A Strange Perspective –
Preliminary Results from
the STAR Detector at
RHIC
Helen Caines
The Ohio State
University
Stony Brook –
Nucl. Seminar
STAR
May 2001
The STAR Collaboration
Brazil: Universidade de
Sao Paolo
Spokesperson: John Harris
China: IHEP - Beijing,
IPP - Wuhan
U.S. Labs: Argonne,
Berkeley, Brookhaven
National Labs
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
STAR
Russia: MEPHI –
Moscow, LPP/LHE
JINR–Dubna, IHEPProtvino
Institutions: 36
Collaborators: 415
Students:
~50
Stony Brook 2001
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
Helen Caines
STAR STRANGENESS!(Preliminary)
L

W-+
W +
K
f
+
K 0s
L
X +
STAR
XK*
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Helen Caines
Introduction
Chemical content
– Yields
When is
Strangeness
Produced –
Flow – How much
and when does it
start?
Resonances
Chemical Freeze-out Ratios
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Thermal Freeze-out
– Radii and Inverse
slopes
Helen Caines
Previous Strangeness Highlights
WA97
Multi- Strange Particles appear to
freeze out at a cooler temperature/
earlier or have less flow
STAR
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The Phase Space Diagram
TWO different phase transitions at work!
Deconfinement transition
– Particles roam freely over a
large volume
Chiral transition
– 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:
STAR
Tc ~ 150-170 MeV
ec ~ 0.5-0.7 GeV/fm
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Helen Caines
STAR Pertinent Facts
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
STAR
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The STAR Detector (Year-by-Year)
Magnet
Coils
TPC Endcap &
MWPC
Time
Projection
Chamber
Silicon
Vertex
Tracker *
FTPCs (1 + 1)
ZCal
ZCal
Endcap
Calorimeter
Barrel EM
Calorimeter
RICH
Vertex
Position
Detectors
Central Trigger
Barrel
+ TOF patch
* yr.1 SVT ladder
•
Year
2000,
year
2001,
year-by-year
until
2003,
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Helen Caines
installation
in 2003
Triggering/Centrality
• “Minimum Bias”
~30K Events
|Zvtx| < 200 cm
ZDC East and West thresholds set to
lower edge of single neutron peak.
• “Central”
CTB threshold set to upper 15%
REQUIRE:
Coincidence ZDC East and West
REQUIRE:
STAR
Min. Bias + CTB over threshold
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Particle ID Techniques - dE/dx
12
dE/dx (keV/cm)
dE/dx
p
8
d
K

4

e
0
dE/dx PID range:
~ 0.7 GeV/c for K/
~ 1.0 GeV/c for K/p
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High Pt K+ & K- Identification Via “Kinks”
+/-
n
K+/STAR
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Particle ID Techniques - Topology
Decay vertices
L
Ks   + +  L  p + L  p + 
+
Vo
X-  L +  X+ L +  +
W  L +KX+
“kinks”:
K  + n
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Finding V0s
proton
Primary
vertex
pion
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In case you thought it was easy…
Before
STAR
After
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Particle ID Techniques Combinatorics
K* combine all K+ and -
Combinatorics
dn/dm
Ks  + + -
f  K+ + K-
L  p + -
L  p + +
pairs
(x 10-5)
f from K+ K- pairs
background
subtracted
m inv (GeV)
m inv
dn/dm
K+
K- pairs
same event dist.
mixed event dist.
Breit-Wigner fit
Mass & width
consistent w. PDG
m inv
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Particle Freeze-out Conditions
3. freeze-out
time
Kinetic Freezeout: elastic scattering
2. hot / dense stops
1. formation
Chemical Freezeout: inelastic scattering
stops
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_
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)
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Strange Baryon Ratios
Reconstruct:_
~0.84 L/ev, ~ 0.61 L/ev
Reconstruct: _
~0.006 X-/ev, ~0.005 X+/ev
STAR Preliminary
Ratio = 0.73 ± 0.03 (stat)
STAR
Ratio = 0.82 ± 0.08 (stat)
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Helen Caines
Preliminary L/L Ratio
Central events
_
L/L= 0.73  0.03 (stat)
|y|<0.5
STAR
Ratio is flat as a function of pt and y
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Anti-baryon/Baryon Ratios versus s
_
Baryon-pair production
increases dramatically with
s – still not baryon free
_
_
_
_
_
_
STAR preliminary
¯
Ypbar
Ypair

 0.65
Yp
Ypair + YTr
Y pair
 2
Y Tr
Pair production is larger
than baryon transport
2/3 of protons from pair production , yet pt dist. the same
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2001
– Another indication
thermalization
Helen Caines
L and L from mixed event Studies
Good cross-check with
_
L/L= 0.77
 0.07 (stat)
standard V0 analysis.
Low pt measurement
where there is no V0 analysis
High efficiency (yields
are ~10X V0 analysis yields)
Background
determined by mixed event
The ratio is in agreement with
“standard” analysis
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K+/K- vs pt
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K+/K- Ratio - Nch
Kinks
dE/dx
•K+/K-= 1.08±0.01(stat.)± 0.06(sys.) (dE/dx). (The kink
method is systematically higher.)
•K+/K- constant over measured centrality.
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K-/- Ratios
STAR preliminary
SP
S
STAR
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K-/- ratio is
enhanced by almost
a factor of 2 in
central collisions
when compared to
peripheral collisions
Helen Caines
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
L
L
X
X
 ud s   u   s  p

   * *
 uds   u   s  p
 us s   u   s  L
 uss    u  *  s  * L
+
 uu d 
p

  D*
p
 uud 
Predict
 ud s 
L

  D*
L
 uds 
Predict
 u   s  K  us 
D    *   -  
 u   s  K  us 
STAR
D=1.12
D=1.12
Measure
D=1.08± 0.08
System consistent with having a de-confined phase
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_
K0* and K0* Identification
First measurement in heavy ion collisions
Short lifetime (ct =4fm) – sensitive to the evolution of
the system?
STAR
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K0*/h-
Represents a 50% increase
compared to K0*/
measured in pp at the ISR.
Aim to measure in pp
ourselves this year.
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Helen Caines
Comparing to SPS
K+/K-(kink) = 1.2 ±
K+/K-(dE/dx) = 1.08 ±0.01 (stat.)±
0.06 (sys.)
K-/-
= 0.15 ± 0.02 (stat.)
¯K*/h-
= 0.06 ± 0.006 (stat.)±
0.01 (sys.)
K*/h-
= 0.058 ± 0.006 (stat.)±
0.01 (sys.)
= 0.6  0.02 (stat.)
 0.06 (sys.)
= 0.73 ± 0.03 (stat.)
 0.82 ± 0.08 (stat.)
¯p/p
STAR
L̄/L
X/X
¯
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Particle Ratios and Chemical Content
-
N i   g je
E j j
ji
j= Quark Chemical
Potential
T = Temperature
Ej – Energy required to
add quark
gj– Saturation factor
Use ratios of particles to determine , Tch and saturation factor
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T
Chemical Fit Results
Not a 4-yields fit!
gs  1
2  1.4
Thermal fit to preliminary data:
Tch (RHIC) = 0.19 GeV
 Tch (SPS) = 0.17 GeV
q (RHIC) = 0.015 GeV
<< q (SPS) = 0.12-0.14 GeV
s (RHIC) < 0.004 GeV
 s (SPS)
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Chemical Freeze-out
early universe
P. Braun-Munzinger, nucl-ex/0007021
Chemical Temperature Tch [MeV]
250
RHIC
200
quark-gluon plasma
SPS
150
AGS
Lattice QCD
deconfinement
chiral restauration
thermal freeze-out
100
SIS
hadron
gas
50
neutron stars
atomic nuclei
0
0
200
400
600
800
1000
1200
Baryonic Potential B [MeV]
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“Kink” Rapidity Distribution
Mid-y K+ dN/dy =
35 ±3(stat.)±5(sys.)
STAR
Mid-y K- dN/dy =
30±2.5(stat.)±4(sys.)
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“Kink” mt Distributions
STAR
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K- Inverse Slope Results
Kink
dE/dx
Increasing
centrality
h- mid rapidity dN/dh
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mt slopes vs. Centrality
mid-rapidity
Tp = 565 MeV
TK = 300 MeV
T = 190 MeV
• Increase with collision
centrality
•  consistent with radial flow.
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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)

= 0.6c
Tfo (RHIC)

= 0.1-0.12 GeV
STAR
ßr (SPS/AGS)
= 0.4 - 0.5c
Tfo (SPS/AGS)
= 0.12-0.14 GeV
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f Identification
STAR
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Radial Flow and the f
Central collisions
NA49 – 290 MeV
NA50 – MeV
STAR
Doesn’t follow “radial flow systematics”
early kinetic freezeout?
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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
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Conclusions
• 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
Thermal freeze out same as at SPS
 f
The f does not seem to flow with the other particles.
Reduced rescattering for the kaons from f decay and/or
f feels less flow
STAR
More than we ever
runCaines
!!!
Stony hoped
Brook 2001 for after the first
Helen
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
1.1 < 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 resonaces due
to improved tracking Should measure spectra for all particles this
year.
HBT with strange particles
Exotica
FTPC: Strange particles at high y
STAR
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Helen Caines