Heavy-Ion Physics
description
Transcript of Heavy-Ion Physics
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Heavy-Ion Physics
Raimond Snellings
XXIII Physics in Collision
Zeuthen, GermanyJune 26-28, 2003
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Outline
Brief introduction to Heavy-Ion Physics CERN SPS: a new state of matter BNL Relativistic Heavy Ion Collider BRAHMS, PHOBOS, PHENIX and STAR (a few selected) RHIC results from year 1-3 Summary
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Collisions of “Large” nuclei convert beam energy to temperatures above 200 MeV or 1,500,000,000,000 K
~100,000 times higher temperature than the center of our sun.
“Large” as compared to mean-free path of produced particles.
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QCD Phase Diagram
We normally think of 4 phases: Plasma Gas Liquid Solid
Phase diagram of waterPhase diagram of nuclear matter
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F. Karsch, hep-lat/0106019
QCD on the Lattice
3/5.1~
200150~
fmGeV
MeVT
c
c
•Z. Fodor and S.D. Katz, hep-lat/01060002
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Schematic Space-Time Diagram of a Heavy Ion Collision
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space
timeSchematic Time Evolution
e
Hard Scattering
AuAu
E
xpan
sion
----
------
-
Hadronization
Freeze-out
jet J/
QGP?Thermalization?
ep K
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CERN SPS: A New State of Matter?
J/ suppression indication of deconfinement? Strangeness enhancement Melting of the
NA50
Are hadronic scenarios ruled out? Co-mover absorption? canonical suppression?
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SPS, NA49: Indications of a Phase Transition at ≈ 30 GeV ?
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A New Era for Heavy Ion Physics: The Relativistic Heavy Ion Collider at BNL
3.83 km circumference Two independent rings
120 bunches/ring 106 ns crossing time
Capable of colliding ~any nuclear species on ~any other species
Energy: 200 GeV for Au-Au
(per N-N collision) 500 GeV for p-p
Luminosity Au-Au: 2 x 1026 cm-2 s-1
p-p : 2 x 1032 cm-2 s-1 (polarized)
`
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Hadron PID over broad rapidity acceptance
Two conventional beam line spectrometers Magnets, Tracking Chambers, TOF, RICH
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Charged Hadrons in Central Spectrometer
Nearly 4 coverage multiplicity counters
Silicon Multiplicity Rings
Magnetic field, Silicon Pad Detectors, TOF
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Electrons, Muons, Photons and Hadrons Measurement Capabilities
Focus on Rare Probes:
J/, high-pT
Two central spectrometers with tracking and electron/photon PID
Two forward muon spectrometers
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Hadronic Observables over a Large Acceptance
Event-by-Event Capabilities
Solenoidal magnetic field Large coverage Time-
Projection Chamber Silicon Tracking, RICH,
EMC, TOF
Online Level 3 Trigger Display
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Heavy-ion Physics at RHIC• RHIC different from previous (fixed target)
heavy ion facilities• ECM increased by order-of-magnitude
• Accessible x (parton momentum fraction) decreases by ~ same factor
• Study pp, pA to AA• Comprehensive set of detectors
• All final state particles measured with overlap between the detectors
• Study QCD at high density with probes generated in the medium
• If QGP produced at RHIC most likely to live longer than at the SPS and therefore easier to observe and study its properties
p
s
2 tx
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Event Characterization Cannot directly measure the impact parameter b! but we can distinguish peripheral collisions from central
collisions!
Ncoll
Npart
Impact Parameter (b)
•b
5% Central
STAR
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Soft Physics
Particle Yields Spectra shapes Elliptic Flow
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Particle distributions (PHOBOS)
dN
ch/d
19.6 GeV 130 GeV 200 GeV
PHOBOS PreliminaryCentral
Peripheral
•central collisions at 130 GeV: 4200 charged particles !•mid rapidity plateau
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Energy Density (Bjorken estimate)
dy
dE
RdyR
dE
dzR
dE
V
E T
TT
T
T
T
222
1~~
3/6.4
2
0/
fmGeVo
yT
BJ R
dydE
Bjorken Estimate
R
0ddET 503±2 GeV
(130 GeV)
PRL 87 (2001)
•preliminary
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Particle spectra at RHIC Superimposed on the thermal (~Boltzmann) distributions:
Collective velocity fields from
Momentum spectra ~
‘Test’ by investigating description for different mass particles:
Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061)
0j,0T μBμ
μνμ
)mp
v(fpd
)dn(Thermal~
pddn
HYDRO
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Particle spectra at the SPS
Rather well described by Hydro motivated fit
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Particle ratios: chemical potentials and freeze-out temperature
pdedn E 3/)(~ Tμ Assume distributions described by
one temperature T and one ( baryon) chemical potential One ratio (e.g.,p / p ) determines / T A second ratio (e.g., K / ) provides T
Then predict all other hadronic yields
and ratios:
TμTμ
Tμ/2
/)(
/)(
ee
e
p
pE
E
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Where is RHIC on the phase diagram?
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Three Forms of Collective Motion
Only type of transverse flow in central collision (b=0) is transverse flow.
Integrates pressure history over complete expansion phase
x
y
Elliptic flow, caused by anisotropic initial overlap region (b > 0).
More weight towards early stage of expansion. x
y
Directed flow, sensitive to earliest collision stage (pre-equilibrium, b > 0) z
x
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What makes elliptic flow an unique probe?
Non central collisions coordinate space configuration anisotropic (almond shape). However, initial momentum distribution isotropic (spherically symmetric).
Only interactions among constituents (mean free path small) generate a pressure gradient which transforms the initial coordinate space anisotropy into the observed momentum space anisotropy
Multiple interactions lead to thermalization -> limiting behavior hydrodynamic flow
)(tan,)(2cos 12
x
yr p
pv
1
2
3
3
cos212
1
nrn
tt
nvdydpp
Nd
pd
NdE
y
x
coordinate space
py
px
Momentum space
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Elliptic Flow at the SPS (NA49 and CERES)
•Clearly deviates from ideal hydrodynamic model calculations
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Hydrodynamic limit
STAR
PHOBOS
Hydrodynamic limit
STAR
PHOBOS
Compilation and Figure from M. Kaneta
Integrated Elliptic Flow
First time in Heavy-Ion Collisions a system created which at low pt is in quantitative agreement with hydrodynamic model predictions for v2 up to mid-central collisions
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Differential Elliptic Flow
Hydro calculation: P. Huovinen et. al.
Typical pt dependence Heavy particles more
sensitive to velocity distribution (less effected by thermal smearing) therefore put better constrained on EOS
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Soft Physics
Energy density estimate well above critical Lattice values
Particle yields are well described in a thermal model Spectra shapes are consistent with thermal boosted
distributions Elliptic flow reaches hydrodynamical model
predictions First time in heavy-ion collisions
Observables consistent with strong early partonic interactions and approaching early local equilibrium
However, size measurements (HBT) are not completely understood yet
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Hard probes and the produced medium
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p+p->0 + X
hep-ex/0305013 S.S. Adler et al.
Hard probes
At RHIC energies different mechanisms are responsible for different regions of particle production.
Rare process (Hard Scattering or “Jets”), a calibrated probe
Hard Scatterin
g
“Well Calibrated”
Thermally-shaped Soft Production
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Hard Probes and the Produced Medium Hard scatterings in
nucleon collisions produce jets of particles.
In the presence of a color-deconfined medium, the partons strongly interact losing much of their energy “Jet Quenching”
32
2,glu
jetGLV eR S
EE C d r og
LL
hadrons
q
q
hadrons leadingparticle
leading particle
schematic view of jet production
22
4R s
BD eM gluS LC
E v
2 24ln
3TG F SD
EE C T L
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Jets at RHICp+p jet+jet
(STAR@RHIC)Au+Au X
(STAR@RHIC)
find this in this
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Find partonic energy loss with leading hadrons
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
Binary collision scaling p+p reference
Energy loss softening of fragmentation suppression of leading hadron yield
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Relative to UA1 p+p
nucl-ex/0305015
BRAHMS preliminary
Measurements of jet suppression
nucl-ex/0304022
Binary scaling
Participant scaling
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Elliptic Flow at higher-pt
M. Gyulassy, I. Vitev and X.N. Wang
•R.S, A.M. Poskanzer, S.A. Voloshin, •STAR note, nucl-ex/9904003
STAR preliminary
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Back to back “jets” at the SPS (CERES)
Cronin Effect:
Multiple Collisions broaden high PT
spectrum
•Centrality 24-30%
•Centrality 11-15%
•SPS. CERES: Away side jet broadening, no disappearance
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near side
away side
peripheral central
Disappearance of back to back “jets”
PRL 90, 082302 (2003)
2( )
22( ) 2(1 cos(2 ))
Au Au
p p
D
D B v
•In central Au+Au collisions the away-side “jet” disappears !!
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High-pt phenomena: Initial state or final state effect?
pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching
nucl-ex/0305015
Final state
Initial state
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Theory expectations for d+Au
If Au+Au suppression is initial state (KLM saturation: 0.75)
1.1-1.5
pT
RA
B
1
Inclusive spectraIf Au+Au suppression is final state
~2-4 GeV/c
All effects strongest in central d+Au collisions
pQCD: no suppression, small broadening due to Cronin effect
0 /2 (radians)
0
High pT hadron pairs
saturation models: suppression due to mono-jet contribution?
suppression?
broadening?
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Comparison of Au+Au to d+Au (PHOBOS and BRAHMS)
central Au+Au PHOBOS d+Au: nucl-ex/0306025
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Comparison of Au+Au to d+Au (PHENIX and STAR)
Dramatically different behavior of Au+Au observables compared to d+Au observables.
Jet Suppression is clearly a final state effect.
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Back to back “jets” in d+Au
“PHENIX Preliminary” results, consistent with
STAR data in submitted paper
?
Central Au+Au
d+Au
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Summary
High-pt probes are a new unique tool at RHIC to understand heavy-ion collisions
New phenomena have been found: Suppression of the inclusive yields (“jet quenching”) Large elliptic flow Disappearance of the away-side “jet”
Pointing at very dense (≈ 30x nuclear densities) and strongly interacting matter
Low-pt (bulk) and high-pt observables consistent with expectations from a QGP (but not as proof, still more work to be done. RHIC program just started)
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Thanks
Many figures on the slides are “borrowed” from: W. Zajc, P. Steinberg, N. Xu, P. Jacobs, F. Laue,
P. Kolb, U. Heinz, T. Hemmick, G. Roland, I. Bearden, M. van Leeuwen and many others
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Time Evolution in a Hydro Calculation
Elliptic Flow reduces spatial anisotropy -> shuts itself off
Calculation: P. Kolb, J. Sollfrank and U.Heinz
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Structure Functions