HBT results from microscopic transport approaches

1
HBT results from microscopic transport approaches

• Marcus Bleicher Qingfeng Li (FIAS)
• Institut für Theoretische Physik
• Goethe Universität Frankfurt
• Germany

2
Thanks to the UrQMD group

• Hannah Petersen
• Diana Schumacher
• Stephane Haussler
• Mohamed Abdel-Aziz
• Qingfeng Li

• Katharina Schmidt
• Manuel Reiter
• Sascha Vogel
• Xianglei Zhu
• Daniel Krieg
• Horst Stoecker

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Outline

• HBT How, Why, What?
• UrQMD model calculations
• HBT puzzle(s)
• Model explanations on the HBT puzzle
• Summary and outlook

4
The tool

• UrQMD Ultra-Relativistic Quantum Molecular
  Dynamics
• out-of-equilibrium transport model, (rel.
  Boltzmann equation)
• Particles interact via - measured and
  calculated cross sections - string
  excitation and fragmentation - formation
  and decay of resonances
• Provides full space-time dynamics of heavy-ion
  collisions

5
What can transport models do?

• Provide baseline calculations,including
  resonances, jets, flow,Study energy/centrality
  dependence
• Provide a look behind the curtain,i.e. what is
  the origin of the observed effect
• Study acceptance effects, i.e. how does limited
  detector coverage and the trigger conditions
  influence the results

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Why the HBT technique is important to probe the
QGP?

• We know, the transition can only take place in a
  very small space-time.
• Correlations of two final-state particles are
  closely linked to the space-time of the region of
  homogeneity (the relevant volume for particles of
  a given velocity, not the entire source, which
  can give partly the message of the source.
• A non-trivial structure in the excitation
  function of HBT might be seen IF there is a
  (phase) transition.

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HBT shows the message at freeze-out
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Motivation
At RHIC look for signals of freely moving
partons.At FAIR/SPSlook for the mixed phase
and the onset of deconfinement
E. Bratkovskaya, M.B. et al., PRC 2005
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The promise

• long life times in the mixed phase

10 fold increase in life time during the mixed
phase
Rischke, Gyulassy, Nucl.Phys.A608479-512,1996
Energy density
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Why use the UrQMD model?

• Hydrodynamics failed to explain the decrease of
  HBT radii with kT (see, e.g. nucl-th/0305084)
• Might be due to the Corona effect at late stage?
• Transport model, considering the full
  rescattering process, might throw light on what
  other mechanisms generate the observed
  kT-dependence of the HBT radii

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Whats the HBT technique?
The quotient of two-particle and one-particle
spectra
The two-particle correlator C(q,K) is related to
the emission function S(x,K), Which is the
Wigner phase-space density of the particle
emitting system and can be viewed as the
probability that a particle with average momentum
K is emitted from the space-time point x in the
collision region.
For identical bosons,
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Gaussian Parameterization

• To better understand the three-dimensional
  spatio-temporal source distribution. Although the
  realistic source deviates from a standard
  Gaussian, it provides the standard description of
  experimental data.
• There exist quite a few different types of
  Gaussian parameterization under different
  coordinate system (CMS, LCMS, YKP, etc).

Yano-Koonin parametrization
Nucl-ex/0505014
From one- to two- to three dimensional
parameterization (e.g. nucl-th/0510049 for
reviews)
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LCMS Gaussian Parameterization

• Longitudinal co-moving system (out-side-long)

• is the incoherence or chaoticity factor, lies
  between 0 (complete coherence)
• and 1 (complete incoherence) in the real
  reactions.
• it will be affected by many factors other than
  
• the quantum statistics (bosons 1, fermions
  -1 ),
• for example,
• misidentified particles(contamination),
• the (long-lived) resonance,
• different technical details of Coulomb
  corrections
• RL,O,S are B-P radii, Rol is the cross term and
  vanishes at mid-rapidity.

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The out-side-long system sketch

• Long parallel to beam, and the longitudinal
  components of the pair velocity vanishes.(Kz0)
• Side perpendicular to beam and average pair
  momentum K.
• Out perpendicular to Long and Side.

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The survey of Pratt radii RL,RO, and RS

• RR(KT, Eb, b, (A,B), y, ?, (m1,m2))

Quite a few model endeavors Hydrodynamics
models matter in the collision region is taken
as an ideal,
locally thermalized fluid with the zero mean free
path (hydro/PYTHIA)UrQMD, RQMD hadronic
dynamics model with string degree of freedom.
Having potentials for
baryons at low beam energies. From
UrQMD ver2.0, the PYTHIA (v6.1) was added in
order to consider the
hard process. MPC Molnars Parton Cascade, (with
the stiffest effective EoS) AMPT A Multi-Phase
Transport model (hadronstringparton) HRM
Hadronic Rescattering Model (no
strings/partons) etc
Next, we show the results of the source of two
negatively (except otherwise stated) charged
pions using UrQMD model.
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How to calculate numerically?

• Standard UrQMD (v2.2) output of freeze-out
  particles
• (http//www.th.physik.uni-frankfurt/urqmd
  )
• CRAB (v3.0) used to analyze the
  (three-dimensional LOS) correlation of two
  identical particles.
• (http//www.nscl.msu.edu/pratt/freecode
  s/crab/home.html)
• Three-dimensional Gaussian fitting.

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World HBT data 1
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World HBT data 2

• First (up to now only) systematic comparison
  between transport model (RQMD) and experimental
  data(Mike Lisa, 2005)

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Systematic analysis is needed

• Hydro is known to fail for HBT radii
• Transport models can provide a baseline
• ?Use UrQMD for a systematic study

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• Transverse momentumdependence of the HBT radii
  at various energies

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UrQMD vs. data _at_ AGS

• Good agreement
• Deviations at small kT for RL and RS

lt11?T
lt5 ?T
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The mass dependence of lifetime of resonances
better agreement
The green lines We consider the Mass
dependence Of lifetime of Resonances.
Time from phase shift?
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UrQMD _at_ SPS-NA49
Note the effect of short formation times more
early pressure
lt7.2 ?T
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UrQMD _at_ SPS-CERES
lt5 ?T
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R.vs.KT_at_RHIC
Deviation for RO!
lt15 ?T
lt15 ?T
lt10 ?T
lt5 ?T
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R.vs.b_at_RHIC GeV
RO problems grow towards central collisions.
lines shifted by 5 fm each
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pp-correlations at RHIC
Q. Li, M.B., H. Stoecker, nucl-th/0602032 Data
STAR

• Correlations are well described except for
  most central reactions

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The HBT puzzle?

• Model calculations of RO/RS or (RO2-RS2)1/2 are
  obviously larger than the experimental data

It is related to duration time (in the absence
of flow)
How to improve it? ---hadronic potentials should
also be studied more carefully?
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The (argued) disadvantages in the UrQMD
calculations

• Hadronic potentials for baryons in the above
  calculations.
• No string-string interaction although the string
  degree of freedom exists.
• Or, no deconfined quarks nor gluons and the
  interactions between them.

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More collisions by setting zero formation time
for strings
It is very time consuming e.g. SPS-E1603events/
h RHIC-s2001event/d
The difference Between C(qo) and C(qs) almost
disappears after considering zero formation
time for string.
A larger early pressure especially in the
sideward direction leads to larger Rs
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Ro/Rs at SPS(Eb160 A GeV)
Early stage, Early state!
Zero-formation time Leads to much smaller Ro/Rs
ratio mainly due to a larger Rs.
32
Flow puzzle---thanks to H. Petersen and X. Zhu
At Eblt10 A GeV, the flow can be well reproduced
with a specified potential.
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At RHIC How about other approaches?
AMPTHBT is sensitive to The parton-scattering Cr
oss sections.
HRM considering only the hadron
rescattering (with sudden collisions ), No parton
degree of freedom
From nucl-ex/0505014 by M. Lisa
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How to solve the HBT puzzle

• reduce (or 0) string formation time for more
  pressure?
• (tremendous number of collisions
• make it almost impossible to
• calculate the HBT interferometry at
  RHIC)
• ?the idea in HRM and checked
• for elliptic flow and HBT at SPS in
  UrQMD
• consider Partons?
• ?the idea in AMPT
• Not yet in UrQMD model
• ? with the help of another model
  qMD,
• consider optical potential for pions (chiral
  symmetry)
• see PRL94, 102302(2005), PRC73, 024901(2006)
• ?and, hadronic potential should be also paid
  attention.

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• Rapidity studies

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R.vs.y_at_NA49
good agreement
Weak y-dependence in all HBT radii For RS, it
decreases slowly with rapidity.
Weak y-dep, Why? strong x-p correlation
NA49 data For kTlt100 MeV/c
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Possible reason
Different particle sources as function of
rapidity Direct production vs. decay
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• Energy dependence

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R.vs.Eb_at_small KT

• Overall reasonable agreement
• But, Ro/Rs too big
• Difference between CERES and NA49
  (acceptance?)

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CERES and theory freeze-out
Roughly, the evolution of Vf can be
reproduced However, the decrease of Vf at AGS is
not obtained.
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Mean free path

• UrQMD seems to supports the finding of a (nearly)
  constant mfp.
• However, this is inconsistent with a microscopic
  analysis (mfp 5 fm)

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Summary and outlook

• Good (quantitatively) agreement of the calculated
  HBT radii with data from AGS to RHIC.
• The decay of resonances affects the HBT radii
  (mainly at low kT).
• HBT puzzle is also seen by the comparison of our
  calculations with data, especially at RHIC
  energies (flow and HBT puzzles are twin.)
• A relativistic EoS for nuclear matter should be
  considered in the UrQMD model.
• It seems essential to consider the interactions
  between new degrees of freedom.

How much can we trust hydro for FAIR?