-Real time display Field:
0.25 T (Half Nominal value)
(slightly worse resolution at higher p, lower pt acceptance)
TPC:
Inner Radius – 50cm
(pt>75 MeV/c)
Length – ± 200cm
( -1.5< h < 1.5)
Events:
~300,000 “Central” Events –top 8% multiplicity
~160,000 “Min-bias” Events
L3-Real time display

13. Needle in the Hay-Stack!
How do you do tracking in this regime?
Solution: Build a detector so you can zoom in close and “see” individual tracks
high resolution
Clearly identify individual tracks
Good tracking efficiency
Pt (GeV/c)

14. Particle ID Techniques - dE/dx
dE/dx PID range:
~ 0.7 GeV/c for K/
~ 1.0 GeV/c for K/p
dE/dx
6.7%
Design
7.5%
With calibration
9 %
No calibration
Resolution:
Even identified anti-3He !

15. Particle ID Techniques - Topology
Decay vertices
Ks  p + + p -
L  p + p -
L  p + p +
X-  L + p -
X+ L + p +
W  L + K - L Vo
X+
“kinks”:
K  + 

16. High Pt K+ & K- Identification Via “Kinks”
m+/- nm
K+/-

17. Particle ID Techniques Combinatorics
Breit-Wigner fit
Mass & width
consistent w. PDG
K* combine all K+ and p-
pairs
(x 10-5)
m inv (GeV)
Ks  p+ + p- f  K+ + K-
L  p + p- L  p + p+
f from K+ K- pairs
K+ K- pairs
m inv
same event dist.
mixed event dist.
background subtracted
dn/dm

18. STAR STRANGENESS!
(Preliminary) K+
W-+ L̅
W̅+ f
K0s L X-
X̅+ K*

19. Triggering/Centrality
• “Minimum Bias”
ZDC East and West thresholds set to lower edge of single neutron peak.
~30K Events
|Zvtx| < 200 cm
• “Central”
CTB threshold set to upper 15%
REQUIRE:
Coincidence ZDC East and West
Min. Bias + CTB over threshold

20. The Collisions
The End Product

21. Baryon Stopping/Transport
Anti-baryons - all from pair production
Baryons - pair production + transported
B/B ratio =1 - Transparent collision
B/B ratio ~ 0 - Full stopping, little pair production
Measure p/p, L/L , K-/K+
(uud/uud) (uds/uds) (us/us) _ _ _ _
- - -
- - - - -

22. _ p/p Ratio Ratio is flat as function of pt and y
Slight fall with centrality
Phys. Rev. Lett March 2001
Ratio = 0.65 ±0.03(stat) ±0.03(sys)
Still finite baryon number

23. Strange Baryon Ratios Ratio = 0.73 ± 0.03 (stat)
Reconstruct:
Reconstruct: _ _
~0.84 L/ev, ~ 0.61 L/ev
~0.006 X-/ev, ~0.005 X+/ev
STAR Preliminary
Ratio = 0.73 ± 0.03 (stat)
Ratio = 0.82 ± 0.08 (stat)

24. Preliminary L̅/ Ratio
L/L=  0.03 (stat) _
Central events
|y|<0.5
Ratio is flat as a function of pt and y

25. L and L̅ from mixed event Studies
Good cross-check with
standard V0 analysis.
L/L= 0.77
 0.07 (stat) _
Low pt measurement
where there is no V0 analysis
High efficiency (yields are ~10X V0 analysis yields)
Background determined by mixed event
STAR preliminary
The ratio is in agreement with “standard” analysis

26. Anti-baryon/Baryon Ratios versus s_
Baryon-pair production increases dramatically with s – still not baryon free
Pair production is larger
than baryon transport
STAR preliminary
2/3 of protons from pair production , yet pt dist. the same
– Another indication of thermalization

27. Particle Freeze-out Conditions
time
3. freeze-out
Kinetic Freeze out: elastic scattering stops
2. hot / dense
Chemical Freeze out: inelastic scattering stops
1. formation

28. K+/K- Ratio - Nch Kinks dE/dx
STAR Preliminary
Kinks
dE/dx
STAR preliminary
STAR preliminary
K+/K-= 1.08±0.01(stat.)± 0.06(sys.) (dE/dx). (The kink method is systematically higher.)
K+/K- constant over measured centrality

29. K0* and K0* Identification_
K0* and K0* Identification
First measurement in heavy ion collisions
Short lifetime (ct =4fm) – sensitive to the evolution of the system?

30. K0*/h- Represents a 50% increase compared to K0*/p measured
in pp at the ISR.
Strangeness Enhancement?
Also look at K*/K
From spin counting
K*/K = vector meson/meson
= V/(V+P)
=0.75
e+e-(LEP)K*/K = 0.32 ±0.02
pp (ISR)K*/K = 0.6 ± .09 ± .03
Au-Au (STAR)= 0.42

31. f Identification STAR Preliminary
Mass and width are consistent with PDG book convoluted with TPC resolution

32. f/h- p-p A-A f/h- ratio increases with beam energy for A-A collisions
Appears flat for p-p collisions
Strangeness Enhancement?

33. Comparing to SPS
K+/K-(dE/dx) = 1.08 ±0.01 (stat.)± (sys.)
f/h = ± (stat.)± (sys.)
K*/h = 0.06 ± (stat.)± (sys.)
K*/h = ± (stat.)± (sys.)
p/p = 0.6  0.02 (stat.)
 0.06 (sys.)
/ = ± 0.03 (stat.)
X/X = ± 0.08 (stat.)

34. Simple Model
Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models
No free quarks so all quarks have to end up confined within a hadron
Predict
D=1.12
Predict
D=1.12
D=1.08± 0.08
Measure
System consistent with having a de-confined phase

35. Particle Ratios and Chemical Content
mj= Quark Chemical Potential
T = Temperature
Ej – Energy required to add quark
gj– Saturation factor
Use ratios of particles to determine m, Tch and saturation factor

36. Chemical Fit Results s (RHIC) < 0.004 GeV Not a 4-yields fit!
2  1.4
Thermal fit to preliminary data:
Tch (RHIC) = 0.19 GeV
 Tch (SPS) = 0.17 GeV
q (RHIC) = GeV
<< q (SPS) = GeV
s (RHIC) < GeV
 s (SPS)
Chem Fit

37. Chemical Freeze-out early universe quark-gluon plasma hadron gas
Baryonic Potential B [MeV]
Chemical Temperature Tch [MeV]
200
250
150
100 50
400
600
800
1000
1200
AGS
SIS
LEP/ SppS
SPS
RHIC
quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinement
chiral restauration
Lattice QCD
atomic nuclei
P. Braun-Munzinger, nucl-ex/
Production systematic

38. Kinetic Freeze-out and Radial Flow
Want to look at how energy distributed in system.
Look in transverse direction so not confused by longitudinal expansion
Look at mt = (pt2 + m2 ) distribution
A thermal distribution gives a linear distribution
dN/dmt  e-(mt/T) mt
1/mt d2N/dydmt
Slope = 1/T
Slope = 1/Tmeas
~ 1/(Tfo+ mo<vt>2)
If there is transverse flow

39. mt slopes vs. Centrality
Tp = 190 MeV
TK = 300 MeV
Tp = 565 MeV
mid-rapidity
Increase with collision centrality
 consistent with radial flow.

40. Inverse slope for f and L
T=352±6(stat) MeV
15% Most Central
e(-mt/T)
Similar slopes for similar masses

41. Radial Flow and Strange Particles
Simple model no longer works!!!
STAR Preliminary
Evidence that strange particles don’t “feel” the flow?

42. Interpreting the mt spectra
Little overlap between the f and p measurements.
Possible agreement where data do overlap. Same for L
Many complicating factors:
Strong flow leads to a curvature in the mt slopes
Range of fitting becomes VERY important.
Still looking at how to describe data consistently
however this is evidence of a strong flow at RHIC

43. Measuring the Source “Size” (HBT)x1 x2 y1 y2
~1 m
1D: overall
rough “size”
~5 fm K
Rout
Rside
3D decomposition of relative momentum provides handle on shape and time as well as size

44. K0s-K0s Correlations No coulomb repulsion No 2 track resolution
Few distortions from resonances
K0s is not a strangeness eigenstate - unique interference term that provides additional space-time information
l = 0.7 ±0.5
R = 6.5 ± 2.3
K0s Correlation will become statistically meaningful once we have ~10M events (aim for this year)

45. What have we “learnt” so far
Mapping out “Soft Physics” Regime
Net-baryon  0 at mid-rapidity! ( y = y0-ybeam ~ 5 )
Chemical parameters
Chemical freeze-out appears to occur at same ~T as SPS
Strangeness saturation similar to SPS
Kinetic parameters
Higher radial flow than at SPS – More complicated picture than seen at the AGS and SPS
Thermal freeze out similar SPS (pions have similar inverse slope)
Strange Particles
Some evidence of strangeness enhancement – f and K*
More than we ever hoped for after the first run !!!
Looking forward to imminent next run

46. This Year – RICH,TOF Patch,SVT,FTPC
RICH and TOF:
Increase K identification in pt over a limited geometric acceptance
Centered at mid-rapidity they provide complimentary pt coverage
TOF patch 0.3< pt <1.5 GeV/c
RICH < pt < 3.0 GeV/c
Overlaps with the TPC kink and dE/dx measurement
kink pt < 5 GeV, dE/dx pt < 0.8 GeV
SVT: Increased efficiency for all strange particles and resonances due to improved tracking
Should measure spectra for all particles this year.
HBT with strange particles
Exotica
FTPC: Strange particles at high y

47. The Silicon Vertex Tracker
Radii – 6,10 15 cm
Length ±12.4 cm
± 18.6 cm
± 21.7cm
(-1 < h < 1)

48. SVT STAR detector gets new silicon heart – CERN Courier
SVT installed and operational April 2001!!
91% live (out of 103,680 channels)

49. SVT being Assembled
Radiation Length 1.5%/layer (including Electronics+Cooling)
216 wafers on 36 Ladders
0.7m2 Silicon
Half assembled.
Fully assembled

50. Principle of Operation
Ionizing particle
Y-position from readout anode number
SDD
X-position from drift time X
Electron cloud
SDD gives unique position in X-Y
* 6.3 cm x 6.3 cm area
* 280 mm thick n-type Si wafer
* 20 mm position resolution

51. SVT Performance Noise 1ch=2mV Cosmic Ray Event–L3 Trigger
Threshold at 4mV 6% live
Hits from Au-Au Event

52. Conclusion
Lots done
Lots still to be done
The future looks bright and exciting

53. Comparison of AGS, SPS, RHIC
Energy density 1GeV/fm3 5.3GeV/fm3 17GeV/fm3
Multiplicity 1,000 3, ,000
Baryon chemical potential mb 520MeV MeV MeV
Freeze-out temperature (T) 120MeV MeV MeV

54. Radial Flow: mt - slopes versus mass
Naïve: T = Tfreeze-out + m  r 2
where  r  = averaged flow velocity
Increased radial flow at RHIC
ßr (RHIC)  ßr (SPS/AGS)
= 0.6c = c Tfo (RHIC)  Tfo (SPS/AGS)
= GeV = GeV

55. K-/p- Ratios
STAR preliminary
K-/p- ratio is enhanced by almost a factor of 2 in central collisions when compared to peripheral collisions
SPS

56. Two-particle interferometry (HBT)
Correlation function for identical bosons:
1d projections of 3d Bertsch-Pratt
12% most central out of 170k events
Coulomb corrected
|y| < 1, < pt < 0.225
qout
STAR preliminary
qlong
STAR preliminary

57. Radii dependence on centrality and kt
low kT
central collisions
x (fm)
y (fm) p- p+
STAR preliminary
Radii increase with multiplicity - Just geometry (?)
Radii decrease with kt – Evidence of flow (?)
“multiplicity”

58. Pion HBT Excitation Function
Compilation of world 3D -HBT parameters as a function of s
STAR Preliminary
Central AuAu (PbPb)
Decreasing  parameter
Decreased correlation strength
More baryon resonances ?
Saturation in radii
Geometric or dynamic (thermal/flow) saturation
No jump in effective lifetime
No significant rise in size of the  emitting source
Lower energy running needed!

59. Tomášik, Heinz nucl-th/9805016
The ROut/RSide Ratio
Emission duration for transparent sources:
STAR Preliminary
Tomášik, Heinz nucl-th/
=0.0
=0.5
opaqueness
Small radii + short emission time + opaqueness  short freeze-out

60. How a TPC works
420 CM
Tracking volume is an empty volume of gas surrounded by a field cage
Drift gas: Ar-CH4 (90%-10%)
Pad electronics: amplifier channels with 512 time samples
Provides 70 mega pixel, 3D image

61. Calibration - Lasers
Using a system of lasers and mirrors illuminate the TPC
Produces a series of
>500 straight lines criss-crossing the TPC volume
Determines:
Drift velocity
Timing offsets
Alignment

62. Calibration – Cosmic Rays
Determine momentum resolution
dp/p < 2% for most tracks

63. K+/K- vs pt

64. K- Inverse Slope Results
Kink
dE/dx
Increasing centrality
h- mid rapidity dN/dh

66. Large h- multiplicity
Nearly Boost invariant

67. The central rapidity region
Almost net-baryon free
dNnet-B/dy ~ 30
Large particle multiplicity
dN/dh ~ 800
hadrons
C. Bernard et al, PRD 55, 6861 (1997)
Excited
Vacuum
hadrons

69. In case you thought it was easy…
After
Before

70. Finding V0s
proton
Primary vertex
pion

71. Welcome to BNL- RHIC!