Recent HBT results by CERES

1
Recent HBT results by CERES
Nucl. Phys. A714 (2003) 124
Phys. Rev. Lett. 90 (2003) 022301

• Harald Appelshäuser
• IKF
• Uni Frankfurt

2
Why HBT?

• the most expensive way to measure the number 6
• space-time evolution vector mesons
• s-production
• generation of flow

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Hydro models
M. Lisa, S. Pratt, R. Soltz, U. Wiedemann,
nucl-ex/0505014
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Why HBT?

• the most expensive way to measure the number 6
• space-time evolution vector mesons
• s-production
• generation of flow
• the long-lived source

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HBT Interferometry
p1
r1
Bose-Einstein statistics of identical bosons
leads to short-range correlations in momentum
space
r2
p2
First application with photons size of stars (R.
Hanbury-Brown, R.Q. Twiss, 1956)
?p
In heavy-ion reactions pions, kaons,
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HBT in expanding systems

• space-momentum correlations
• pions with similar momenta (small q) are
• preferentially emitted from close-by regions
• length of homogeneity

? thermal vs. geometrical length scale

• HBT radii depend on mt
• differential HBT-analysis as function of mt
• contains information about source
• expansion

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CERES Experiment
TPC
0 lt f lt 2p 2.1 lt h lt 2.6
Pb-Au 158 AGeV/c
TPC age 1999 40 AGeV/c 8M Pb-Au events 2000
80 AGeV/c 1 M Pb-Au events 2000 158 AGeV/c
35M Pb-Au events
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Bertsch-Pratt Parametrization
in LCMS


Rlong longitudinal expansion and lifetime
tf Rside transverse size, radial
expansion Rout as Rside but also duration
of emission Dt
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Analysis of correlation function
Coulomb repulsion
CERES 158A GeV/c Pb-Au Nucl. Phys. A714 (2003)
124
q lt 0.03 GeV/c
? Rlong, Rside, Rout
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Sensitivity to Coulomb treatment
Coulomb repulsion
CERES 158A GeV/c Pb-Au Nucl. Phys. A714 (2003)
124
Boxes no Coulomb Triangles full
Coulomb Open circles w(kt) 1 Full circles
final result
? Strong sensitivity of Rout to the treatment of
Coulomb repulsion
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Momentum resolution
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Momentum resolution
Rlong Rside Rout
H. Tilsner PhD Thesis Heidelberg
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Rlong longitudinal expansion and lifetime
? Hubble-Expansion
for Tf 160 120 MeV
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Rside transverse expansion and geometry
CERES Pb-Au Nucl. Phys. A714 (2003) 124
gt15
10-15
5-10
0-5
hf2 strength of transverse
expansion (U. Heinz, B. Tomasik, U. Wiedemann)
lt vt gt 0.5-0.6c
for Tf 160 120 MeV
1/vmt (1/vGeV)
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Rout emission duration
CERES Pb-Au Nucl. Phys. A714 (2003) 124
gt15
10-15
5-10
0-5
Generally
At 158 AGeV Short but finite emission duration
from
kt (GeV/c)
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HBT radii AGS SPS RHIC
CERES Nucl. Phys. A714 (2003) 124
Rlong increase SPS ? RHIC?
but by and large a weak Ebeam dependence
? is there a universal freeze-out criterion?
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Freeze-out condition
This compilation D. Brinkmann M.Gazdzicki QM1995
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Freeze-out volume
freeze-out volume Vf
Centrality dependence
CERES Phys. Rev. Lett. 90 (2003) 022301
since
? ????
only at given beam energy!!
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Freeze-out volume vs. beam energy
freeze-out volume Vf

• surprise non-monotonic behaviour
• minimum between AGS and SPS

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Particle multiplicity vs. beam energy
CERES Phys. Rev. Lett. 90 (2003) 022301

• particle number increases monotonically
• ? freeze-out at constant density excluded
• chemical composition changes
• cross sections are very different
• ? try mean free path

vs(GeV)
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Mean free path
CERES Phys. Rev. Lett. 90 (2003) 022301
in a composed medium
vs(GeV)

• also Ns shows non-monotonic behaviour
• dominated by baryons at low energy and mesons at
  high energy

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Universal pion freeze-out
CERES Phys. Rev. Lett. 90 (2003) 022301

• ? Ns follows Vf
• data are consistent with constant
• mean free path at freeze-out

independent of beam energy and centrality
vs(GeV)
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Universal freeze-out II

• universal pion freeze-out
• independent of beam energy and
• system size
• in A-A lf ltlt R
• why no dependence on beam energy and system
  size?
• why so small?

lf (fm)
sqrt(sNN) (GeV)
..implies f.o. at constant density
(R(dNch/dh)1/3) if particle ratios are constant!
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Rlong rapidity dependence
Stefan Kniege (NA49) Kromeriz 2005 nucl-ex/0601
024
NA49 20 AGeV 30 AGeV 40 AGeV 80 AGeV 158
AGeV kt 50 MeV/c
NA49 20 AGeV 30 AGeV 40 AGeV 80 AGeV 158
AGeV kt 50 MeV/c
Rlong (fm)
Rlong (fm)
ypp - ymid
ypp - ybeam
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Rlong rapidity dependence
Rlong (fm)

• delayed freeze-out in the
• fragmentation region due to baryons?
• relativistic effect?

ypp - ybeam
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?
?
Rlong (fm)
(dNch/dh)1/3
RHIC
LHC
SPS
AGS
ypp - ybeam