New results from the NA57 experiment

1
New results from the NA57 experiment

• Giuseppe E. Bruno
• Università degli Studi di Bari and INFN, Bari,
  Italy

For the NA57 Collaboration
2
Outline

• Aim of the experiment and experimental layout
• Results on the hyperon yields
• Blast wave analysis
• Conclusions

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Aim of the experiment

• NA57 studies the production of strange
• and multi-strange baryons in heavy ion
• interactions as a function of
• collision centrality
• collision energy

??-
p ?-
??-
p ?-
?K-
p ?-
c.c.
A sketch of hyperon decay and detection in the
NA57 set-up
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Layout of the NA57 experiment at CERN SPS
 
 
Target 1 Pb Scintillator Petals
centrality trigger MSD Multiplicity Silicon
Detector Tracking device silicon
pixel planes (5 x 5 cm2 ) Lever arm
double side mstrips
Data samples collected by NA57
Sample size (trigger x 106)
Beam momentum
Data taking year
System
Pb-Pb
160 A GeV/c
230 230
1998 2000
Pb-Pb
40 A GeV/c
260
1999
p-Be
40 GeV/c
60 110
1999 2001
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Hyperon yield measurements

• Data corrected for acceptance and also for
  detector and reconstruction efficiency by Monte
  Carlo simulation
• In the acceptance window
• Yield
• (i.e. particle per event)
• Transverse mass spectra (Tapp)
• Extrapolation to a common window
• one unit of rapidity about ycm
• full range of pT

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Centrality of the collision
Centrality determination from charged particle
multiplicity measurement Nwound from trigger
cross section (Glauber model calculations)
Pb-Pb 160 A GeV
0
I
II
III
IV

• 160 GeV 56 most central events in 5 classes
  (bins 0 to IV)
• ltNwoundgt 62?4, 121?4, 209?3, 290?2, 349?1
• 40 GeV determination of classes close to that
  for 160 GeV,
• results presented here for bins I-IV

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Hyperon yields at 40 and 160 A GeV/c
Yield calculated for 42 most central Pb-Pb
collisions (class I-IV)
systematic errors

• Particle yields are closer, a significant factor
  is observed for ?-
• Anti-particle yields larger by a factor 5 at
  160 GeV/c

Indication for larger baryon density at 40 A GeV/c
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Comparison with higher energy (STAR at RHIC)
Centrality 5 for L
10 for X,W (to be consistent with STAR)
STAR
NA57

• anti-hyperon/hyperon ratios
• increase with energy
• energy dependence weaker for
• species with higher strangeness

STAR 130 GeV data from C.Adler et al.,
Phys.Rev.Lett. 89 (2002), J.Castillo,
nucl-ex/0210032, C.Suire, nucl-ex/0211017 and
reference therein
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Centrality dependence of the yields at 160 A GeV/c
New all available data on ?- and ?
3x increase of statistics
Factor ? 20 for ?
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mT spectra in Pb-Pb at 160 A GeV/c
Hydro-dynamical picture the mT spectra are
sensitive to the transverse flow
In the relativistic domain (mT ? m0)
Blue shifted temperature same slope for
different m0
Non relativistic particles (pT ? m0)
Linear increase of the inverse slope with m0
Most central 55 of ?I
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A blast wave analysis
Ref E. Schnedermann, J. Sollfrank and U. Heinz,
Phys. Rev. C48 (1993) 2462.
In the framework of model based on thermalization
and hydro-dynamical transverse flow description
Bessel functions
Two parameters in this model

• It assume a constant profile for the transverse
  flow velocity
• Other models can be implemented (e.g.
  ??(r)?Sr/Rn )
• A nice overview of different models can be found
  in
• G. Torrieri and J. Rafelski arXivnucl-th/02120
  91

Thermal freeze-out temperature
Collective transverse flow velocity
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Results of fit to NA57 data in full centraliy
region
Global fit with two parameters ! (i.e. thermal
freeze-out temperature and transverse flow
velocity)
T161?7 MeV ??0.395?0.015 ?2/ndf26.1/39
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Centrality dependence (i)
??
T (MeV)
?2/ndf
Centrality
0-I 23?56 163?16 0.35?0.03 37.8/37
II-III 5?23 167?13 0.40?0.03 32.6/37
IV 0?5 131?10 0.47?0.02 37.4/37

• Suggested trend the more central the collision
  the
• higher ?? and the lower the freeze-out
  temperature

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Centrality dependence (ii)
Preliminary results consistent with NA49 using
the same approach and with the WA97 HBT
analysis of negative hadrons
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Summary

• Particle yields and ratios in Pb-Pb collisions
• energy dependence is weaker for ? and stronger
  for ?
• indication of a larger baryon density at 40 GeV/c
• Strangeness enhancement at 160 A GeV/c
• enhancement hierarchy with strangeness content
  (up to
• a factor ?20 for ?)
• significant centrality dependence of the
  enhancement for
• all observed particles but for ? (most
  peripheral class 0
• essential)
• ?Very near future enhancement pattern at 40
  GeV/c
• Transverse mass spectra at 160 A GeV/c
• all spectra well described by a common freeze-out
• temperature and transverse flow velocity
• hints for a centrality dependence of the thermal
  freeze-out

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The NA57 Collaboration
Physics Department, University of Athens, Greece
Dipartimento IA di Fisica dell'Università e del
Politecnico di Bari and INFN, Bari, Italy
Fysisk Institutt , Universitetet i Bergen,
Bergen, Norway Høgskolen i Bergen, Bergen,
Norway University of Birmingham, Birmingham, UK
Comenius University, Bratislava, Slovakia
University of Catania and INFN, Catania, Italy
CERN, European Laboratory for Particle Physics,
Geneva, Switzerland Institute of Experimental
Physics Slovak Academy of Science, Kosice,
Slovakia P.J. Safárik University, Kosice,
Slovakia Fysisk institutt, Universitetet i
Oslo, Oslo, Norway University of Padua and
INFN, Padua, Italy Collège de France, Paris,
France Institute of Physics, Prague, Czech
Republic University La Sapienza'' and INFN,
Rome, Italy Dipartimento di Scienze Fisiche
E.R. Caianiello'' dell'Università and INFN,
Salerno, Italy State University of St.
Petersburg, St. Petersburg, Russia IReS/ULP,
Strasbourg, France Utrecht University and
NIKHEF, Utrecht, The Netherlands