Tiziano Virgili*

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Hyperon enhancement at 40 and 158 A GeV/c

• Tiziano Virgili

For the NA57 Collaboration
Università degli Studi di Salerno and INFN,
Italy.
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Contents
Physics motivation The NA57 experimental
apparatus Multiplicity and centrality
measurement (new this year) Strangeness
production transverse mass spectra Strangeness
production ratios and enhancements (new this
year) Conclusions
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Physics motivation

• NA57 physics goals
• study of the dependence of hyperon enhancements
  on
• interaction volume down to Nwound ? 50
  collision energy (data at both 40 and 158 A
  GeV/c Data samples

System Beam momenta Sample size
Pb-Pb 158 A GeV/c (230230) x 106 evts
Pb-Pb 40 A GeV/c 240 x 106 evts
p-Be (WA97) 158 A GeV/c 120 x 106 evts
p-Be 40 A GeV/c (60110) x 106 evts
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Experimental apparatus
B1.4 T
Target 1 Pb-Pb, 8 p-Be Scintillator petals
centrality trigger (Pb-Pb) MSD 1 2
multiplicity detector (Pb-Pb) Tracking
device silicon pixel planes (5 x 5 cm2 cross
section, 50 x 500 mm2 cell size, total of ? 1
M channels) Lever arm pixels double sided
mstrips
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Strange particle detection
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Selected signals (Pb-Pb)
40 A GeV
160 A GeV
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Multiplicity measurement
ds/Nch (barn)
158 A GeV/c
Nch
ds/Nch (barn)
Multiplicity measured in the range 2lt?lt4 (158 A
GeV/c) and 1.9lt?lt3.6 (40 A GeV/c) Measurement
of dN/d? at the peak
40 A GeV/c
Nch
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Centrality of the collision
bin of stot ltNwoundgt r.m.s.
0 40-53 58 18
1 23-40 117 30
2 11-23 204 37
3 5-11 287 36
4 0-5 349 28
ds/Nch (barn)
Data are divided in 5 centrality classes N. of
participants (wounded nucleons) are determined
within the Glauber model.
Nch
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dN/d? measurement
General agreement at 158 A GeV/c disagreement
with NA50 at 40 A GeV/c Agreement for the
participants between NA50 and NA57 disagreement
with NA49 (larger values at the same centrality)
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Ratio of dN/d? (158 A GeV/40 A GeV)
ln(17.3)/ln(8.8)
(Corrected for centrality)
Ratio close to a scaling with log(s)
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Strangeness production Transverse mass spectra,
Yields and Enhancements

• Double differential cross sections
• for each particle fitted to
• assuming flat rapidity distribution in the
  selected acceptance regions
• Inverse slope of mT distribution ( T Tapp )
  free parameter of the fit.

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Transverse mass spectra (158
A GeV/c)
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Inverse slopes (158 A GeV/c)
Pb-Pb
p-Be p-Pb 0 1 2 3 4
K0 1974 2176 23915 2398 2337 2448 2349
L 1802 1966 23719 27413 28212 31514 30515
L 1572 18311 27719 26411 28310 31314 29514
X- 20213 23514 29020 29011 2959 30411 29912
X 18217 22421 23229 31123 29418 34628 35631
W-W 169 40 33499 2743428 26823 26823
W- 26419 26419 26419 26419 26419
W 28428 28428 28428 28428 28428
In central and semi-central Pb-Pb collisions
(bin 1,2,3,4) we measure compatible
slopes for particle and its anti-particle
This symmetry is less evident in p-Be
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Blast wave analysis
Hydro-dynamical picture the mT spectra are
sensitive to the transverse flow
Blast wave description of the spectra
Transverse flow velocity
Transverse source size
Surface velocity
Ref E Schnedermann, J Sollfrank and U Heinz,
Phys. Rev. C48 (1993) 2462
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Blast wave fit to strange particles
53 most central events
n1

• T 144 7(stat) 14(syst) MeV
• ltb?gt 0.381 0.013(stat) 0.012(syst)

• T and ltß?gt depend weakly on n0, ½, 1
• n2 case disfavoured by data (bad ?2)

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Centrality dependence of the thermal freeze-out
in Pb-Pb at 158 A GeV/c
1s contours

• With increasing centrality
• Transverse flow velocity increases
• Freeze-out temperature decreases
• Earlier decoupling for peripheral collisions ?

n1
J. Phys. G, nucl-ex/0403016
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Energy dependence of ratios
Centrality regions NA57 0 - 11
STAR 0 - 11
Ref Physics Letters B 567 (2003), 167.
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Enhancements at 158 A GeV/c
Factor ? 20 for ?
10
10
1
1

• Evidence of significant centrality dependence of
  enhancements in Pb-Pb (measurements in bin 0
  essential)
• Saturation for the two-three most central bins ?

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Enhancements at 40 A GeV/c

• Enhancements are still there at 40 A GeV/c,
• with the same hierarchy as at 158 A GeV/c
  E(?) lt E(?)

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Conclusions (I)

• Multiplicity measurement
• Disagreement at 40 A GeV/c with NA50
• Agreement with other experiments at 158 A GeV/c
• Ratio close to simple scaling with ln(s)

• Transverse mass spectra in Pb-Pb at 158 A
  GeV/c
• Symmetry between hyperon and anti-hyperon in
  central
• and semi-central Pb-Pb collisions (bins
  1,2,3,4), not in p-Be
• Description by common freeze-out adequate
• Evidence for a centrality dependence of the
  thermal
• freeze-out parameters

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Conclusions (II)

• Energy dependence of ratios
• weaker dependence for particles with lower
• strangeness content (larger baryon density
  at
• lower energy)

• Strangeness enhancement 40 vs 158 A GeV/c
• Enhancement of midrapidity yields at 40 A GeV/c
  of the same order as at 158 A GeV/c, and with
  the same hierarchy E(?)ltE(?)

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The NA57 Collaboration
Athens, Greece Bari, Italy Bergen, Norway
Birmingham, UK Bratislava, Slovakia Catania,
Italy CERN, Switzerland Kosice, Slovakia Oslo,
Norway Padua, Italy Paris, France Prague,
Czech Republic Rome, Italy Salerno, Italy St.
Petersburg, Russia Strasbourg, France Utrecht,
The Netherlands.