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STAR
Helen CainesThe Ohio State
University
March 2001
Crossing a New Threshold
First Results from the Relativistic Heavy Ion Collider
Science is a wonderful thing if one does not have to earn one's living at it – Einstein (1879—1955)
Helen Caines
OSU – March 2001STAR
Motivation
Why Relativistic Heavy Ion Collisions?To study a hadronic matter at high energy
densityEarly universeCenter of stars
To study the deconfined state of QCDWhere is the phase transition?What order is it?
To study the Vacuum – Chiral symmetry restoration
Origin of (hadronic) mass
Helen Caines
OSU – March 2001STAR
The Phase Space DiagramTWO different phase transitions at work!
– Particles roam freely over a large volume
– Masses changeCalculations show that these occur at approximately the same pointTwo sets of conditions:High TemperatureHigh Baryon DensityLattice QCD calc. Predict:
Tc ~ 150-170 MeV
c ~ 0.5-0.7 GeV/fm
Deconfinement transition
Chiral transition
Helen Caines
OSU – March 2001STAR
most dangerous event in
human history: - ABC
News –Sept ‘99
Don’t Panic!!!"Big Bang machine could
destroy Earth" -The
Sunday Times – July ‘99
the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99
- New Scientist
Will Brookhaven
Destroy the Universe? –
NY Times – Aug ‘99
No… the experiment will not tear our region of space to subatomic shreds.- Washington Post – Sept ‘99
Apocalypse2 – ABC News – Sept
‘99
Helen Caines
OSU – March 2001STAR
Welcome to BNL- RHIC!
Helen Caines
OSU – March 2001STAR
The Collisions
The End Product
Helen Caines
OSU – March 2001STAR
The STAR Detector (Year-by-Year)
• Year 2000, year 2001, year-by-year until 2003, installation in 2003
ZCal
Silicon Vertex Tracker *
Central Trigger Barrel+ TOF patch
FTPCs (1 + 1)
Time Projection Chamber
Vertex Position Detectors
Magnet
Coils
RICH * yr.1 SVT ladder
Barrel EM Calorimeter
TPC Endcap & MWPC
Endcap Calorimeter
ZCal
Helen Caines
OSU – March 2001STAR
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: 140000 amplifier
channels with 512 time samples – Provides 70 mega pixel, 3D image
Helen Caines
OSU – March 2001STAR
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
Good tracking efficiency
Clearly identify individual tracks
high resolution
Pt (GeV/c)
Helen Caines
OSU – March 2001STAR
Spectators – Definitely going down the beam lineParticipants – Definitely created moving away from beamline
Triggering/Centrality
ImpactParameter
Spectators
SpectatorsZero-Degree Calorimeter
Participants
Several meters
• “Minimum Bias”ZDC East and West thresholds set to lower edge of single neutron peak.
REQUIRE:Coincidence ZDC East and West
• “Central”CTB threshold set to upper 15%
REQUIRE: Min. Bias + CTB over threshold
~30K Events |Zvtx| < 200 cm
Helen Caines
OSU – March 2001STAR
Au-Au Event at 130 A-GeV
Peripheral EventFrom real-time Level 3 display.
Helen Caines
OSU – March 2001STAR
Au- Au Event 130 A-GeV
Mid-Central EventFrom real-time Level 3 display.
Helen Caines
OSU – March 2001STAR
Au -Au Event 130 A-GeV
Central EventFrom real-time Level 3 display.
Helen Caines
OSU – March 2001STAR
STAR Pertinent FactsField: 0.25 T (Half Nominal value) worse resolution at higher p lower pt acceptance
TPC: Inner Radius – 50cm
(pt>75 MeV/c)
Length – ± 200cm ( -1.5 1.5)
Events: ~300,000 “Central” Events –top 8% multiplicity~160,000 “Min-bias” Events
Helen Caines
OSU – March 2001STAR
Particle ID Techniques - dE/dx
dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p
12
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
12
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
dE/dx
6.7%Design7.5%With calibration9 %No calibration
Resolution:
Even identified anti-3He !
Helen Caines
OSU – March 2001STAR
Particle ID Techniques - Topology
Decay vertices Ks + + -
p + -
p + +
- + -
+ + +
+ K -
“kinks”: K +
Vo
Helen Caines
OSU – March 2001STAR
STAR STRANGENESS!
K0s
K+
(Preliminary)
Helen Caines
OSU – March 2001STAR
Physics Measurements
•dN/dfor h- (||<= ~1.5) particle density, entropy•Flow early dynamics, pressure•p/p, / stopping•Particle spectra temperature, radial flow•Particle ratioschemistry•Particle correlations geometry, collective flow•High Pt jet quenching
__
•Neutral particle decays ,K0s, strangeness production
Helen Caines
OSU – March 2001STAR
The Serious Predictions>factor 2 variation in yields Radii increase from SPS
R0/Rs >= 1.6 (long lifetime)
Little Stopping –Net proton yield = 4 –
20
Transverse flow – Same a SPS - much
higherHeavier particles not
see flow
Helen Caines
OSU – March 2001STAR
Negative Hadrons: Distribution and Multiplicity
h-
Full efficiency corrections
h-
Increased particle production per participant pair:43% compared to Pb+Pb @ 17.2 GeV30% compared to pp @ 200 GeV
dN(h-)/d = 264 1 18 (extrap. to all pt) At low end of predictions – Kills many models
More than just pp happening
Helen Caines
OSU – March 2001STAR
Transverse Energy
PHENIX Preliminary
Phenix Electromagnetic Calorimeter measures transverse energy in collisionsCentral Events:
Lattice predicts transition at
~ 5.0 GeV/fm3
critical ~ 0.5-0.7 GeV/fm3
Have the Energy Density!!
dydE
RBjt
02 2
11
Helen Caines
OSU – March 2001STAR
Is there Thermalization?
Almond shape overlap region in coordinate space
y2 x2 y2 x2
2cos2 v
x
y
pp
atan
Origin: spatial anisotropy of the system when created and rescattering of evolving system
Look at “Elliptic” Flow
Helen Caines
OSU – March 2001STAR
Hydro Calculation of Elliptic Flow
P. Kolb, J. Sollfrank, and U. Heinz
Equal energy density lines
• Elliptic flow observable sensitive to early evolution of system
• Large v2 is an indication of early thermalization
First time in Heavy-Ion Collisions a system created which approaches hydrodynamic model predictions
Flow: A pressure build up -> Explosion with azimuthal asymmetry
•zero for central eventsHydrodynamics: Assumes continuum matter with local equilibrium
•Locally equilibrated or “thermalized”.
|| < 1.3
0.1 < pt < 2.0
Hydro Calculations
STARPRL 86 (2001) 402
Helen Caines
OSU – March 2001STAR
OK
•Have a high enough energy density to cause transition•Have a source that is consistent with being thermalized and has a large elliptic flowBut what did we create?
Helen Caines
OSU – March 2001STAR
Baryon Stopping/Transport
Anti-baryons - all from pair productionBaryons - pair production + transported
B/B ratio =1 - Transparent collision
B/B ratio ~ 0 - Full stopping, little pair production
Measure p/p, / , K-/K+
(uud/uud) (uds/uds) (us/us)
_
_
_ _
- - - - - - - -
Helen Caines
OSU – March 2001STAR
p/p Ratio_
Phys. Rev. Lett March 2001
Ratio = 0.65 ±0.03(stat) ±0.03(sys)
Ratio is flat as function of pt and y
Slight fall with centrality
Helen Caines
OSU – March 2001STAR
Strange Baryon Ratios
Ratio = 0.73 ± 0.03 (stat)
~0.84 /ev, ~ 0.61/ev
Reconstruct: Reconstruct: _
STAR Preliminary
~0.006 /ev, ~0.005/ev
Ratio = 0.82 ± 0.08 (stat)
Helen Caines
OSU – March 2001STAR
¯______
_
Anti-baryon/Baryon Ratios versus s
STAR preliminary
Baryon-pair production increases dramatically with s – still not baryon free
65.0
Trpair
pair
p
pbar
YYY
YY
2Tr
pair
YY
2/3 of protons from pair production , yet pt dist. the same
– Another indication of thermalization
Pair production is larger than baryon transport
Helen Caines
OSU – March 2001STAR
Simple ModelAssume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models
*Duds
sdu*ss
uu
ussssu
pp*D
uudduu
pp*
ss
uu
udssdu
D=1.12
D=1.12
No free quarks so all quarks have to end up confined within a hadron
Predict
Predict
D=1.08± 0.08
susu
KK
ss
uuD
Measure
System consistent with having a de-confined phase
Helen Caines
OSU – March 2001STAR
Kinetic Freeze-out and Radial Flow
If there is transverse flow
Look at mt = (pt2 + m2 )
distributionA thermal distribution gives a linear distribution
dN/dmt e-(mt/T)
mt
1/m
t d2 N
/dyd
mt
Slope = 1/T
Slope = 1/Tmeas
~ 1/(Tfo+ 0.5mo<vt>2)
Want to look at how energy distributed in system.Look in transverse direction so not confused by longitudinal expansion
Helen Caines
OSU – March 2001STAR
T = 190 MeV
T = 300 MeV
Tp = 565 MeV
mid-rapidity
mt slopes vs. Centrality
• Increase with collision centrality
consistent with radial flow.
Helen Caines
OSU – March 2001STAR
Radial Flow: mt - slopes versus mass
Naïve: T = Tfreeze-out + m r 2 where r = averaged flow velocity
Increased radial flow at RHICßr (RHIC) ßr (SPS/AGS) = 0.6c = 0.4 - 0.5cTfo (RHIC) Tfo (SPS/AGS) = 0.1-0.12 GeV = 0.12-0.14 GeV
Helen Caines
OSU – March 2001STAR
Particle Ratios and Chemical Content
j= Quark Chemical PotentialT = TemperatureEj – Energy of quarkj– Saturation factor
Use ratios of particles to determine Tch and saturation factor
ij
i ejNT
jjE
)(
Helen Caines
OSU – March 2001STAR
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
Helen Caines
OSU – March 2001STAR
P. Braun-Munzinger, nucl-ex/0007021
Chemical Freeze-out
Baryonic Potential B [MeV]
Chem
ical T
empe
ratu
re T
ch [M
eV]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
LEP/
Spp
S
SPS
RHIC quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinementchiral restauration
Lattice QCD
atomic nuclei
Helen Caines
OSU – March 2001STAR
OK (2)
Shown that the collision region:
•Some evidence that source is thermalized•Particles kinetically freeze-out with common T•Large transverse flow -
common to all species•Particles chemically freeze out earlier (higher T)•Near y axis on phase diagram•Relative particle production consitant with having
had free quarks
Helen Caines
OSU – March 2001STAR
K
RoutRside
Measuring the Source “Size” (HBT)
222111 xyipxyip ee~
~5 fm
x1
x2
y1
y2 ~1 m 122211 xyipxyip ee
)xpcos(1~)p,p(P *21
C (Q
inv)
Qinv (GeV/c)
1
2
0.05 0.10
Width ~ 1/R
1D: overallrough “size”
3D decomposition of relative momentum provides handle on shape and time as well as size
Helen Caines
OSU – March 2001STAR
HBT and the Phase Transition
withouttransition
“”
withtransition
c
Rischke & GyulassyNPA 608, 479 (1996)
Generic prediction of 3D hydrodynamic models
Primary HBT “signature” of QGP
~ emission
timescale
Phase transition longer lifetime; Rout/Rside ~ 1 + ()/Rside
Helen Caines
OSU – March 2001STAR
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, 0.125 < pt < 0.225
qout
STAR preliminary
STAR preliminary
qlong
fmRfmRfmR
Long
Side
Out
)21.012.007.7()16.009.047.5()23.011.086.5(
03.001.050.0
Helen Caines
OSU – March 2001STAR
Radii dependence on centrality and kt
•Radii increase with multiplicity - Just geometry (?)•Radii decrease with kt – Evidence of flow (?)
low kT central collisions
“multiplicity”
STAR preliminary
x (fm)
y (f
m)
Helen Caines
OSU – March 2001STAR
Pion HBT Excitation Function
• 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!
STAR PreliminaryCompilation of world 3D -HBT parameters as a function of s
Helen Caines
OSU – March 2001STAR
2/)( 21TTT ppK
STAR Preliminary
Tomášik, Heinz nucl-th/9805016
=0.0
=0.5
opaqueness
The ROut/RSide RatioEmission duration for transparent sources:
TSideOut RR 22
Small radii + short emission time + opaqueness short freeze-out
Helen Caines
OSU – March 2001STAR
K0s-K0
s Correlations
= 0.7 ±0.5
R = 6.5 ± 2.3
•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
K0s Correlation will
become statistically meaningful once we have ~10M events
Helen Caines
OSU – March 2001STAR
Hard Probes in Heavy-Ion Collisions
a) formation phaseparton scattering
b) hot and dense phaseQuark Gluon PlasmaHadron Gas
c) freeze-outemission of hadrons
• “hard” probes: cc, bb and jets– during formation phase parton
scattering processes with large Q2 – create high mass or high momentum
objects– penetrate hot and dense matter– sensitive to state of hot and dense
matter
color screening: J/suppression
dE/dx jet quenching
QGP
vacuum
Helen Caines
OSU – March 2001STAR
Negative Hadrons: pt - distributions
Power Law
A (1 + pt /p0) - n
p0 = 2.74 ± 0.11 GeV/c
n = 13.65 ± 0.42
STAR
<pt> = 0.514 ± 0.012 GeV/c
NA49
<pt> = 0.414 ± 0.004 GeV/c
UA1
<pt> = 0.392 ± 0.003 GeV/cSTAR preliminary
Mean pt higher than SPS and pp
Helen Caines
OSU – March 2001STAR
Au+Au/pp: Compare pt - distributions
• “Hard” Scaling• Nuclear Overlap
Integral• TAA = 26 mb-1 for 5%
most central• NAA / Npp= Nbin coll =
1050
• “Soft” Scaling• NAA / Npp= ( 344 / 2 )
Jet Quenching:First hint for QGP formation at RHIC ?
STAR preliminary
Helen Caines
OSU – March 2001STAR
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 SPSThermal freeze out same as at SPS
Unexpected: small HBT radii Strong elliptic flow Pion phase-space density at freeze-out seems to be universal
• Promising results from “Hard Physics” pt spectra from central collisions show clear deviation from p-p
extrapolation high-pt data are consistent with “jet quenching” predictions !
More than we ever hoped for after the first run !!!
Helen Caines
OSU – March 2001STAR
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
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 Institutions: 36 Collaborators: 415
The GroupProfs: PostDocs: Students:T.Humanic Me S.BekeleM.Lisa B.Neilson M.Lopez-
NoriegaE.Sugarbaker R.Wells
R.Wilson
The STAR Collaboration
Helen Caines
OSU – March 2001STAR
0 0.1 0.2 0.3 0.4 0.5 0.6pt
0
0.05
0.1
0.15
0.2
0.25
0.3
<f>
Pion Phase Space Density
NA49
STAR PreliminarySTAR
Radius Fits
fBE;no flowT0=99.5 MeVT0=94.3 MeVT0=89.7 MeV
fBE;flowT0=94.3 MeVT0=89.7 MeV
The Phase Space Density
• “Universal” phase space density observed at SPS appears to hold at RHIC as well• Consistent with thermal distribution (T94MeV) and strong collective flow ( 0.58)• Fundamental phase space saturation may relate increases in geometry,
temperature, multiplicity
pion occupation of cell in coordinatemomentum space:
LSO
1/2
T
2
Tπ
3
T RRRπ) (λ
myN
mE2)()m(
dddcf
Helen Caines
OSU – March 2001STAR
Calibration – Cosmic Rays
Determine momentum resolution
p/p < 2% for most tracks
Helen Caines
OSU – March 2001STAR
Calibration - Lasers
Using a system of lasers and mirrors illuminate the TPC Produces a series of>500 straight lines criss-crossing the TPC volumeDetermines:
• Drift velocity• Timing offsets• Alignment
Helen Caines
OSU – March 2001STAR
QGP prediction: Enhancement > > > h
Evidence for Strangeness Enhancement
WA97
Helen Caines
OSU – March 2001STAR
What about the Chemical Freeze-out?
Yields of hadrons characterised by a few simple parameters
T, V, q (or expq/T), S
Absolute abundances require more sophisticated descriptions including such details as flow effects and the fact that the fire-ball
isn’t at rest.
Perform a least-squared fit to the data with T, V, q /T and S as free parameters
Made simpler by taking particle ratios.
Helen Caines
OSU – March 2001STAR
Energy Density Estimate
What is the energy density reached?Is it high enough to cause phase transition?Is there thermalization?Bjorken formula for thermalized energy density dy
dERBj
t
02 2
11
R2
2c0
Measure Et at y=0
Assume 0 = 0.5 fm/cAssume full overlap
Helen Caines
OSU – March 2001STAR
Elliptic Flow of Pions and Protons
• Hydro calculations: P. Huovinen, P. Kolb and U. Heinz
Mass dependence of v2(pt) shows a behavior in agreement with hydro calculations
Helen Caines
OSU – March 2001STAR
Elliptic Flow Excitation Function
STAR, PRL 86 (2001) 402
Helen Caines
OSU – March 2001STAR
v2(pt) for high pt particles
M. Gyulassy, I. Vitev and X.N. Wang, nucl-th/00012092
Helen Caines
OSU – March 2001STAR
BeforeAfter
In case you thought it was easy…
Helen Caines
OSU – March 2001STAR
Particle ID Techniques Combinatorics
Ks + + - K+ + K-
p + - p + +
Combinatorics
from K+ K- pairs
K+ K- pairs
m inv
m inv
same event dist.mixed event dist.
background subtracted
dn/dm
dn/dm Breit-Wigner fit Mass & width consistent w. PDG
K* combine all K+ and -
pairs (x 10-5)
m inv (GeV)
Helen Caines
OSU – March 2001STAR
Charged particle anisotropy 0< pt< 4.5 GeV/c
Around pt > 2 GeV/c the data starts to deviate from hydro. However, v2 stays large.
Only statistical errors
Systematic error 10% - 20% for pt = 2 – 4.5 GeV/c