Transport Model Analysis of UltraRelativistic AA interactions

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Transport Model Analysis of Ultra-Relativistic
AA interactions

• Marcus Bleicher
• Institut für Theoretische Physik
• Goethe Universität Frankfurt
• Germany

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Thanks to

• Elena Bratkovskaya
• Sascha Vogel
• Xianglei Zhu
• Stephane Haussler
• Hannah Petersen
• Diana Schumacher

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Contents

• Introduction
• Strangeness as QGP signal
• Experimental facts the horn
• Strange fluctuations
• Summary

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The tool UrQMDv2.2

• Non-equilibrium transport model
• Hadrons and resonances
• String excitation and fragmentation
• Cross sections are parameterized
• via AQM or calculated by detailed balance
• pQCD hard scattering at high energies (not in
  v1.3)
• Generates full space-time dynamics of hadrons
  and strings

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Collision Spectrum
1fm/c
10fm/c

• Initial stage scattering before 1.5 fm/c
  Baryon stopping, meson production,
• Thermalization stage (1.5 6 fm/c) Cooking
  QCD matter, may be QGP formation
• Hadronic freeze-out stage (6 10 fm/c)
  Elastic and pseudo-elastic hadron scatterings

PbPb _at_ 160 AGeV
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Where do we expect QGP?

• 1st Order phase transition at high
• No P.T. at low
• Search for irregularities around Ebeam 10-40
  GeV
• Flow, strangeness, E-by-E

Plot adapted from L. Bravina
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Strangeness enhancement
T0.2-0.3 GeV
? QGP has lower threshold for strangeness
production ? relative strangeness enhancement
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PP Excitation functions

• PP works nicely
• Perturbative QCD is used for hard scatterings
  above 50 GeV

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AA Excitation functions

• 4 and mid-y abundancies OK
• Energy dependence OK
• Hadron-string models work well

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Excitation functions ratios

• Horn in the ratio not reproduced
• well reproduced
• relative strange baryon
  enhancement reproduced

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Fluctuation studies Ratios
Taken from Christoph Roland
NA49 Preliminary
NA49 Preliminary

• K/p fluctuations increase towards lower beam
  energy
• Significant enhancement over hadronic cascade
  model
• p/p fluctuations are negative
• indicates a strong contribution from resonance
  decays

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Baryon-Strangeness Correlations I
Definition
Idea Strangeness and baryon numbercarriers are
different in QGP and hadron gas.
First suggested by V. Koch et al., 2005

• HG strangeness is decoupled from baryon number
  (mesons) ? small CBS correlation
• QGP strangeness is fixed to baryon number
  (strange quark)? large CBS correlation

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Baryon-Strangeness Correlations 2

• Limiting cases for CBS
• Large mB CBS ?3/2
• large acc. window CBS ?0Explored with help
  of increasing rapidity window inAuAu reaction
  at RHIC

• Present models yield similar results for small
  rapidity window
• Different handling of the fragmentation
  region/spectators influences results at large
  rapidities

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Baryon-Strangeness Correlations 3
Energy dependence of CBS allows to study the
onset of deconfinement transition Note that the
QGP result is for m0 Here ymaxlt0.5

• Deviations from the HG are expected around high
  SPS energy region, due to QGP onset.

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Baryon-Strangeness Correlations 4
Centrality dependence of CBS allows to study the
critical volume needed for QGP formation. Note
that the QGP result is for m0 ymaxlt0.5,
Ecm200AGeV

• Hadron-string transport models predict no
  centrality dependence of CBS
• A QGP transition leads to a strong centrality
  dependence

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Summary

• Hadron yields are well reproduced in models
• Most ratios can be understood in transport
  models
• Model K/p ratios do not reproduce the strong
  peak observed in data
• In data, the dynamical fluctuation of K/p
  increase strongly towards 20 AGeV beam energy
  (not present in hadron-string models)
• Baryon-strangeness correlations allow to pin
  down the onset of the QGP transition.

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Conclusion
Most interesting energy range, because

• K/p ratio enhanced
• Large dynamical fluctuations
• Latent heat is big

Will be explored by new GSI accelerator (near
Frankfurt)