Modern Nuclear Physics with STAR @ RHIC: Recreating the Creation of the Universe

1
Modern Nuclear Physics with STAR _at_
RHICRecreating the Creation of the Universe

• Rene Bellwied
• Wayne State University
• (bellwied_at_physics.wayne.edu)
• Lecture 1 Why and How ?
• Lecture 2 Bulk plasma matter ?
• (soft particle production)
• Lecture 3 Probing the plasma
• (via hard probes)

2
What is our mission ?

• Discover the QGP
• Find transition behavior between an excited
  hadronic gas and another phase
• Characterize the states of matter
• Do we have a hot dense partonic phase and how
  long does it live ?
• Characterize medium in terms of density,
  temperature and time
• Is the medium equilibrated (thermal, chemical)

3
The idea of two phase transitions

• Deconfinement
• The quarks and gluons deconfine because energy
  or parton density gets too high
• (best visualized in the bag model).
• Chiral symmetry restoration
• Massive hadrons in the hadron gas are massless
  partons in the plasma. Mass breaks chiral
  symmetry, therefore it has to be restored in the
  plasma
• What is the mechanism of hadronization ?
• How do hadrons obtain their mass ?
• (link to LHC and HERA physics)

4
What do we measure in a collider experiment ?

• particles come from the vertex. They have to
  traverse certain detectors but should not change
  their properties when traversing the inner
  detectors
• DETECT but dont DEFLECT !!!
• inner detectors have to be very thin (low
  radiation length) easy with gas (TPC), challenge
  with solid state materials (Silicon).
• Measurements - momentum and charge via high
  resolution tracking in SVT and TPC in
  magnetic field (and FTPC) - PID via
  dE/dx in SVT and TPC and time of flight in
  TOF and Cerenkov light in RICH - PID of decay
  particles via impact parameter from SVT and
  TPC
• particles should stop in the outermost detector
• Outer detector has to be thick and of high
  radiation length (e.g. Pb/Scint calorimeter)
• Measurements - deposited energy for event and
  specific particles - e/h separation via shower
  profile - photon via shower profile

5
What do we have to check ?

• If there was a transition to a different phase,
  then this phase could only last very shortly. The
  only evidence we have to check is the collision
  debris.
• Check the make-up of the debris
• which particles have been formed ?
• how many of them ?
• are they emitted statistically (Boltzmann
  distribution) ?
• what are their kinematics (speed, momentum,
  angular distributions) ?
• are they correlated in coordinate or momentum
  space ?
• do they move collectively ?
• do some of them melt ?

6
Signatures of the QGP phase
For more detail see for example J. Harris and B.
Müller, Annu, Rev. Nucl. Part. Sci. 1996
4671-107 (http//arjournals.annualreviews.org/doi
/pdf/10.1146/annurev.nucl.46.1.71)
Phase transitions are signaled thermodynamically
by a step function when plotting temperature
vs. entropy (i.e. of degrees of freedom). The
temperature (or energy) is used to increase the
number of degrees of freedom rather than heat the
existing form of matter. In the simplest
approximation the number of degrees of freedom
should scale with the particle multiplicity. At
the step some signatures drop and some signatures
rise
7
Evidence Some particles are suppressed

• If the phase is very dense (QGP) than certain
  particles get absorbed

If things are produced in pairs then one might
make it out and the other one not.
If things require the fusion of very heavy rare
quarks they might be suppressed in a dense medium
8
Evidence Some particles are enhanced

• Remember dark matter ? Well, we didnt find
  clumps of it yet, but we found increased
  production of strange quark particles

9
How do we know what happened ?

• We have to compare to a system that did
  definitely not go through a phase transition (a
  reference collision)
• Two options
• A proton-proton collision compared to a Gold-Gold
  collision does not generate a big enough volume
  to generate a plasma phase
• A peripheral Gold-Gold collision compared to a
  central one does not generate enough energy and
  volume to generate a plasma phase

10
Kinematic variables of choice
y -6 0
6
11
0.) Global observablesA.) particle
productionB.) particle spectraC.) particle
flowD.) particle correlations
12
Lattice QCD

• There are two order parameters

• Quarks and gluons are studied on a discrete
  space-time lattice
• Solves the problem of divergences in pQCD
  calculations (which arise due to loop diagrams)

T 150-200 MeV e 0.6-1.8 GeV/fm3
13
Assessing the Initial Energy Density Calorimetry
Bjorken-Formula for Energy Density PRD 27, 140
(1983) watch out for typo (factor 2)
Time it takes to thermalize system (t0 1 fm/c)
6.5 fm
pR2
Central AuAu (PbPb) Collisions 17 GeV eBJ ?
3.2 GeV/fm3 130 GeV eBJ ? 4.6 GeV/fm3 200 GeV
eBJ ? 5.0 GeV/fm3
Note t0 (RHIC) lt t0 (SPS) commonly use 1 fm/c in
both cases
14
Assessing the Initial Energy Density Tracking
Bjorken-Formula for Energy Density
Gives interestingly always slightly smaller
values than with calorimetry (15 in NA49 and
STAR).
15
The Problem with eBJ

• eBJ is not necessarily a thermalized energy
  density
• no direct relation to lattice value
• requires boost invariance
• t0 is not well defined and model dependent
• usually 1fm/c taken for SPS
• 0.2 0.6 fm/c at RHIC ?
• system performs work pdV ? ereal gt eBJ
• from simple thermodynamic assumptions
• ? roughly factor 2

16
Boost invariance based on rapidity distributions
17
So what is e now ?

• At RHIC energies, central AuAu collisions
• From Bjorken estimates via ET and Nch e gt 5
  GeV/fm3
• From energy loss of high-pT particles e 15
  GeV/fm3
• From Hydromodels with thermalization ecenter
  25 GeV/fm3
• All are rough estimates and model dependent (EOS,
  t0, ... ?) , no information about thermalization
  or deconfinement. Methods not completely
  comparable
• But are without doubt good enough to support that
  e gtgt eC 1 GeV/fm3

18
How do we use hadrons ?

• Discovery probes
• CERN Strangeness enhancement/equilibration
• RHIC Elliptic flow
• RHIC Hadronic jet quenching
• Characterization probes
• Chemical and kinetic properties
• HBT and resonance production for timescales
• Fluctuations for dynamic behavior

19
Particle Identification in STAR
20
Basic Idea of Statistical Hadronic Models

• Assume thermally (constant Tch) and chemically
  (constant ni) equilibrated system
• Given Tch and ? 's ( system size), ni's can be
  calculated in a grand canonical ensemble

• Chemical freeze-out
• (yields ratios)
• inelastic interactions cease
• particle abundances fixed (except maybe
  resonances)
• Thermal freeze-out
• (shapes of pT,mT spectra)
• elastic interactions cease
• particle dynamics fixed

21
Particle productionStatistical models do well
We get a chemical freeze-out temperature and a
baryochemical potential out of the fit
22
Ratios that constrain model parameters
23
Statistical Hadronic Models Misconceptions

• Model says nothing about how system reaches
  chemical equilibrium
• Model says nothing about when system reaches
  chemical equilibrium
• Model makes no predictions of dynamical
  quantities
• Some models use a strangeness suppression factor,
  others not
• Model does not make assumptions about a partonic
  phase However the model findings can complement
  other studies of the phase diagram (e.g.
  Lattice-QCD)

24
Thermalization in Elementary Collisions ?
Seems to work rather well ?!
Beccatini, Heinz, Z.Phys. C76 (1997) 269
25
Thermalization in Elementary Collisions ?

• Is a process which leads to multiparticle
  production thermal?
• Any mechanism for producing hadrons which evenly
  populates the free particle phase space will
  mimic a microcanonical ensemble.
• Relative probability to find a given number of
  particles is given by the ratio of the
  phase-space volumes Pn/Pn fn(E)/fn(E) ?
  given by statistics only. Difference between MCE
  and CE vanishes as the size of the system N
  increases.

This type of thermal behavior requires no
rescattering and no interactions. The collisions
simply serve as a mechanism to populate phase
space without ever reaching thermal or chemical
equilibrium In RHI we are looking for large
collective effects.
26
Statistics ? Thermodynamics
pp
Ensemble of events constitutes a statistical
ensemble T and µ are simply Lagrange multipliers
Phase Space Dominance
AA

• We can talk about pressure
• T and µ are more than Lagrange multipliers

27
Are thermal models boring ?
Good success with thermal models in ee-, pp, and
AA collisions. Thermal models generally make tell
us nothing about QGP, but (e.g. PBM et al.,
nucl-th/0112051) Elementary particle
collisions canonical description, i.e. local
quantum number conservation (e.g.strangeness)
over small volume. Just Lagrange multipliers, not
indicators of thermalization. Heavy ion
collisions grand-canonical description, i.e.
percolation of strangeness over large volumes,
most likely in deconfined phase if chemical
freeze-out is close to phase boundary.
28
T systematics
Satz Nucl.Phys. A715 (2003) 3c
filled AA open elementary

• it looks like Hagedorn was right!
• if the resonance mass spectrum grows
  exponentially (and this seems to be the case),
  there is a maximum possible temperature for a
  system of hadrons
• indeed, we dont seem to be able to get a system
  of hadrons with a temperature beyond Tmax 170
  MeV!

29
Does the thermal model always work ?
Data Fit (s) Ratio

• Particle ratios well described by Tch 160?10
  MeV, mB 24 ?5 MeV
• Resonance ratios change from pp to AuAu ?
  Hadronic Re-scatterings!

30
Strange resonances in medium

• Short life time fm/c
• K lt ?lt ?(1520) lt ?
• 4 lt 6 lt 13 lt 40

Rescattering vs. Regeneration ?
Medium effects on resonance and their decay
products before (inelastic) and after chemical
freeze out (elastic).
Red before chemical freeze out Blue after
chemical freeze out
31
Resonance Production in pp and AuAu
Life time fm/c ? (1020) 40 L(1520)
13 K(892) 4 ?
1.7
Thermal model 1 T 177 MeV mB 29 MeV
UrQMD 2
1 P. Braun-Munzinger et.al., PLB 518(2001) 41
D.Magestro, private communication 2 Marcus
Bleicher and Jörg Aichelin Phys. Lett.
B530 (2002) 81-87. M. Bleicher, private
communication
Rescattering and regeneration is needed !
32
Resonance yields consistent with a hadronic
re-scattering stage

• Generation/suppression according to x-sections

p
p
D
p
Preliminary
r/p
p
p
D
L
D/p
More D
K
Chemical freeze-out
p
p
f Ok
f/K
r
p
p
Less K
K/K
p
r
K
Less L
L/L
K
K
f
0.1
0.2
0.3
K
33
Strangeness Two historic QGP predictions

• restoration of c symmetry -gt increased production
  of s
• mass of strange quark in QGP
  expected to go back to
  current
  value (mS 150 MeV Tc)
• copious production of ss pairs,
  mostly by gg fusion
  
• Rafelski Phys. Rep. 88 (1982) 331
• Rafelski-Müller P. R. Lett. 48 (1982) 1066
• deconfinement ? stronger effect for multi-strange
• by using uncorrelated s quarks produced in
  independent partonic reactions, faster and more
  copious than in hadronic phase
• strangeness enhancement increasing with
  strangeness content
• Koch, Müller Rafelski Phys. Rep. 142 (1986)
  167
• Strangeness production depends strongly on baryon
  density
• (i.e. stopping vs. transparency, finite
  baryo-chemical potential)

34
Strangeness enhancement in B/B ratios

• Baryon over antibaryon production can be a QGP
  signature as long as the baryochemical potential
  is high (Rafelski Koch, Z.Phys. 1988)

• With diminishing baryochemical potential
  (increasing transparency) the ratios approach
  unity with or without QGP, and thus only probe
  the net baryon density at RHIC.

35
New RHIC data of baryon ratios

• The ratios for pp and AA at 130 and 200 GeV are
  almost indistinguishable. The baryochemical
  potentials drop from SPS to RHIC by almost an
  order of magnitude to 50 MeV at 130 GeV and 20
  MeV at 200 GeV.

36
Strangeness enhancementWroblewski factor
evolution
Wroblewski factor
dependent on T and mB dominated by Kaons
37
Strangeness enhancement

• K/p the benchmark for abundant strangeness
  production

38
The SPS discovery plot (WA97/NA57)Unusual
strangeness enhancement
N(wounded)
N(wounded)
39
The switch from canonical to grand-canonical(Tou
nsi,Redlich, hep-ph/0111159, hep-ph/0209284)
The strangeness enhancement factors at the SPS
(WA97) can be explained not as an enhancement in
AA but a suppression in pp. The pp phase space
for particle production is small. The volume is
small and the volume term will dominate the
ensemble (canonical (local)). The grand-canonical
approach works for central AA collisions, but
because the enhancements are quoted relative to
pp they are due to a canonical suppression of
strangeness in pp.
40
Strangeness enhancement factors at RHIC
No Npart-scaling in Au-Au at RHIC -gt lack of
Npart scaling no thermalization ? Alternatives
no strangeness saturation in peripheral
collisions (gs 1) non-thermal jet
contributions rise with centrality
41
Identified particle spectra p, p,
K-,, p-,, K0s and L
42
Identified Particle Spectra for Au-Au _at_ 200 GeV

• The spectral shape gives us
• Kinetic freeze-out temperatures
• Transverse flow
• The stronger the flow the less appropriate are
  simple exponential fits
• Hydrodynamic models (e.g. Heinz et al., Shuryak
  et al.)
• Hydro-like parameters (Blastwave)
• Blastwave parameterization e.g.
• Ref. E.Schnedermann et al, PRC48 (1993) 2462
• Explains spectra, flow HBT

43
Thermal Spectra
Invariant spectrum of particles radiated by a
thermal source
where mT (m2pT2)½ transverse mass (Note
requires knowledge of mass) m b mb s
ms grand canonical chem. potential T temperature
of source Neglect quantum statistics (small
effect) and integrating over rapidity gives
R. Hagedorn, Supplemento al Nuovo Cimento Vol.
III, No.2 (1965)
At mid-rapidity E mT cosh y mT and hence
Boltzmann
44
Thermal Spectra (flow aside)

• Describes many spectra well over several orders
  of magnitude with almost uniform slope 1/T
• usually fails at low-pT
• (? flow)
• most certainly will fail
• at high-pT
• (? power-law)

N.B. Constituent quark and parton recombination
models yield exponential spectra with partons
following a pQCD power-law distribution. (Biro,
Müller, hep-ph/0309052) ? T is not related to
actual temperature but reflects pQCD parameter
p0 and n.
45
Thermal spectra and radial expansion (flow)

• Different spectral shapes for particles of
  differing mass? strong collective radial flow
• Spectral shape is determined by more than a
  simple T
• at a minimum T, bT

46
Thermal Flow Traditional Approach
Assume common flow pattern and common temperature
Tth
1. Fit Data ? T
2. Plot T(m) ? Tth, bT

• is the transverse expansion velocity. With
  respect to T use kinetic energy term ½ m b2
• This yields a common thermal freezeout
  temperature and a common b.

47
Hydrodynamics in High-Density Scenarios

• Assumes local thermal equilibrium (zero
  mean-free-path limit) and solves equations of
  motion for fluid elements (not particles)
• Equations given by continuity, conservation laws,
  and Equation of State (EOS)
• EOS relates quantities like pressure,
  temperature, chemical potential, volume direct
  access to underlying physics

Kolb, Sollfrank Heinz, hep-ph/0006129
48
Hydromodels can describe mT (pT) spectra

• Good agreement with hydrodynamic prediction at
  RHIC SPS (2d only)
• RHIC Tth 100 MeV, ? bT ? 0.55 c

49
Blastwave a hydrodynamic inspired description of
spectra
Spectrum of longitudinal and transverse boosted
thermal source
bs
R
Ref. Schnedermann, Sollfrank Heinz, PRC48
(1993) 2462
Static Freeze-out picture, No dynamical evolution
to freezeout
50
The Blastwave Function

• Increasing T has similar effect on a spectrum as
  
• increasing bs
• Flow profile (n) matters at lower mT!
• Need high quality data down to low-mT

51
Heavy (strange ?) particles show deviations in
basic thermal parametrizations
52
Blastwave fits

• Source is assumed to be
• In local thermal equilibrium
• Strongly boosted

• ?, K, p Common thermal freeze-out at T90 MeV
  and lt??gt0.60 c
• ? Shows different thermal freeze-out behavior
• Higher temperature
• Lower transverse flow
• Probe earlier stage of the collision, one at
  which transverse flow has already developed
• If created at an early partonic stage it must
  show significant elliptic flow (v2)

AuAu ?sNN200 GeV
STAR Preliminary
? 68.3 CL
? 95.5 CL
? 99.7 CL
53
Blastwave vs. Hydrodynamics
Mike Lisa (QM04) Use it dont abuse it ! Only
use a static freeze-out parametrization when the
dynamic model doesnt work !!
54
Collective Radial Expansion
From fits to p, K, p spectra

• lt?r gt
• increases continuously
• Tth
• saturates around AGS energy

• Strong collective radial expansion at RHIC
• high pressure
• high rescattering rate
• Thermalization likely

Slightly model dependent here Blastwave model
55
Elliptic Flow (in the transverse plane)for a
mid-peripheral collision
Flow
Y
Out-of-plane
In-plane
Reaction plane
Flow
X
Dashed lines hard sphere radii of nuclei
Re-interactions ? FLOW Re-interactions among
what? Hadrons, partons or both? In other words,
what equation of state?
56
v2 measurements (Miklos Favorite)
Multistrange v2 establishes partonic collectivity
?
57
Lifetime and centrality dependence from ?(1520)
/? and K(892)/K
G. Torrieri and J. Rafelski, Phys. Lett. B509
(2001) 239

• Model includes
• Temperature at chemical freeze-out
• Lifetime between chemical and thermal
  freeze-out
• By comparing two particle ratios (no
  regeneration)
• results between
• T 160 MeV gt ?? gt 4 fm/c (lower limit !!!)
• ?? 0 fm/c gt T 110-130 MeV

Life time K(892) 4 fm/c L(1520) 13 fm/c
?(1520)/? 0.034 ? 0.011 ? 0.013
K/K- 0.20 ? 0.03 at 0-10 most central AuAu
58
Time scales according to STAR data
Balance function (require flow)
Resonance survival
Rout, Rside
Rlong (and HBT wrt reaction plane)
dN/dt
time
1 fm/c
5 fm/c
10 fm/c
20 fm/c
Chemical freeze out
Kinetic freeze out
59
Summary global observables

• Initial energy density high enough to produce a
  QGP
• e ? 10 GeV/fm3
• (model dependent)
• High gluon density
• dN/dy 800-1200
• Proof for high density matter but not for QGP

60
Summary of particle identified observables

• Statistical thermal models appear to work
  well at SPS and RHIC
• Chemical freeze-out is close to TC
• Hadrons appear to be born
• into equilibrium at RHIC (SPS)
• Shows that what we observe is
• consistent with thermalization
• Thermal freeze-out is common
• for all particles if radial flow
• is taken into account.
• T and bT are correlated
• Fact that you derive T,bT is
• no direct proof but it is consistent with
  thermalization

61
Conclusion

• There is no in bulk matter
  properties
• However
• So far all pieces point
• indeed to QGP formation
• - collective flow
• radial
• - thermal behavior
• - high energy density
• - strange particle production enhancement

elliptic