PowerPoint-Pr

1
Rapidity dependence of Bose-Einstein
correlations at SPS energies
Stefan Kniege Institut für KernphysikJohann
Wolfgang Goethe Universität Frankfurt a.M.
for the NA49 collaboration
Workshop on Particle Correlations and
Femtoscopy Kromeriz , 15-17.09.2005
2
Outline
Analysis

• NA49 detector and data sets
• correlation function
• ? fit method and parametrisation
• systematic errors

Results

• kt- dependence of the HBT radii
• ?Blastwave analysis
• (HBT radii particle mt- spectra)
• Y- dependence of HBT radii

3
Analysis Detector and datasets
Setup of the NA49 detector
Data sets
Beam energy (AGeV) Number of events (k events) Centrality ()
20 360 7.2
30 420 7.2
40 580 7.2
80 300 7.2
158 890 7.2
4
Detector and data sets
Similar coverage of phase space for different
energies due to scaled magnetic field
Phase space (h- -h- pairs)
kinematic interval Ycm (0.0-2.5)
5 bins
kt (0.0-0.6)GeV/c 5 bins
for all energies
5
correlation function
Signal distribution
Momentum differences of
h- h- pairs of tracks
from the same event
Measurement
Background distribution mixed event pairs
Bertsch-Pratt Parametrisation
Fit function
coulomb weight Phys Lett. B 432 (1998)
Basic considerations
- extract ? - ? - correlations
and ? as a physics parameter .
?determine the purity p of the source by
VENUS/GEANT simulation. - coulomb interaction
iterative calculation of mean pair separation
r. - consider different
statistics in S(q) and B(q) by a normalisation
factor n
6
BP- projections kt-dependence
kt dependence
Projections of the correlation function onto qout
(50 MeV/c) at midrapidity.
fit
data
7
BP- projections Y-dependence
Ycm dependence
Projections of the correlation function onto qout
(50 MeV/c) kt(0.0-0.1)GeV/c.
fit
data
8
Systematic errors
9
Systematic errors two track distance (ttd)
Two track distance ttd is determined at each pad
row of the TPCs
(72 (VTPC), 90 (MTPC))

• pair accepted if ttd gt ttdcut
• for a given
  number of pad rows ( nrows )

determination of cut criterion variation of
ttdcut and nrows
ttd
?Look directly at the correlation function
passed pad rows nrows
10
Systematic errors Two Track Resolution
Undershoot of the correlation function in case of
to loose cuts (left plot)
In the plateau region (blue) results vary by
less than 0.2 fm
Common cut criterion for all energies
ttdcut2.2 cm nrows 50
11
Systematic errors Momentum resolution
Basic consideration
Momentum resolution is determined for each
out-side-long -bin by a reconstruction of
simulated events ?qlt 5 MeV/c (158 AGeV)

?qlt 10 MeV/c (20 AGeV)
Cideal obtained by weighting mixed event
pairs according to a
theoretical Bose-Einstein correlation function
(radii taken from fit to the
data in the corresponding bin) Csmeared q
value shifted according to momentum resolution
?q before weighting
12
Systematic errors Momentum resolution
Preliminary results still uncorrected
Very small correction ( lt0.2fm) at low energies
13
Further systematic uncertanties

• Missing particle identification
• strong influence on ?
• ?Coulomb-correction
• strong influence on Rout and ?
• ?Normalisation
• strong influence on ? and the Radii
• in case of small statistics or narrow
  acceptance

Systematic error on the radii ? 1fm
14
Results kt-dependence Blastwave parametrisation
15
Results Blastwave analysis
Emission function (Retiere/Lisa
nucl-th/0312024)
source velocity - longitudinal boost
invariance - linear increasing transverse
flow ?0
freezeout time
emission duration
Fit parameters
weighting function - Transverse radius -
surface diffuseness (box- profile assumed)
16
Results Blastwave analysis
calculation of observables
pt-spectra
azimuthal angle of momentum
HBT radii (longitudinal boost invariant source
, analysed in the LCMS)
Average over the emission function
Combined fit of HBT-radii and particle spectra
17
Results Blastwave analysis
Fit to mt-spectra only
18
Results Blastwave analysis
Fit to mt-spectra only
19
Results Blastwave analysis
Combined fit to spectra and HBT radii at
midrapdity
Only p and and p - spectra were used to make
the results comparable to AGS and RHIC data.
Good approximation to the data except for slight
deviations in Rside at high kt
20
Results Blastwave analysis fit results
open symbols results from fit to mt - spectra
only
TgtTchem ! at AGS energies
Tchem J. Cleymans, K. Redlich, Phys. Rev.
C60, 054908 (1999)

• Further possible improvements
• add further
• particles
• Take into account
• resonance decays

• slightly increasing temperature with beam energy
  at SPS energies
• approximately constant transverse geometrical
  Radii (Rbox? 2Rgauss)
• finite emission duration at SPS energies

21
Results Ycm-dependence
22
Results Ycm-dependence
Non vanishing cross term for non boost invariant
expansion expected ( Phys.Rev.Lett.
74,4400-4403(1995) )
158 AGeV kt (0.0-0.1) GeV/c
? shift of the correlation function in the qlong
- qout- plane
23
Results Ycm-dependence
increasing Routlong
kt (0.0-0.1) GeV/c
24
Results Ycm-dependence
increasing Routlong
kt (0.1-0.2) GeV/c
25
Results Ycm-dependence
increasing Routlong
kt (0.2-0.3) GeV/c
26
Summary and Conclusion

• Bose-Einstein correlations at NA49
• 1. measured over a wide region of phase
  space
• 2. measured at 5 different beam energies

? source parameters extracted from transverse
momentum dependence of the HBT radii show no
pronounced energy dependence at SPS
energies.
? HBT-Radii Rside, Rout show no
pronounced rapidity dependence at SPS energies

• ?rapidity dependence of R outlong reflects
• non boost invariant longitudinal expansion
• ? further investigation (YKP
  parametrisation)

27
Thanks for your attention