1
Hot quarks! : What do we see in Relativistic Heavy Ion
collisions
“How to cook the primordial soup”
Manuel Calderón de la Barca Sánchez
Indiana University
How does the strong force, QCD, behave at high energy?
How can we study it?
What tools do we use?
What have we learned?
2Manuel Calderón de la Barca, P408/508 Seminar
Disclaimermany of the concepts I will present are
“works in very active progress”
an unavoidable consequence of exciting, cutting edge science
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Heavy Ions: How does nuclear matter look at high temperature?
~ 1-3 GeV/fm3
High Density QCD Matter in Laboratory
Determine its properties
QCD Prediction: Phase Transitions
Deconfinement to Q-G Plasma
Chiral symmetry restoration
Relevance to other research areas?
Quark-hadron phase transition in early Universe
Cores of dense stars
High density QCD
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The phase diagram of water
Analogous graphs• superfluids• superconductors• metal/insulator• …
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Generating a deconfined state
Nuclear Matter(confined)
Hadronic Matter(confined)
Quark Gluon Plasmadeconfined !
Present understanding of Quantum Chromodynamics (QCD)• heating• compression deconfined color matter !
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Expectations from Lattice QCD
/T4 ~ # degrees of freedom
confined:few d.o.f.
deconfined:many d.o.f.
TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core]
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How can we study it?
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Imagine…
• You know that ice exists…
• Your theory friends with huge computers tell you that there is something called water…
• You don’t have a way to heat ice…
• So you put millions of ice cubes in an ice-accelerator
• Send them at 99.995% of the speed of light to collide
• Generating thousands of ice-cube+ice-cube collisions per second…
• And you watch it all from the vicinity of Mars!
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Key Idea: Bulk Matter
• We must create/compress/heat a bulk (geometrically large) system
– freeze/melt a single H20 molecule?
– fundamental distinction from particle physics
• Only achievable through collisions of the heaviest nuclei (Au, Pb) at the highest available energy– at Relativistic Heavy Ion Collider (RHIC)
1000’s ofparticles producedin each collision
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What tools do we use?
How fast? How massive? How long?
Detectors and accelerators are our “bread-and-butter”.
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“RHIC is big”
as seen by the Landsat-4 satellite…
• big facility• big detectors• big collaborations• “big” collisions
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RHIC BRAHMSPHOBOS
PHENIXSTAR
AGS
TANDEMS
Relativistic Heavy Ion Collider (RHIC)
1 km
v = 0.99995c = 186,000 miles/sec
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RHIC BRAHMSPHOBOS
PHENIXSTAR
AGS
TANDEMS
Relativistic Heavy Ion Collider (RHIC)
PHENIX~ 500 collaborators
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RHIC BRAHMSPHOBOS
PHENIXSTAR
AGS
TANDEMS
Relativistic Heavy Ion Collider (RHIC)
STAR ~500 Collaborators
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The STAR ExperimentSTAR: Solenoidal Tracker at RHIC
multipurpose detector system for hadronic measurements
• large coverage (geometrical acceptance)• tracking of charged particles in high multiplicity environment• measure correlations of observables• study of hard processes (jet physics)
Solenoidal Tracker At RHIC
goal: track “all” charged hadrons (bags of quarks) emitted in each collision
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Operation of a Time Projection Chamber
SCA/ADCDAQ
Copper pads ~ 1cm2
Amplifying and digitizing electronics connected to each pad
Anode wires with +HVsitting ~5 mm above pads
Charged particle fliesthrough TPC gas…
..generating a clusterof liberated electrons
Electricfield
“Avalanche” as electronsapproach anode wire...
..capacitively inducinga signal on nearby pads... ..which is amplified, digitized, and recorded for later analysis
V
t AD
Cbucket #
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One collision seen by STAR
TPCMomentum determined by trackcurvature in magnetic field…
…and by direction relative to beam
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Particle momentum from tracking…
… how to get particle space-timeinformation??
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Impact parameter & Reaction plane
Impact parameter vector b :
beam direction
connects centers of colliding nuclei
b = 0 “central collision”many particles produced
“peripheral collision”fewer particles produced
b
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Impact parameter & Reaction plane
Impact parameter vector b :
beam direction
connects centers of colliding nuclei
b = 0 “central collision”many particles produced
“peripheral collision”fewer particles produced
b
b
Reaction plane:
spanned by beam direction and b
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Collision Geometry and Centrality
x
z
y
Non-central Collisions
Reaction plane
sNN = 200 GeV
Multiplicity
centralperipheral
5% Central
STAR
Multiplicity (arb. units)
ZD
C
Paddles/BBCZDC ZDC
Au Au
Paddles/BBC Central
Multiplicity Detectors
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What sorts of things have we learned?
• We hoped to create bulk matter,
– do we see evidence for collective behaviour?
• We hoped to create high density matter,
– do we see evidence for dissipative behaviour?
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How do semi-central collisions evolve?
b
1) Superposition of independent p+p:momenta pointed at randomrelative to reaction plane
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How do semi-central collisions evolve?
b
1) Superposition of independent p+p:
2) Evolution as a bulk system
momenta pointed at randomrelative to reaction plane
highdensity / pressure
at center
“zero” pressurein surrounding vacuum
Pressure gradients (larger in-plane) push bulk “out” “flow”
more, faster particles seen in-plane
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How do semi-central collisions evolve?
1) Superposition of independent p+p:
2) Evolution as a bulk system
momenta pointed at randomrelative to reaction plane
Pressure gradients (larger in-plane) push bulk “out” “flow”
more, faster particles seen in-plane
N
-RP (rad)0 /2 /4 3/4
N
-RP (rad)0 /2 /4 3/4
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Azimuthal distributions at RHICSTAR, PRL90 032301 (2003)
b ≈ 4 fm
“central” collisions
b ≈ 6.5 fm
midcentral collisions
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Azimuthal distributions at RHICSTAR, PRL90 032301 (2003)
b ≈ 4 fmb ≈ 6.5 fmb ≈ 10 fm
peripheral collisions
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Elliptic flow – collectivity& sensitivity to early system
STAR, PRL90 032301 (2003)
“v2”
“Elliptic flow”
• evidence ofcollective motion
• quantified by v2
• geometrical anisotropy momentum anisotropy
• sensitive to early pressure
• evidence for• early thermalization
• QGP in early stage
Hydrodynamiccalculation ofsystem evolution
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A more direct handle?• elliptic flow (v2) and other measurements (in a few slides)
evidence towards QGP at RHIC– indirect connection to geometry
• Are there more direct handles on the space-time geometry of collisions?
– yes ! Even at the 10-15 m / 10-23 s scale !
• What can they tell us about the QGP and system evolution?
– Let’s put Quantum Mechanics to good use!
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222111 p)xr(i22
p)xr(i11T e)p,x(Ue)p,x(U
probing source geometry through interferometry
12 ppq
2
21
2121 )q(~1
)p(P)p(P)p,p(P
)p,p(C
C (Q
inv)
Qinv (GeV/c)
1
2
0.05 0.10
Width ~ 1/R
Measurable! F.T. of pion source
)xx(iq2
*21
*1T
*T
21e1UUUU
Creation probability (x,p) = U*U5 fm
1 m source
(x)
r1
r2
x1
x2
{2
1
}e)p,x(Ue)p,x(U 212121 p)xr(i21
p)xr(i12
p1
p2
The Bottom line…if a pion is emitted, it is more likely to emit another
pion with very similar momentum if the source is small
experimentally measuring this enhanced probability: quite challenging
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Correlation functions for different colliding systems
C2(
Qin
v)
Qinv (GeV/c)
STAR preliminary p+pR ~ 1 fm
d+AuR ~ 2 fm
Au+AuR ~ 6 fm
Different colliding systems studied at RHIC
Interferometry probes extremely small scales!
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beam directi
on
More detailed geometryRelative momentum between pions is a vector can extract 3D shape information
1 2q p p
p2
p1
q1 2K p p
R long
RsideRout
Rlong – along beam direction
Rout – along “line of sight”
Rside – “line of sight”
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Timescales
• Evolution of source shape
– suggests system lifetime is shorter than otherwise-successful theory predicts
• Is there a more direct handle on timescales?
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Disintegration timescaleRelative momentum between pions is a vector can extract 3D shape information
1 2q p p
p2
p1
q1 2K p p
Rout
Rlong – along beam direction
Rout – along “line of sight”
Rside – “line of sight”
Rside
increases with emission timescale
OUT
SIDE
R
R
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Disintegration timescale - expectation
3D 1-fluid Hydrodynamics Rischke & Gyulassy, NPA 608, 479 (1996)
withtransition
withtransition
“” “”
Long-standing favorite signature of QGP:
• increase in , ROUT/RSIDE due to deconfinement confinement transition
• expected to “turn on” as QGP energy threshold is reached
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Disintegration timescale - observation
4
6
8
4
6
8
1.0
1.25
1.5R
O (
fm)
RS (
fm)
RO
/ R
S
increasing collision energy
RHIC
• no threshold effect seen
• RO/RS ~ 1
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Disintegration timescale - observation
• no threshold effect seen
• RO/RS ~ 1
• toy model calculations suggest very short timescales
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What have we learned (part I)?
• There is evidence that the system behaves collectively!
• Space-momentum correlations consistent with
– a very rapidly expanding source
– developed in extremely short timescales ~1fm
(so short that no models can account for it)
Do we know that the system is at high densisty?
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How can you know it’s high density?
• High energy collisions: access to high-momentum transfer processes
– High pT particle production
– High Mass particle production (Heavy Flavors!)
• Name of the game:
– Theorists can calculate this for p+p collisions
– Check how it is modified in Au+Au collisions
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Jets in high energy collisions
nucleon nucleon
parton
jetFree quarks and gluons not observed: high transverse energy partons fragment into collimated sprays (jets) of hadrons
• fundamental expectation of QCD • occurs in all high energy collisions: e++e-, p+pbar, Au+Au,…
Hard scattering of partons: quarks or gluons drawn from wavefunction of colliding projectiles
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42Manuel Calderón de la Barca, P408/508 Seminar
Partonic energy loss in dense matter
Multiple soft interactions:
Strong dependence of energy loss on gluon density gluemeasure color charge density at early hot, dense phase
Gluon bremsstrahlung
Opacity expansion:
glueSmediumT
SR
kq
LqC
E
2
2
ˆ
ˆ4
L
ELogrdCCE jet
glueSaA 23 2
,
Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,…
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Jets at RHIC
nucleon nucleonparton
jet
p+p jet+jet (STAR@RHIC)
Find this …
Au+Au ??? (STAR@RHIC)
in this !!!
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Partonic energy loss via leading hadrons
-
Energy loss softening of fragmentation suppression of leading hadron yield
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
Binary collision scaling p+p reference
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Leading hadrons at lower energy
Multiple scattering in initial state(“Cronin effect”)
p+A collisions:
Central Pb+Pb collisions at CERN SPS (s=20 GeV)
SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow,…
tppA ppA
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Au+Au and p+p: inclusive charged hadrons
p+p reference spectrum measured at RHIC
PRL 89, 202301
nucl-ex/0305015, PRL in press
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Suppresion of inclusive hadron yield
• central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT
nucl-ex/0305015
Au+Au relative to p+p Au+Au central/peripheralRAA RCP
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PHENIX observes a similar effect (0s)
nucl-ex/0304022
Factor ~5 suppression for central Au+Au collisions
lower energy Pb+Pb
lower energy
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PHOBOS ( ~ 0.8) and BRAHMS ( ~ 2.0)
PHOBOS: nucl-ex/0302015BRAHMS: nucl-ex/0307003
Also have = 0
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High-pt Evidence
• The yield of high pt particles is significantly suppressed in central Au+Au collisions.
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Jets and two-particle azimuthal distributions
p+p dijet • trigger: highest pT track, pT>4 GeV/c
• distribution: 2 GeV/c<pT<pTtrigger
• normalize to number of triggers
trigger
Phys Rev Lett 90, 082302
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Azimuthal distributions in Au+Au
Au+Au peripheral Au+Au central
Near-side: peripheral and central Au+Au similar to p+p
Strong suppression of back-to-back correlations in central Au+Au
pedestal and flow subtracted
Phys Rev Lett 90, 082302
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High-pt Evidence
• The yield of high pt particles is significantly suppressed in central Au+Au collisions.
• We see an away side jet in p+p, but not in central Au+Au collisions.
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What might all this mean?
?
Conjecture: core of reaction volume is opaque to jets
surface emission
Consequences: near-side corrleations unchanged suppression of back-to-back correlations suppression of inclusive rates azimuthal correlations with reaction plane
Compelling picture, but is it right?
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Is suppression initial or final state effect?
Initial state?
Final state?
partonic energy loss in dense medium generated in collision
strong modification of Au wavefunction initial jet production rates suppressed for heavy nuclei (e.g. gluon saturation at low xBjorken)
56Manuel Calderón de la Barca, P408/508 Seminar
nucl-ex/0305015RCP
Inclusive suppression: theory vs. data
pQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93
pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching
Final state
Initial state
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Is suppression initial or final state effect?
Initial state?
Final state?gluon
saturation
partonic energy loss
How to discriminate? Turn off final state d+Au collisions
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d+Au vs. p+p: Theoretical expectations
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?
If Au+Au suppression is initial state(KLM saturation: 0.75)
1.1-1.5
pT
RA
B
1
Inclusive spectra
If Au+Au suppression is final state
~2-4 GeV/c
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d+Au inclusive yields relative to binary-scaled p+p
Phys Rev Lett 91, 072304
STARSTARddpdT
ddpdNR
Tpp
AB
TAB
AB /
/
• d+Au : enhancement
• Au+Au: strong suppression
Suppression of the inclusive yield in central Au+Au is a final-state effect
pT
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PHENIX, PHOBOS, BRAHMS find similar results
Phys Rev Lett 91, 072302/3/5 (2003)
PHENIX
BRAHMS
PHOBOS
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Azimuthal distributions
pedestal and flow subtracted
Near-side: p+p, d+Au, Au+Au similarBack-to-back: Au+Au strongly suppressed relative to p+p and d+Au
Suppression of the back-to-back correlation in central Au+Au is a final-state effect
Phys Rev Lett 91, 072304
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Summary of STAR high pT
measurements hadrons at pT>~3 GeV/c are jet fragments central Au+Au:
strong suppression of inclusive yield at pT>5 GeV/c suppression factor ~ constant for 5<pT<12 GeV/cstrong suppression of back-to-back hadron pairs
Possible interpretation:Hard scattered partons (or their fragments) interact strongly with mediumObserved fragments are emitted from the surface of the hot & dense zone created in the collision
?
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Experiment: the strong suppression of the inclusive yield and back-to-back correlations at high pT observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.
Theory: pQCD models which reproduce the inclusive suppression in central Au+Au collisions require color charge densities a factor 30-50 greater than that of cold nuclear matter
RHIC is generating very high energy density matter
What are the main lessons so far…
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Have we found the Quark Gluon Plasma at RHIC?
We now know that Au+Au collisions generate a medium that
• is dense (pQCD theory: many times cold nuclear matter density)
• is dissipative
• exhibits strong collective behavior :we do create bulk matter
Not yet, there is still work to do
We have yet to show that:
• dissipation and collective behavior both occur at the partonic stage
• the system is deconfined and thermalized
• a transition occurs: can we turn the effects off ?
This represents significant progress in our understanding of strongly
interacting matter
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Extra Slides
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• “observe” the source from all angles relative to the reaction plane
• expect oscillations in radii for non-round sources
big RSIDE
small RSIDE
Source shapeR
SID
E
-RP (rad)0 /2 /4 3/4
reaction plane
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STAR, PRL93 012301 (2004)
Measured final source shape
centralcollisions
mid-centralcollisions
peripheralcollisions
Expected evolution:
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“Big” and “Small” – in meters
100 106 1012 1018 102410-610-1210-18
1,000,000,000,000,000,000,000,000 m
0.000000000000000001 m
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“Short” and “long” – in seconds
Today’s lecture
100 106 1012 1018 102410-610-1210-1810-24
as many yoctoseconds (10-24 s ~ 3 fm/c) in a secondas seconds in 10 thousand trillion years
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How can we check if it’s deconfined?
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The Original Idea …Matsui & Satz (PLB 178 (1986) 416J/ suppression by QGPColor screening of static potential between heavy quarks:
4/)( ,
3
4 )( 2 rgr
rrV qq
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The Original Idea …Matsui & Satz (PLB 178 (1986) 416J/ suppression by QGPColor screening of static potential between heavy quarks:
4/)( ,
3
4 )( 2 rgr
rrV qq
D. Kharzeev and H. Satz, Phys. Lett. B477 (2000): Hard gluons needed for breaking J/ not available in hadron gas
S. Digal et al., Phys. Rev. D 64, 094015 (2001)C.-Y. Wong, Phys. Rev. C 65, 034902 (2002)Calculations using lattice potentials: sequential suppression’, c dissolves below TC
J/ dissolves at 1.2 TC
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Heavy Quarks in the Lattice• Profile of the action density between static quarks at
distance 1.5 fm, Phys. Rev. D51 (1995) 5165
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Static free energy in full QCD
3F: Improved staggered Asqtad action, lattices
K. Petrov, P.P, PRD 70 (04) 054503
String breaking
screening
Vacuum physics
2F: Improved staggered p4 action lattices,
O. Kaczmarek et al, Progr. Theor. Phys. Suppl 153 (04) 287
P.Petrevczky
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Taking into account the entropy and the internal energy
Kaczmarek, Karsch, P.P., Zantow, hep-lat/0309121see talk by Kaczmarek
Potential energyIn the Lattice shows screening!
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… and its Confirmation …CERN/SPS s 17 GeV• mid-rapidity• large statistics (beyond RHIC)
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Electron PID with MRPC TOF/TPC and EMCEMC
1. use TPC for p and dE/dx2. use Tower E p/E3. use SMD shape to reject hadrons4. e/h discrimination power ~ 105
5. works for pT > 1.5 GeV/c
ToF1. use TPC and ToF PID
2. works for pT < 3 GeV/c
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Quarkonium Trigger Development
• Use calorimeter for triggering on 2 high towers with large invariant mass
• J/ Trigger for pp and d+Au
Trigger even for Au+Au
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80Manuel Calderón de la Barca, P408/508 Seminar
Azimuthal anisotropy in non-central collisions
xy z
Initial spatial anisotropy
py
px
Final momentum anisotropy
x
y
p
patan
y 2 x 2
y 2 x 2
v2 px
2 py2
px2 py
2
Reaction plane defined by “soft” (low pT) particles
Elliptic term
)](2cos[21 2 Rvd
dN
Reaction plane azimuthal angle
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Elliptic flow at high pT: theory
Snellings; Gyulassy, Vitev and Wang (nucl-th/00012092)
Jet propagation through anisotropic matter (non-central collisions)
Finite v2: high pT hadron correlated with reaction plane defined by the “soft” part of event (pT<2 GeV/c)
jet
jet
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Elliptic flow at high pT: data
STARSTAR
PRL 90, 032301
• pT<2 GeV: detailed agreement with hydrodynamics • pT >4 GeV: finite v2 in qualitative agreement with jet quenching expectations
STAR preliminary
STARSTAR
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More informationRelative momentum between pions is a vector can extract 3D shape information
1 2q p p
p2
p1 1 2K p p
Rout
Rlong – along beam direction
Rout – along “line of sight”
Rside – “line of sight”
Rside
study as K grows…
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Why do the radii fallwith increasing momentum ??
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Why do the radii fallwith increasing momentum ??
It’s collective flow !!
Direct geometrical/dynamical evidencefor bulk behaviour!!!
Amount of flow consistent with p-space
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Disintegration timescale - observation
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
N()
Heinz & Kolb, hep-ph/0204061
• no threshold effect seen
• RO/RS ~ 1
• toy model calculations suggest very short timescales• rapid, explosive evolution• too explosive for “real” models
which explain all other data
An important space-time“puzzle” at RHIC
- actively under study
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Forces and structures in Nature
1) Gravity• one “charge” (mass)• force decreases with distance
m1 m2
2) Electric (& Magnetic)• two “charges” (+/-)• force decreases with distance
+ -
+ +
Atom
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Atomic nuclei and the “nuclear” force
Nuclei composed of:• protons (+ electric charge)• neutrons (no electric charge)
Does not blow up!? “nuclear force”• overcomes electrical repulsion
• determines nuclear reactions(stellar burning, fusion…)
• arises from fundamental strong force (#3)
• acts on color charge of quarks
proton
neutron
quark
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Strong color fieldEnergy grows with separation !!!E=mc2 !
An analogy… and a difference!to study structure of an atom…
“white” proton
…separate constituents
Imagine our understanding of atoms or QED if we could not isolate charged objects!!
nucleus
electron
quark
quark-antiquark paircreated from vacuum
“white” proton(confined quarks)
“white” 0
(confined quarks)
Confinement: fundamental & crucial (but not understood!) feature of strong force- colored objects (quarks) have energy in normal vacuum
neutral atom
To understand the strong force and the phenomenon of confinement:Create and study a system of deconfined colored quarks (and gluons)