The soft sector at RHIC:
Main message and open issue (IMHO*)
Mike Lisa
Ohio State University
* IMHO = In Mike’s Humble Opinion
Outline
• Importance of the soft sector / geometry
• flow & hydrodynamics - dynamical connection to EoS
• space & momentum probes - proof / characterization
of system
• evolution of the soft sector & multiplicity dependence
• an important open issue
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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R.H.I.C. defined...
• collision of nuclei sufficiently large that nuclear details unimportant
– distinct from nuclear or particle physics
– “Geranium on Linoleum”
• sufficiently large for meaningful bulk &
thermodynamic quantities
• non-trivial spatial scales & geometry
drive bulk dynamics (e.g. flow)
• how big is “sufficient” ?
– important reference issue
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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phase structure of bulk system:
• driving symmetries
• long-range collective behaviour
• “new” physics [superfluidity in l-He]
• relevance
of meaningful EoS
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Why care about the soft sector (the “brown muck”)?
• Well-justified excitement
about high-pT physics
99.5%
• But to study a new type of
matter... bulk system
• large-scale, non-perturbative
(soft) deconfinement
• jets/ect are probes of this
system
• Crucial to understand bulk
properties/dynamics in their
own right
matter of
interest
10 May 2006
hard probes
Strong & Electroweak Matter (SEWM06) - BNL
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Chemistry: liquid-gas PT
liquid-gas phase PT “caloric curve”
Pochodzalla et al,
PRL 75 1040 (1995)
• previous experience at lower energies (also: critical exponents)
• “comforting” plot (though not as simple as it may appear)
• l (liquid) or gaseous (g) phase may be measured in exit channel
 l-g PT observation/characterization easier than h-p PT
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Chemistry: hadron-parton PT
p2
i ~ g i   dp
 p 2 m 2   
1 exp T i i 



• thermochemical fit works well
• Tassymtote ~ Tcrit from Lattice (@ b=0)
• nature of this beast: “exciting” features require theoretical imagination
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Beyond Chemistry: a dynamical connection
bulk (soft sector)
collective flow
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Hydrodynamics: the high-density limit
• local thermal equilibrium*
• solve equations for fluid elements (not particles):
– continuity
– conservation laws
characteristic global spacemomentum correlation pattern
– Equation of State (EoS)
• lower energy collisions
– works qualitatively
– overpredicts collective effects
– RHIC is first time hydro works!
* how?
Strickland...Strong & Electroweak Matter (SEWM06) - BNL
10 MayMrowczynski,
2006
beware such
statements...
lattice QCD input
9
Microexplosions
Femtoexplosions
• energy quickly deposited
s
0.1 J
1 J
• enter plasma phase
17 J/m3
• expand10
5 GeV/fm3 = 1036 J/m3
hydrodynamically
to6 original
phase200 MeV = 1012 K
T• cool back10
K
• do geometric
“postmortem” & infer
momentum
rate
1018 K/sec
1035 K/s
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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The Geometry connection:
Momentum is clear (even if
experimentally challenging).
How to access geometry?
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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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
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Impact parameter & Reaction plane
Impact parameter vector b :
 beam direction
connects centers of colliding nuclei
Reaction plane:
spanned by beam direction and b
b = 0  “central collision”
many particles produced
“peripheral collision”
fewer particles produced
b
10 May 2006
b
Strong & Electroweak Matter (SEWM06) - BNL
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How do semi-central collisions evolve?
1) Superposition of independent p+p:
momenta pointed at random
relative to reaction plane
b
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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How do semi-central collisions evolve?
1) Superposition of independent p+p:
high
density / pressure
at center
momenta pointed at random
relative to reaction plane
2) Evolution as a bulk system
Pressure gradients (larger in-plane)
push bulk “out”  “flow”
“zero” pressure
in surrounding vacuum
more, faster particles
seen in-plane
b
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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How do semi-central collisions evolve?
1) Superposition of independent p+p: N
momenta pointed at random
relative to reaction plane
0
/4
/2
0
/4
/2
3/4

3/4

-RP (rad)
2) Evolution as a bulk system
Pressure gradients (larger in-plane)
push bulk “out”  “flow”
more, faster particles
seen in-plane
10 May 2006
N
Strong & Electroweak Matter (SEWM06) - BNL
-RP (rad)
16
Azimuthal distributions at RHIC
STAR, PRL90 032301 (2003)
b ≈ 6.5 fm
b ≈ 4 fm
“central” collisions
midcentral
collisions
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Azimuthal distributions at RHIC
STAR, PRL90 032301 (2003)
b ≈ 10 fm
b ≈ 6.5 fm
b ≈ 4 fm
peripheral collisions
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Elliptic flow – collectivity & sensitivity to early system
“Elliptic flow” v2
STAR, PRL90 032301 (2003)
• collective motion
“v2”
• geometrical anisotropy
 momentum anisotropy
• sensitive to early pressure
and thermalization
• characteristic mass ordering
• sensitive to EoS
STAR (20-60% centrality)
hydro:Huovinen et al (2001)
• “QGP” PT preferred (?)
Hydrodynamic
calculation of
system evolution
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Trends, soft sector, and RHI history
A. Wetzler (2005)
Finally, we
understand it!
Just one
event!
M Gyulassy ‘95
10 May 2006
Then: p-dependent potentials, surface
effects, hard-cores, absoption...
Strong & Electroweak Matter (SEWM06) - BNL
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R.H.I. History - beware single-point agreement
STAR, PRL86 402 (2001)
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v2: demands 1st-order PT [or something like it]?
• sensitive to EoS, esp. heavy particles
• but only (non-realistic) strong
1st-order P.T. needed to fit data?
• “Lattice” fits as poorly as “pure hadronic” EoS
• Detailed agreement between ideal hydro and
measured v2(mass,pt) an accident? (Hirano
and Gyulassy arXiv:nucl-th/0506049)
H:
pure hadronic
qp: “lattice”
Q:
1st order [“standard”]
T:
“1st order” + slow
approach to SB limit
10 May 2006
Huovinen,
Strong & Electroweak Matter (SEWM06) - P.
BNL
NP A761 296 (2005)
22
Geometrical consequences of flow: femtoscopy (“HBT”)
C2(Qinv)
separation
distribution
from
momentum
correlations
pa
xa
xb
STAR preliminary
pa
pb
xa
xb
p+p
R ~ 1 fm
R~1/QCORR
10 May 2006
pb
Au+Au
R ~ 6 fm
Strong & Electroweak Matter (SEWM06) - BNL
Qinv (GeV/c)
23
Geometric substructure?
random (non-)system:
all observers measure the
“whole source”
10 May 2006
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Flow-generated substructure
random (non-)system:
all observers measure the
“whole source”
Specific predictions of
bulk collective flow:
• space-momentum (x-p) correlations
• faster (high pT) particles come from
•smaller source
•closer to “the edge”
10 May 2006
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Flow-dominated
“Blast-wave” model
PRC70 044907 (2004)
R (fm)
Spectra
v2

HBT
STAR PRL 91 262301 (2003)
K
mT (GeV/c)
space-momentum
substructure mapped in detail
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Prejudices: Winter Workshop, 1999
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Paradigm shift
• many did not anticipate what turned out to be THE story of the bulk
– “RHIC will be different than anything seen before”
– keep an open mind
Harris & Mueller
AnnRevNuclPartSci ‘96
P. Kolb, J. Sollfrank,
and U. Heinz, Phys.
Rev. C. C62 054909
(2000).
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Strong & Electroweak Matter (SEWM06) - BNL
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Paradigm shift
• many did not anticipate what turned out to be THE story of the bulk
– “RHIC will be different than anything seen before”
– keep an open mind
• instead: “evolution rather than revolution” (the story of R.H.I.C.)
ARNPS 55 357 (2005)
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
Lisa, Winter Wkshp ‘99
29
What drives the soft sector ?
H. Caines (STAR) QM05
NA57 (open)
STAR (filled)
LPSW nucl-ex/0505014
10 May 2006
NA57 (open)
STAR (filled)
S. Manly (PHOBOS) QM05
Strong & Electroweak Matter (SEWM06) - BNL
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Paradigm shift
• many did not anticipate what turned out to be THE story of the bulk
– keep an open mind
• instead: “evolution rather than revolution” (the story of R.H.I.C.)
• “everything” scales only with multiplicity (indep A,B, s...)?!
– low-energy central collision ~ high-energy peripheral collision
– entropy is driving factor (more hydro-dominance)?
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Paradigm shift
• many did not anticipate what turned out to be THE story of the bulk
– keep an open mind
• instead: “evolution rather than revolution” (the story of R.H.I.C.)
• “everything” scales only with multiplicity (indep A,B, s...)?!
– low-energy central collision ~ high-energy peripheral collision
– entropy is driving factor (more hydro-dominance)?
• next logical step...
– again (like low s case) : is RHIC so different from pp ? bulk behaviour?
– again, keep an open mind
p+p, 7 May 2006
10 May 2006
Au+Au 2002
Strong & Electroweak Matter (SEWM06) - BNL
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pp = “a small AA” ? ; Is geometry meaningful?
Collectivity is meaningless without geometry
Energy loss of energetic partons in quark-gluon plasma:
Possible extinction of high pT jets in hadron-hadron collisions
J.D. Bjorken, 1982
p+p, 7 May 2006
10 May 2006
Au+Au 2002
Strong & Electroweak Matter (SEWM06) - BNL
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D. Kharzeev
Transverse Dynamics Workshop
March 2003
sure, but how would we know?
• theoretical prejudice
• experimental signatures
10 May 2006
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Zbigniew Chajecki QM05
R Z(fm)
1. Heisenberg
uncertainty?
Z0 decay @ LEP
•2.e.g.
G. fragmentation?
Alexander
String
(Lund)
••3.“plausible”
ineffects?
z-direction
pResonance
maybe (??)
T dependence
•• unlikely
in transvrseprobably no
dependence
• mass
e.g. Wiedemann
& DELPHI
Heinz ‘97
[Andersson,
Moriond 2000]
• maybe,
but presumably
 different effect
significantly
femtoscopy in p+p @ RHIC
p+p and A+A measured in same
experiment
• unique opportunity to compare physics
• what causes pT-dependence in p+p?
• same cause as in A+A?
than for Au+Au
R (fm)
• under investigation

K
p 
STAR preliminary
hep-ph/0108194
10 May 2006
m, mT (GeV)
mT (GeV)
Strong & Electroweak Matter (SEWM06) - BNL
mT (GeV)
35
Zbigniew Chajecki QM05
3. Resonance effects?
4. Bulk system („hydro”)
in pp?
DELPHI
flow not expected in such
a small system as p+p
R (fm)
e.g. Shuryak: hep-ph/0405066

 Csorgo et al.:
K Buda-Lund
treatment of p+p collision as
d+Au : Rlong doesn’t change
bulk
system
with centrality
p 
(w/ temperature gradients)
hep-ph/0406042
femtoscopy in p+p @ RHIC
p+p and A+A measured in same
experiment
• unique opportunity to compare physics
• what causes pT-dependence in p+p?
 in A+A?
• same cause as
1/(2mT)d2n/(dmTdy)
R Z(fm)
1. Heisenberg
uncertainty?
Z0 decay @ LEP
2. String fragmentation? (Lund)
K
p
mT-m (GeV)
RSIDE
ROUT
RLONG
STAR preliminary
hep-ph/0108194
10 May 2006
m, mT (GeV)
mT (GeV)
Strong & Electroweak Matter (SEWM06) - BNL
mT (GeV)
mT (GeV)
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Surprising („puzzling”) scaling
Ratio of (AuAu, CuCu, dAu) HBT
radii by pp
 All pT(mT) dependences of
HBT radii observed by STAR
scale with pp although it’s
expected that different origins
drive these dependences
HBT radii scale with pp
Scary coincidence
or something deeper?
pp, dAu, CuCu - STAR preliminary
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Other soft
“resembles data”
v2
Is the physics similar?
Is the data trying to tell us something?
PYTHIA 6.4 Minbias
(Single nondiffractive)
p+p @ 200 GeV
N.B.: Pythia mocks up the soft sector
Cannot, itself, (dis)prove flow/non-flow
What can we tell from the data?
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Summary (1/2)
• Importance of the soft sector
• elliptic flow:
– sensitive to high-density (early) aspects of EoS
– hydro works
•experimental basis of the big story (“perfect liquid”) in soft
sector
•single-point agreement : common in our field, and worrysome
– unique connection to underlying physics under construction
(likely to require details a la nuclear EoS)
• flow mapped in detail through space-momentum substructure
• no doubt: AA ≠ N*(p+p)
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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Summary (2/2)
• evolution (not revolution) of soft sector
• multiplicity (entropy?) dominance
• pp = “little AA” ??
– besides the idea that “it shouldn’t be”, what points against it?
– is “matter” created in such a small system?
– baryon/meson splitting in v2, etc...
– “universality”: route to deep physical insight, or cruel joke?
• IMHO, this is the most important open issue in the bulk sector - do
we know our “reference”?
– deserves much more attention
– RHIC (& LHC) programs might make unique contributions to
particle and HI physics here
10 May 2006
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The
End
Flow-dominated
“Blast-wave” model
PRC70 044907 (2004)
R (fm)
Spectra
v2

HBT
STAR PRL 91 262301 (2003)
K
mT (GeV/c)
space-momentum
substructure mapped in detail
10 May 2006
Strong & Electroweak Matter (SEWM06) - BNL
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here i am
• now say:
• flow is established (hope you are convinced!)
• flow is important (IF it agrees with hydro, then basis of perfect fluid statement)
• but... flow was not universally accepted.
– even though ALL (ALL!) other HIC showed flow, there was the idea that “RHIC
physics will be different”
– well, it is, but only quantitatively
– but OK data showed different so now we know better
• now something harder:
– we all assume “A+A *must* be different than p+p”
– really?
• mult scaling
• HBT(pT)
• Dima/Bjorken
• keep an open mind! This field IS full of surprises, but they lie in the detailed
systematics, NOT in sharp “differences”!
10 May 2006
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Raimond’s Summary of HIF 2005
• Elliptic flow is a very powerful observable
– Its magnitude in agreement with prediction from ideal hydrodynamics and ideal hydro + hadron cascade
– Mass dependence of elliptic flow in agreement with common collective velocity and favors soft effective EOS
• The basis of the ideal fluid statement
• The break down of hydro behavior at more peripheral collisions higher transverse momenta and away from
midrapidity can be understood and was qualitatively predicted in hydro+hadron cascade calculations (Teany and
Shuryak)
• However, the observed monotonic behavior provides not very satisfying proof, like to see at least some change
in slope of energy dependence v2/ (slope of v2(pt) perhaps more sensitive?)
• What about v4?
• Still a lot to do and understand (both at RHIC and at the LHC)
10 May 2006
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• so, flow is the big story of the soft sector
– other consequences?
– spectra (BW fits, basically): more massive particles pushed more - no time
– space-time
• after all, geometry is the whole key to this game, right?
• what would nonflowing versus flowing system do? [use hydro Kolb scenario
versus pp superposition]
• femtoscopy (HBT and pi-K) can distinguish : QUANTITATIVE agreement
– definitely definitely definitely collective, bulk flow in AA - it is “big enough”
• note: before RHIC turn-on, many folks did not expect flow: “RHIC will be so
different”
– show Zajc slide from 99 workshop
– show flow excitation functions - we shoulda thought (Yogi Bera)
– point: many things are similar to “reference” - systematics counts!
• similarly, let us keep an open mind: is SOFT SECTOR p+p REALLY...
– understood?
– so different?
• MULT SCALING: (incl strange) - and LHC “prediction”...
• No time for
– BW fits to spectra
multiplicity
scaling
10–
May
2006
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Outline
•
•
•
•
•
•
What drives the soft sector @ RHIC (and where is it driving us)?
soft sector is over”flow”ing with systematics (i.e. bulk signatures are “the story”)
dN/d\eta is god
p+p-->A+A “evolution not revolution”
pp @ high roots ~ AA @ low roots (mult) at least in soft sector
is this a “new” puzzle (puzzle in soft sector, that is. No need to view it as HBT
puzzle per se)
• What argues against saying that pp = “little AA” (with flow and e-thing)?
– BULK measurement HBT (and non-id)? NO
– BULK measurement v2 (NO)
– BULK measurement spectra (NO)
– strangeness enhancement NO (b/c seems smooth with mult. “enhancement is
DEFINED as AA/pp, so smoothness is what to look for. DON’T use
ASSUMPTion that pp is nonQGP, or else you are assuming your answer.)
– ANY “sudden” change a la P.T.? (NO)
– jet suppress? Well, it’s also smooth, and anyhow we want to look @ soft
sector for EoS (P.T.) effects
– our prejudice? YES!
10 May 2006
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But what is this pp “non-flow” which we subtract?
• pp collisions produce azimuthal
correlations (might be flow)
•  contribute to v2 of larger system
• remove this contribution
(unfortunately called “non-flow”)
to determine bulk flow of the larger
system
v2 “raw”
v2 “non-flow subtracted”
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“Corrected” v2
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use
•
•
•
•
•
Heavy Ion Forum stuff on HBT (recent, Mercedes, etc.)
Talk to Helen. Use her stuff
Talk to Peter. Use spectra stuff.
Talk to Aihong. Get “v2” from p+p.
My “seeing flow” pictures (declan et al). Compare “QGP” search/details
to EoS search/details (include picture of Gyulassy -- “one event!”)
10 May 2006
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include
• Paul’s (or Raimond’s?) 3/2 v2 scaling from HADRONIC model
• Peter: need big cross-section and instant therm (uRQMD and Molnar)
• Aihong and Paul: v2 subtraction - subtract “primordial” v2 of p+p, but that
does NOT mean that this primordial v2 is NOT flow!
• Define Relativistic Heavy Ion Physics:
– limit where study of nuclear collisions does not depend anymore on
details of the colliding nuclei: “Geranium on Linoleum”
10 May 2006
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analogy b/t flow at low (high) roots to
extract EoS of nuclear (strong) matter
• Gyulassy plot from my Berkeley School talk (“just one event”)
– at the end, needed huge range of systematics (not just excitation
function, but also (impt) asymmetric systems (Jerry Chance paper
from EOS)
• Even now, can we make direct connection b/t Lattice and dynamics [nice
plot to make is to use poster, which has Lattice result and STAR event,
and make an arrow between them]
– remember Huovinen: sharp F.O. PT works best (though by now is not
considered realistic anymore)
– point: we will have significant more work to disentangle details of this
“matter”
• should NOT be satisfied with dynamic theories (some hydros!)
which work only on very limited range of conditions-- that would not
have worked in the low-roots case
10 May 2006
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Linear scale
ubQ*  AA  v bv pM AA  ubQ*  pp
vs transverse momentum
Msub=5
Msub=480
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Linear scale
ubQ*  AA  v bv pM AA  ubQ*  pp
The plot above, showing the rise and fall of azimuthal correlations ( M<uu*>)
can be explained only by flow: no any other known source of the azimuthal
correlation is able to give such a dependence (including the “saturation picture”;
the latter, from “flow” point of view being just “usual” non-flow)
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Log scale
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Varying the equation of state
P. Huovinen, Nucl.Phys.A761:296-312,2005
H: pure hadronic
• yes, sensitive to EoS
qp:“lattice”
• but only (non-realistic) strong 1st-order P.T. needed to fit data
Q: 1st order [“standard”]
• “Lattice” fits as poorly as “pure hadronic” EoS
T: “1st order” + slow
approach to SB limit
10 May 2006
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pp Event in STAR 7May 2006
10 May 2006
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Thermal eq
elementary
10 May 2006
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Thermalization in Elementary Collisions ?
•
•
Thermal Model:
e+e-  qq hadronic jets ~ hadron gas = fireball (2 jets = 2 fireballs)
•
Correlated jets: small systems + quantum numbers conservation 
canonical form
•
Recipe:
• Assume thermal and chemical equilibrium
• canonical ensemble to describe partition function
• input: measured particle yields
• output: T, V, s  determined by fit
(s to account for incomplete saturation of strangeness)
•
Studies performed at several s and various systems: pp , pp, e+e-
•
10 May 2006
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Thermalization in Elementary Collisions ?
•
Seems to work rather
well ?!
Beccatini, Heinz, Z.Phys. C76 (1997) 269
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Thermalization in Elementary Collisions ?
•
Beccatini, Heinz, Z.Phys. C76 (1997) 269
• T of fireball from fitting hadron yields does not (or only weakly) depend on s
• T  170 MeV
• Universal hadronization mechanism at critical values ?
10 May 2006
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Thermalization in Elementary Collisions ?
• Does the agreement with data mean we that the assumption of thermal and
chemical equilibrium are correct ? NO !
• Is a process which leads to multiparticle production thermal?
• Microcanonical Ensemble vs Canonical Ensemble:
– Difference between MCE and CE vanishes as the size of the system N increases
• Any mechanism for producing hadrons which evenly populates the free particle
phase space will mimic a microcanonical ensemble
•  thermal behavior even if there is no interaction between particles
This type of “thermal” behaviour requires no rescattering and no
interactions. The collisions simply serve as as a mechanism to
populating phase space without ever reaching thermal or chemical
equilibrium
In RHI we are looking for large collective effects.
10 May 2006
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Statistical Thermal Models in Heavy-Ion
Collisions: SPS
Assume:
• thermally and chemically equilibrated fireball at hadro-chemical freeze-out
Recipe:
• grand canonical ensemble to describe partition function  density of particles of species I

gi
p 2dp
i  2 
2 0 exp[( Ei  b Bi  s Si ) / T ]  1
• fixed by constraints: Volume V, , strangeness chemical potential S, isospin
• input: measured particle ratios
• output: temperature T and baryo-chemical potential B
• SPS: Tch = 160-170 MeV, B = 270 MeV
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