peter steinberg cipanp 2003 dynamics of soft particle production in heavy ion collisions peter...
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Peter Steinberg CIPANP 2003 Question 1 Where does the entropy come from? What are the degrees of freedom of a Au+Au collision at RHIC?TRANSCRIPT
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Peter Steinberg CIPANP 2003
Dynamics of Soft Particle Production in Heavy Ion Collisions
Peter SteinbergBrookhaven National Laboratory
Visiting Fulbright Professor at University of Cape Town, South Africa
CIPANP May 19-24 2003New York City, NY USA
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Peter Steinberg CIPANP 2003
1. Can we understand the early dynamics?2. Is the initial state modified before freezeout?3. Can simple regularities in the data teach us how to
disentangle energy & geometry?
Statistical Mechanic
sHydrodynamic
s Geometry QCDPartonsaturation
BjorkenHydro
GlauberModel
Statistical/ThermalModelsstopping multiplicity
spectraRadial flowElliptic
flow
Strangeness
HBT
EnergyDensity
A Briefer History of Time
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Peter Steinberg CIPANP 2003
Question 1
Where does the entropy come from?
What are the degrees of freedom ofa Au+Au collision at RHIC?
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Peter Steinberg CIPANP 2003
Energy & Geometry
3/4
121
part
N
icoll NNpart
4ib
Participant
BinaryCollisions
“Glauber Model”
sNN/2
sNN/2
Nucleon-Nucleon
CMS Energy
Short distance,Incoherent
Long distance,Coherent
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Peter Steinberg CIPANP 2003
Nucleon Structure & Nuclear Collisions• With increasing energy: quarks partons
• Nuclei act as overlapping layers of nucleons increased density
Proton Quark Model High Energy
Et /~ “snapshot”of vacuumfluctuations
High EnergyProton
High EnergyProton in a
Nucleus
Figures from H. Satz, QM2002
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Peter Steinberg CIPANP 2003
Parton Saturation2 2
2 2
( , ) ( ) 1xG x Q QR Q
22 2
2
2
1 / 3
( , )( )
1~
ss s
xG x QQ Q A
R
fmA
• Density (thickness) momentum scale Qs
• Below Qs, target is “black”, cross section saturates• Theoretical approach: “Color Glass Condensate”
• Weak coupling Strong fields! 2 2, ~ 1/s sxG x Q Q
“Packing Factor”
Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc.
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Peter Steinberg CIPANP 2003
Saturation Phenomenology• Qs controls low-x physics: applies to HERA & RHIC
• Golec-Biernat-Wusthoff energy scaling of p cross section
• Rapidity (geometric scaling)
• Centrality – Npart scaling (sources) modified by thickness
• McLerran-VenugopalanMuellerKharzeev/Nardi
2 20
0s
xQ x Qx
2
22
, A spart s
s s
S QdN cN xG x Qdy Q
2sQ W
Geometry QCDInitialFinal
~ .25 3 20 ~ 2
@130 ( )Q GeV
GeV RHIC
2 ~ /s s part coll partQ xG N N
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Peter Steinberg CIPANP 2003
LPHD: How do we “see” saturation?• Saturation calculations
depend on hypothesis:
• “Local parton hadron duality” tested in e+e-• Dokshitzer, Mueller, Khoze,
Ochs, etc.• pQCD mysteriously “works”
at low p
• Hadronization is “soft”• No modification of parton
spectrum and yield
~1chadrons partonsN cN
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Peter Steinberg CIPANP 2003
•Initial state LPHD Final state!
• limiting fragmentation (1-x)4
•Shape seems be found in pp (& e+e-) as well…
Saturation vs. Multiplicity Data• Kharzeev, Levin, Nardi
4
22 2
1 /,
ln / ,TT s
x xx k
k Q W x
3
22 21 23 , ,T s T T
d NE dk x k x p kdp
“Quark counting” at high-x (Phenomenology!)
PHOBOS200 GeV
130 GeV
19.6 GeV
/ / / 2partdN d N
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Peter Steinberg CIPANP 2003
Saturation vs. Spectra• Low-x *p physics
controlled by dimensionless quantity
• RHIC data shows evidence of simliar “geometric scaling”:
• Further evidence that one scale may control much of the observed physics
2
20 0x
Q xQ
3
3 ~ ~ 1T Ts
s s
m md NE fdp p p
n
Schaffner-Bielich, McLerran,Venugopalan, Kharzeev
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Peter Steinberg CIPANP 2003
Implications for Initial State• Initial state Final State
• Coherence lower entropy than pQCD• Qs determines initial physics
• Momentum, Density, Formation Time
• Early formation time large energy densities
0 ~ .2s
fmQ
2 30
/~ ~ 5 18 20TdE dy GeVR fm
PHENIX
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Peter Steinberg CIPANP 2003
Critical remarks• Saturation approach is appealing
• Unifies many features of data with one scale• Trying to reconcile pQCD & unitarity• Qualitative connection to many aspects of data
• However, not a complete physical picture• Phenomenological factors need justification• LPHD is still a hypothesis! – needs testing
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Peter Steinberg CIPANP 2003
Question 2• What happens between the initial state and
final state?
• Can a hydrodynamic description make sense?• Thermalization• Dynamics (EOS)• Observables
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Peter Steinberg CIPANP 2003
Thermal Model Calculations
3
3 ( ) /
112
i
i
Si i s E T
d pN gVe
“StrangenessSuppression”
iSiQiBi SQB
(Cleymans,Redlich, Braun-
Munziger, Stachel,Magestro, Kaneta, Xu…)Grand
CanonicalEnsemble
Chemical FreezeoutTemperature
Baryon ChemicalPotential
Fireball Volume/ 20
B Q fixedS
Excellent fit to RHIC data
Equilibration mechanism?
Conservation laws obeyed globally,not locally!
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Peter Steinberg CIPANP 2003
Thermal Model Systematics
• A+A looks like thermalized hadron gas• So do elementary systems not a hadronic effect• Consensus: “born into” equilibrium well before freezeout
Kaneta & Xu
/ ~ 1E N GeV
LEP
Cleymans & Redlich
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Peter Steinberg CIPANP 2003
Hydrodynamic Approach• Landau (1953)
• Strongly interacting degrees of freedom• Short mean-free path
• Specify initial conditions, and then conserve:• Energy-momentum & “charges” (e.g. baryon #)
• Two basic approaches developed:
Landau (1953):Complete Stopping1D 3D expansiondN/dy ~ Gaussian
Bjorken (1983):Boost
InvariancedN/dy ~ const
ISR data (now RHIC data) seemed to prefer Bjorken…
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Peter Steinberg CIPANP 2003
The Hydro “Machine”
0T
0n x
0p
3
3 ,ii
d NE f x p p ddp
Energy-MomentumConservation
Baryon NumberConservation
Equation ofState (EOS)
Freezeout Hypersurface (x,t)Velocity field u(x,t)
Cooper-Frye Formula
Boost Invariant Initial Conditions
( , ) ( , )s x y or e x y
x
u x
Ideal gas
Lauret, Shuryak, Teaney
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Peter Steinberg CIPANP 2003
Hydro Initial Conditions• Glauber Matching to
final state multiplicity
WN WN
BC BC
e sWN e sBC e s
, (1 ) WN BCs x y x s xs (Kolb/Heinz)
Heinz/Kolb
, WNs x y s (Lauret, Shuryak, Teaney) 30
3
.6 ~ 25 /
~ 100 /~ 350
e fm GeV fm
s fmT MeV
Typical values:
Allows study of centralitydependence of initial state
(WoundedNucleons)
(BinaryCollisions)
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Peter Steinberg CIPANP 2003
Particle Spectra• Centrality dependence radial velocity
• NB: ~ T4 -> (T=120 MeV) << (T~165 MeV)
Pion ‘excess’ reduced by attentionto chemical freezeout conditions
P. Kolb & R. RappHeinz/Kolb
20T T m
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Peter Steinberg CIPANP 2003
Equation of State• EOS encodes all of the bulk dynamics
• 1st order phase transition (a la lattice) leads to softening of EOS: cs0
/ 3p B
/ 3p
~ / 6p
2s
dp cd
(Landau 1953:ideal, massless)
B
(resonance gas,much softer)
(QGP)
(Speed ofsound)
Heinz/Kolb
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Peter Steinberg CIPANP 2003
Elliptic Flow
21 2 cos 2dN vd
PHOBOS data
2 2
22 2
Y X
Y X
MomentumSpace
Glauber relates b to
Solutions to Hydro Equations:
CoordinateSpace
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Peter Steinberg CIPANP 2003
• v2 results have differing sensitivity to EOS
• Heavy particles sensitive to EOS• Less affected by thermal smearing
• Current results prefer 1st order PT!
Elliptic Flow Results
R. Snellings, STAR preliminary
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Peter Steinberg CIPANP 2003
Trouble Down the Hill?
• Trouble for hydro in the longitudinal direction• HBT: Rlong has problems (M. Lisa)• Elliptic flow away from 90o (T. Hirano)
• Where is the problem: initial state or freezeout?• 3D modeling? Viscosity (Teaney)?
• Is boost invariance justified, even at y=0?
T. HiranoHeinz/Kolb
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Peter Steinberg CIPANP 2003
Hydro vs. Saturation• If hydro is truly applicable then
• cf. Saturation + LPHD (parton-hadron duality)
• Interesting that numbers from saturation are not incompatible w/ hydro!• “Bottom up” (BMSS), Eskola, et al
(U.Heinz)
Initial State Final State
Initial State Final State
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Peter Steinberg CIPANP 2003
Critical Remarks• Ambiguities:
• Initial state• Need additional input beyond 2D Glauber
• Which EOS is required • Consistency with broad range of data
• Freezeout conditions • Many variations, incl. “Blast wave” (M. Lisa)
• Assumption of boost-invariance• Hiding important dynamics?
• Systematic studies are crucial!
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Peter Steinberg CIPANP 2003
Question 3• How much does simple “scaling” behavior
in the data teach us?
• What drives the physics?• Energy• Geometry
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Peter Steinberg CIPANP 2003
Simple Behavior of Nch• PHOBOS observes that e+e- sets multiplicity scale
• The rest is linear participant scaling (soft)• Simple argument: reduced leading particle effect
/ 2ch e e eff partN N s N
PHOBOS, QM2002
Nch /
e+ e
- fit
Is this “scaling”?
Au+Aue++e-
p+p
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Peter Steinberg CIPANP 2003
Scaling of Thermal Parameters
Thermal parameters: rapid change “saturation”
1.27 , 4.3165 , ~o o p
a GeV b GeVT MeV m 1 /B
ab s
JC, PBM, KR, etc.
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Peter Steinberg CIPANP 2003
Entropy & Chemistry• Thermodynamics B supresses s
• Increasing energy lowers BPAS, Cleymans, et al
AGS SPS RHIC
B Be p nsT
“Scaling”0B
Additional energy justmakes a “bigger” system:
LHC ~ RHIC
(entropy density)
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Peter Steinberg CIPANP 2003
Strangeness Enhancements
1
0.6
0.4
0.2
0.8
0 100 200 300 400Npart
J. Cleymans, B. Kaempfer, PAS, S. Wheaton, nucl-th/0212335
PHENIX
2s
ssuu dd
J. Cleymans
Energy: B 0, AA is “different”
Geometry: fraction of multiply-struck participantsdrives system towards full chemical equilibrium? NA49, E910
2s f
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Peter Steinberg CIPANP 2003
What have we learned?• RHIC provides extensive systematics in energy,
geometry (& rapidity)!• Which variables control the physics!
• Energy Larger multiplicity, “Saturation” as B0
• Nuclear geometry multiple collisions• Leading particles attenuated (e+e-)• Chemical equilibrium (strangeness)
• Caveat: Beware of coincidences!• Strive for uniqueness, or broad applicability
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Peter Steinberg CIPANP 2003
What is “stopping”?• None of this was predicted we don’t understand
some basic features of the initial state!• Transfer of energy: longitudinal transverse
• 20 years after Busza&Goldhaber: what is stopping?• dE/dx? Or “destroying” nucleons completely!?
GRV-HO
Net
Bar
yon
Bass & Muller, nucl-th/0212103
p p
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Peter Steinberg CIPANP 2003
Status of Soft Dynamics• Saturation is a reasonable picture of initial state
• One scale to rule them all!• Phenomenology many assumptions need justification
• Hydro addresses dynamics after initial state• Final state Information moving beyond R>Rp • Results sensitive to arbitrary initial conditions, EOS, and final
state! Systematics are crucial.• Empirical scaling is a reality check
• Chemistry matters! Nuclear geometry matters!• Beware of accidents: distinguish cause from correlation
• Global dynamics matter!• Strongly interacting, conservative system• Longitudinal dynamics may be very important• Be careful about what we factorize away!
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Peter Steinberg CIPANP 2003
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Peter Steinberg CIPANP 2003
Scope of this talk• Dynamics
• With increasing time, energy scales decrease• Must consider range of dynamical scenarios since the
soft processes are omnipresent!
• Soft particle production• Bulk (99%) of produced particles• These will be the “freezeout” of the QGP• Momentum scales are < 2 GeV
• Heavy Ion Collisions• d+A data is just becoming available• Will be an important contribution
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Peter Steinberg CIPANP 2003
Multiplicity Scaling
WA98WA97/NA57
Phobos
NA49
E917/866
STAR(PRELIMINARY)
RESULTS
E877
PHENIX
BRAHMS
Phobos
• Does the particle density act as a scale?• Elliptic flow “scales” in the same way…
Z. Xu NA49 compilation
Charged Particle Densityat =0
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Peter Steinberg CIPANP 2003
Multiplicity & v2
2 /~ dN dyS
Empirically <pT> is also a function of multiplicity & mass:
Hydro or CGC?Both predict this sort of behavior
Schaffner-Bielich et al
ParticlesArea
v2 data from AGSRHICscales with local particle density
Challenge to hydro?
NA49