Hyperon yields in Pb-Pb collisions Latest results from the WA97 and NA57 collaborations

1
Hyperon yields in Pb-Pb collisionsLatest results
from the WA97 and NA57 collaborations

• Roman Lietava
• University of Birmingham, United Kingdom
• on behalf of the WA97 and NA57 collaborations
• ICHEP02, Amsterdam
  
  24th-31st July 2002

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Talk content

• Physics motivation
• Hyperon yields
• - centrality dependence
• New ? yields at 160 A GeV/c from NA57
  
• - energy dependence
• New ? and ? yields at 40 A GeV/c from
  NA57
• Comparison with STAR
• Conclusions
• Outlook

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Physics motivation

• Enhancement of strange baryons QGP signature
  (PRL42,1982)

WA97 Pb-Pb 160AGeV 40 of ?in (SQM98)
Y Nwound E
Y Nwound
Pb-Pb
p-Pb
Y strange particle yield N wound number of
wounded nucleons in collision
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Study of hyperon enhancement

• Dependence on the centrality of collision
• Dependence on energy of colliding system

• WA97 experiment
• Energy 160 A GeV/c.
• Systems p-Be p-Pb Pb-Pb 40 of ?in,,Nwound gt
  100.
• NA57 experiment
• Energy 160 A GeV/c. Systems Pb-Pb 60 of ?in,
  Nwound gt 50
• Energy 40 A GeV/c. Systems p-Be Pb-Pb

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Collisions centrality

• Centrality selection --gt charged part.
  multiplicity (MSD)
• Nwound from cross sections for each class
  (Glauber model)

Pb-Pb _at_ 160 A GeV
Pb-Pb _at_ 40 A GeV
IV
III
II
I
0
? 40 GeV ?25 most central events
(corresponding to bins IIIIIIV _at_160 GeV)
ltNwoundgt 262 ? 17
?160 GeV 56 most central collisions in 5
bins (from 0 to IV) Most peripheral bin
ltNwoundgt 62 ? 4
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Centrality dependenceWA97 _at_160 A GeV/c
pT? 0,?y-ycm?? 0.5
pT? 0,?y-ycm?? 0.5
Hierarchy of enhancements En(W) gt En(X) gt
En(L) up to ?15 times for W from p-Be to Pb-Pb
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NA57 hyperon yields vs WA97 (160 A GeV/c)
NA57 confirms the enhancements observed by WA97
(black symbols)
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Centrality dependenceNA57_at_160 A GeV/c
black bars systematic errors

• All particle yields per participant, but X,
  decrease with centrality (power law?)
• Significant change of X behaviour in the most
  peripheral bin yield rises from ltNwoundgt 62
  to ltNwoundgt 121 by a factor 2.6 (3.5 s effect)

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Energy dependence of the yields

• ? and ? yields at 40 and 160 GeV/c (NA57)
• ? and ? yields from SPS to RHIC

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? and ? yields _at_ 40 and 160 GeV/c
pT gt 0, ?y-ycm?lt 0.5
Energy dependence in the ?25 most
central sin for both energies
Black bars systematic errors

• from 160 GeV to 40 GeV
• X- about the same, W- down by a
  factor 3, X and W down by a factor 5
• ? Larger baryon density

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X and W yields from SPS to RHIC
QM2002

• Most central events
• 12 for NA57 (bins IIIIV)
• 14 for STAR

STAR results from - J. Castillo in
SQM2001 for L and X - B. Hippolyte, PhD
thesis,Universite de Strasbourg, for W

• from NA57 to STAR energy
• both X- and W- yields increase slower than their
  anti-particles
• ? at RHIC energy about same production rate for
  hyperon and anti-hyperon

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Conclusions

• Enhancements
• NA57 confirms the enhancement observed by WA97
  for L, X and W and their antiparticles at 160 A
  GeV/c.
• Hierarchy of enhancements En(W) gt En(X) gt En(L)
  up to ?15 times for W from p-Be to Pb-Pb.
• Yields per participant decrease with centrality
  (power law?) in Pb-Pb _at_ 160 A GeV/c.
• X and W yields indicates a larger baryon density
  _at_ 40 w.r.t. 160 GeV/c.

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Outlook

• Pb-Pb _at_ 160 A GeV/c
• Increase statistics for X- and X by a factor ?2
  (2000 data set).
• Negatives.
• Pb-Pb _at_ 40 A GeV/c
• L and L yields.
• Negatives.
• p-Be _at_ 40 A GeV/c
• L, L and X- (X?) yields.
• Negatives.
• Enhancements in Pb-Pb relative to p-Be _at_ 40
  GeV/c.