crossing a new threshold first results from the relativistic heavy ion collider
DESCRIPTION
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). Motivation. Why Relativistic Heavy Ion Collisions? To study a hadronic matter at high energy density - PowerPoint PPT PresentationTRANSCRIPT
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 density
Early universe
Center of stars
To study the deconfined state of QCD
Where 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 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:
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 – S
ept
‘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 line
Participants – Definitely created moving away from beamline
Triggering/Centrality
ImpactParameter
Spectators
Spectators
Zero-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 Facts
Field:
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%Design
7.5%With calibration
9 %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 higher
Heavier 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 collisions
Central 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
p
patan
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
thermalizationFirst 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 events
Hydrodynamics:
Assumes continuum matter with local equilibrium
•Locally equilibrated or “thermalized”.
|| < 1.3
0.1 < pt < 2.0
Hydro Calculations
STAR
PRL 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 flow
But what did we create?
Helen Caines
OSU – March 2001STAR
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, / , 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
YY
Y
Y
Y
2Tr
pair
Y
Y
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 Model
Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models
*Duds
sdu*
s
s
u
u
uss
ssu
p
p*D
uud
duu
p
p*
s
s
u
u
uds
sduD=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
su
su
K
K
s
s
u
uD
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 d2N
/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 Potential
T = Temperature
Ej – Energy of quark
j– 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 Tem
pera
ture
Tch
[M
eV
]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
LEP
/ SppS
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
fmR
fmR
fmR
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 Preliminary
Compilation 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 Ratio
Emission 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 quenchingQGP
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 SPS
Thermal 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 0.6
pt
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 RRR
π) (λ
my
N
mE2
)()m(
dd
dcf
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
dE
RBjt
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