A Strange Perspective –
Preliminary Results from
the STAR Detector at
RHIC
ACS - Chicago
Helen Caines
The Ohio State
University
Aug 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
Russia: MEPHI –
Moscow, LPP/LHE
JINR–Dubna, IHEPProtvino
Institutions: 36
Collaborators: 415
Students:
~50
ACS - 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
What are we looking for?
• What is the initial environment like for particle
production?
Baryon / antibaryon ratios
– Net baryon density
• What happens during the initial particle
production?
Strange hadron / h- ratios
– Strangeness production
– Quark coalescence?
Quark-counting ratios
• Are re-interactions significant?
Hadron ratios vs. pt
– Rescattering of hadrons
– Equilibration of strangeness
Strange baryon ratios
– Radial flow
- 2001
Helen Caines
MACS
t slopes
Data Quality 1: Peaks
~0.84 L/ev,
_
~ 0.61 L/ev
~0.006 X-/ev,
~0.005 X+/ev
_
~1.6 K0s/ev
ACS - 2001
Clear peak
Helen Caines
Data Quality 2: Resonances
First measurement in heavy ion collisions
K*
f
_
K*
Mass and width are
consistent with PDG
book convoluted with
TPC resolution
ACS - 2001
Helen Caines
Data Quality 3: Lifetime check
Star Preliminary
L
Star Preliminary
K0s
Lifetime : 8.03 ±0.05 (stat)cm
Lifetime : 2.64 ±0.01(stat)cm
PDG Value : 7.89 cm
PDG Value : 2.68 cm
ACS - 2001
Helen Caines
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 ,
----(uud/uud) (uds/uds)
K-/K+
(us/us)
ACS - 2001
Helen Caines
STAR B/B Ratios
Ratio approaching
1.0 as strangeness
content increases
However still some
baryon number
being transported
from beams
Ratios calculated for
central events at midrapidity, averaged
over experimental
acceptance in pt
ACS - 2001
Helen Caines
Energy Evolution of B/B Ratio
Production of baryons through
pair processes increases dramatically with s – still not baryon free
Ypbar
Ypair

 0.65
Yp
Ypair + YTr
Y pair
2
Y Tr
(ISR)
STAR preliminary
Pair-process production is
larger than baryon transport
ACS - 2001
Helen Caines
Ratios vs pt
0.60.02 (stat.) 0.06 (sys.)
_
L/L
1.1± 0.05 (stat.)
0.73 ± 0.03 (stat.)
pt
Baryon pt distribution the
same as anti-baryon
2/3 of protons from pair processes ,
yet pt dist. the same as antiprotons
ACS - 2001
Much Rescattering!!
Helen Caines
K+/K- Ratio - Nch
K+/K- constant over measured centrality
ACS - 2001
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
 uu d 
p

  D*
p
 uud 
 us s   u   s  L
 uss    u    s  * L
 ud s 
L

  D*
L
 uds 
+
 u   s  K  us 
D       -  
 u   s  K  us 
Predict
D=1.12
Predict
D=1.12
Measure
D=1.08± 0.08
System consistent with having a de-confined phase
ACS - 2001
Helen Caines
Energy Evolution Revisited
RHIC/STAR (Au+Au)
SPS/NA44 (S+S)
SPS/NA49 (Pb+Pb)
AGS/E866 (Au+Au)
YK YK +
Yubar
Ypair


Yu
Ypair +YTr
YK - /YK +
Ypair

YTr 1-YK - /YK +
(ISR)
STAR preliminary
3
YB Yubar 
 


YB  Yu 

Y - 3
K 
 
Y + 
 K 
K-/K+ ratios exhibit similar behavior to
_
p/p as net baryon number drops and p
absorption lessens
ACS - 2001
Helen Caines
Particle Ratios and Chemical Content
  e / T
- 2 -1
q s
2
q s
L u ds  
-4 -2


 q s
L uds  
-
 
X
dss


 -q1 s
L uds   
2
s
2
q s
2
s q s
N i   j e
- ( E - j )
ji
j= Quark Chemical
Potential
T = Temperature
Ej – Energy required to
add quark
j– Saturation factor
Use ratios of particles to determine , Tch and saturation factor
ACS - 2001
Helen Caines
T
Chemical Fit Results
Not a 4-yields fit!
s  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)
ACS - 2001
Helen Caines
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]
ACS - 2001
Helen Caines
Kinetic Freeze-out and Radial Flow
1/mt d2N/dydmt
Want to look at how energy distributed
in system.
Look in transverse direction so not
confused by longitudinal expansion
mt
Look at mt = (pt2 + m2 ) distribution
A thermal distribution gives a linear
distribution
dN/dmt  e-(mt/T)
Slope = 1/T
If there is transverse flow
Slope = 1/Tmeas
~ 1/(Tfo+ mo<vt>2)
ACS - 2001
Helen Caines
Kaon Slope Systematics
K 0s
e(-mT/T)
T~290+-5 MeV
ACS - 2001
Helen Caines
Inverse slope for f and L
e(-mt/T)
_
L
L
T=352+-7 MeV
Similar slopes for similar masses
ACS - 2001
Helen Caines
L Inverse Slope Systematics
Note spectra are not feed-down corrected
Fits are e(-mt/T)
Centrality %
L
T (MeV)
0-5
342 ± 9 ± 20
5-10
336 ± 9 ± 20
10-20
328 ± 7 ± 20
20-35
331 ± 8 ± 20
35-75
295 ± 7 ± 19
T=300-350 MeV
|y|<0.5
Some indication that one slope fit is not appropriate at low and high mt
ACS - 2001
Helen Caines
mt slopes vs. Centrality
mid-rapidity
Tp = 565 MeV
TK = 300 MeV
T = 190 MeV
• Increase with collision
centrality
•  consistent with radial flow.
ACS - 2001
Helen Caines
Radial Flow?
STAR Preliminary
Fitting to p
indicates high
flow.
Fitting to L and
f“same” as
SPS.
L
What’s going
on?
L
L
Depends on fit range
ACS - 2001
Helen Caines
mT dist. from Hydrodynamic type model
s
s
u (t , r , z  0)  (cosh  , er sinh  , 0)
  tanh -1 r
R
 r   s f (r )
flow profile selected
(r =s (r/Rmax)0.5)
ACS - 2001
1/mT dN/dmT (a.u.)
Ref. : E.Schnedermann et al, PRC48 (1993) 2462
mT - m
Helen Caines
Fits to the hydro. model
Kp
p
K-
Tth [GeV]
L
-
Tth [GeV]
1/mT dN/dmT (a.u.)
solid : used for fit
-
L
0
0.4
<r > [c]
0
<r > [c]
0.4
ßr (RHIC) = 0.52c
Tfo (RHIC) = 0.13 GeV
mT - m [GeV/c2]
explosive radial expansion at RHIC  high pressure
ACS - 2001
Helen Caines
The Global picture
Seems to be a limiting
Tfo
As colliding energy
goes up energy goes
into higher and higher
transverse flow.
ACS - 2001
Helen Caines
_
h-, L and L pT distributions
Evidence that B/M ratio > 1
at high pT
Consequence of radial flow ?
or novel baryon dynamics ?
STAR Preliminary
ACS - 2001
Helen Caines
f,L,L fractions of hNote: spectra are not
feed-down corrected
STAR Preliminary
 and L yields are
from fits to
Boltzmann;
_
h- yields are power
law fits
L= (0.042 0.001)h-
All ratios are flat as
functions of centrality
f= (0.02 0.002)h-
ACS - 2001
Helen Caines
K-/- Ratios
K-/- ratio is enhanced
by almost a factor of 2
in central collisions
when compared to
peripheral collisions
STAR preliminary
Similar dependence on
centrality was seen in
SPS and AGS data
SP
S
ACS - 2001
Energy dependence of
the ratio reflects the
changing baryon
chemical potential.
Helen Caines
K0*/h-
Represents a 50% increase
compared to K0*/ measured
in pp at the ISR.
More evidence of
Strangeness Enhancement?
ACS - 2001
Helen Caines
/h- Ratios
Relative
production of
f increasing
with collision
energy in
heavy ion
collisions.
STAR preliminary
HI collisions
p+p collisions
p+p collisions
Strangeness
Enhancement?
ACS
- 2001
Helen Caines
What have we learnt so far?
• What is the initial environment like for particle
Still a significant amount
production?
of baryon number around
– Net baryon density
• What happens during the initial particle
Increasing fraction of
production?
– Strangeness production
particle production with
energy, but not centrality?
– Quark coalescence?
Reasonable predictor
• Are re-interactions significant?Little pt dependence,
significant rescattering?
– Rescattering of hadrons?
– Radial flow?
Slope dependence of m
t
fit range- Large flow
ACS - 2001
Helen Caines
SPARE STUFF-not shown
ACS - 2001
Helen Caines
1/mT dN/dmT (a.u.)
Interpreting the mt spectra
_
L(x2)
p
mT – m0 (GeV/c2)
ACS - 2001
Helen Caines
K0*/hRepresents a 50% increase
compared to K0*/ 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
ACS - 2001
Helen Caines
Event (Centrality) Selection
PRL 86, (2001) 402
5% Central
nch = primary tracks in || < 0.75
ZDC
Au
Au
ACS - 2001
ZDC
Central Multiplicity
Detectors
Helen Caines
Strange particle ratios
ACS - 2001
Helen Caines
The STAR Detector (Year-by-Year)
Magnet
Coils
TPC Endcap &
MWPC
Time
Projection
Chamber
Silicon
Vertex
Tracker *
FTPCs
ZCal
Endcap
Calorimeter
Barrel EM
Calorimeter
ZCal
Vertex
Position
Detectors
Central Trigger
Barrel
+ TOF patch
RICH
* yr.1 SVT ladder
Year 2000, year 2001, year-by-year until 2003,
ACS - 2001
Helen Caines
installation
in 2003
pbar/p ratio
X.N.Wang, Phys.Rev.C 58 (1998) 2321
Identified pbar/p Ratio
• Two Detectors
– TPC
– RICH
pbar/p ratio
• RICH will extend ratio to
5 GeV/c with improved
statistics
• Ratio constant as
function of Pt
ACS - 2001
Helen Caines
Comparing to SPS
K+/K-(dE/dx) = 1.08 ±0.01 (stat.)±
0.06 (sys.)
f/h-
= 0.021 ± 0.001 (stat.)±
0.004 (sys.)
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.)
L̄/L
= 0.73 ± 0.03 (stat.)
X/X0.82±
0.08 (stat.)
¯
¯p/p
ACS - 2001
Helen Caines
SVT Performance
Noise
1ch=2mV
Cosmic Ray Event–L3 Trigger
Threshold at 4mV 6% live
Hits from Au-Au Event
ACS - 2001
Helen Caines
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
= 0.7 ±0.5
R = 6.5 ± 2.3
K0s Correlation will
become statistically
meaningful once we have
~10M events (aim for this
year)
ACS - 2001
Helen Caines
Preliminary L/L Ratio
Central events
_
L/L= 0.73  0.03 (stat)
|y|<0.5
Ratio is flat as a function of pt and y
ACS - 2001
Helen Caines
Strangeness Highlights (2)
SPS
AGS
AGS and SPS > 1
_
Need to consider p absorption
Multi-Strange Particles appear to
freeze out at a cooler temperature/
earlier or have less flow
ACS - 2001
Helen Caines
Previous Strangeness Highlights
Enhancement W> X > L > h
SPS s=17GeV
WA97
|s|
Evidence of strangeness enhancement between pA and
ACS - 2001
Helen
Caines
AA collisions at the SPS
– Not reproducible by
models