Current Status of the RHIC HBT Puzzle

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Current Status of the RHIC HBT Puzzle

• Sergey Panitkin
• Brookhaven National Lab
• La Thuile, March 18, 2005

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Who cares about the soft sector?

• Well-justified excitement about high-pT physics

• But recall that we want to create/study a new
  type of matter ( bulk system)
• large-scale (soft?) deconfinement
• Collective properties (T,p, flow?)
• Jets/ect are probes of this system
• Crucial to understand bulk properties and
  dynamics in their own right

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Collective behavior at RHIC
P. Kolb, J. Sollfrank, U. Heinz

• Hydrodynamics seems to reproduce p-space aspects
  (spectra and elliptical flow) of particle
  emission up to pT2GeV/c
• Bulk system?!Note Hydro provides
  complete space-time evolution. Can be tested!

Heinz Kolb, hep-th/0204061
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Two-particle Correlations
Single particle spectrum is sensitive to momentum
distribution only
Relative momentum distribution of particle pairs
is sensitive to space-time information
Source function
FSI
Intensity interferometry, HBT technique, etc.
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Two particle Interferometry Idealized case
For non-interacting identical bosons
emission function
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Correlation functions for different colliding
systems
STAR preliminary
C2(Qinv)
Qinv (GeV/c)
Correlations have more information One can use
more sophisticated analysis to extract it
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Standard Pratt-Bertsch coordinate system
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HBT for Gaussian sources
Decompose q into components qLong in beam
direction qOut in direction of transverse
momentum KT qSide ? qLong qOut
Radii are related to source variances
Sensitive to emission time
Sensitive to transverse extent
Sensitive to longitudinal extent
In Longitudinally Co-Moving System (LCMS) bl 0
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In Search of the QGP. Naïve expectations
QGP has more degrees of freedom than pion
gas Entropy should be conserved during fireball
evolution Hence Look in hadronic phase for
signs of Large size, Large
lifetime, Expansion
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In Search of the QGP. Expectations.
Energy density

• One step further
• Hydro calculation of Rischke Gyulassy expects
  Rout/Rside 2-gt4 _at_ Kt 350 MeV.
• Looking for a soft spot

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Excitation function of the HBT parameters

• 10 Central AuAu(PbPb) events
• y 0
• kT ?0.17 GeV/c
• no significant rise in spatio-temporal size of
  the ? emitting source at RHIC
• Ro/Rs 1
• Some rise in Rlong

Note 100 GeV gap between SPS and RHIC
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RHIC Energy Scan

• Measurements at 200, 130, 62 GeV
• No significant change with energy
• Ro/Rs 1

Where are signs of phase transition?!
Is HBT sensitive to geometry at all?!
PHOBOS nucl-ex/0409001
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More Experimental Systematics
b?0
STAR PRL 9312301 (2004)
PHENIX nucl-ex/0401003
Centrality dependence
asHBT
Clear sensitivity to source geometry !
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Source expansion at RHIC
STAR Collaboration, nucl-ex/0312009
?initial ?final
STAR preliminary
Expansion at low Pt
Change in eccentricity of the source
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Model Comparison (the puzzle)

• Subset of models shown
• Broad range of physics scenarios explored
• Good description of p-space
• Poor description of HBT data

Does Rout/Rside of unity imply no long-lived
mixed phase?
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x-t Correlation of Source Function
positive!
Why hydro doesnt work?
Positive? Negative?
RoutRside may require positive x-t corr.
t
t
Typical source function from hydro with Bjorken
flow
Hubble like flow? Buda-Lund, AMPT
x
x
Negative x-t correlation
Positive x-t correlation
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BudaLund fits to 130 GeV data
Parameterization, not a model !
PRC54 (1996) 1390, NPA 590 (1995) 465
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Conclusions

• Rich set of experimental HBT results exists at
  RHIC
• Large number of models has been developed to
  explain RHIC physics
• Good description of momentum space observables
• Evidence that matter at RHIC exhibits collective
  bulk properties
• Most models still can not reproduce
  simultaneously experimental observables at low
  Pt v2, spectra, correlations, dN/dy, etc
• We are gaining better understanding of model
  failures
• Bjorken longitudinal boost-invariance assumption
  seems to be one of the sources of discrepancies
  (at least for Hydrodynamical models)
• More detailed correlation measurements will be
  available soon non-identical correlations, HBT
  Ptgt1 GeV/c, Kaons, protons, etc
• Stay tuned!

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The END
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Back Up slides
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Does HBT in rhic make sense?

• YES
• Size R(Npart1/3)
• Shape R( ?)
• Dynamic structure R(mT)
• and NO
• Model disagreement transport
• Too-short timescales BW
• Inconsistent dynamical picture
• No large rise in RO, RL general

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Buda-Lund fits to 130 GeV data
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Fourier coefficients of HBT(?) oscillations

• 0th-order FC centrality kT dependence mirrors
  ?-integrated analyses quantitatively consistent
• Relative amplitudes increase from central to
  peripheral collisions

means
relative amplitudes
STAR Collaboration, nucl-ex/0312009
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Comparison to PHENIX
AuAu 200 GeV 30 Centrality
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HBT with respect to reaction plane

• Elliptic geometry leads to oscillations of the
  radii
• For example Rside

fp90
Rside (small)
Rside (large)
Reaction plane
fp0
out-of-plane extended source
Naïve view with no flow
Heinz, Hummel, Lisa, Wiedemann PRC 044903 (2002)
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Hanbury Brown-Twiss interferometry

• Two-particle interferometry p-space separation ?
  space-time separation