Strange Hadrons Production in Cu+Cu collisions at s NN = 62.4 GeV at RHIC Marcelo G. Munhoz Universidade de São Paulo – Brazil for the STAR Collaboration
Jan 31, 2016
Strange Hadrons Production in Cu+Cu collisions at sNN = 62.4 GeV at RHIC
Marcelo G. Munhoz Universidade de São Paulo – Brazil
for the STAR Collaboration
Motivation• Systematic study of particle
production in relativistic heavy ion collisions as a function of energy and system size
• Allows to investigate the mechanisms behind strangeness production in these collisions
• What is the influence of the system geometry?
• How does strangeness production change as a function of energy?
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In this talk…• First results on K0 short, Λ, Ξ and Ω production
from Cu+Cu collisions at √sNN = 62.4 GeV ▫ special credits to Geraldo Magela – UNICAMP and
Ulisses Gulart – USP• How does strange hadron production change
when one goes from Cu+Cu to Au+Au or from 62.4 to 200 GeV?
• What can we learn from this systematic comparisons?
• Few comparisons between▫different √sNN : 62.4 GeV and 200 GeV▫different colliding systems: Cu+Cu and Au+Au
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Neutral Strange Hadrons
• Particle identification through the topology of the weak decay
• ~10M events analyzed
• Clear peak at the invariant mass spectra for |y| < 0.5 and 0.5 < pt < 4.5 GeV/c
• Good statistics (less than 1% statistical error)
• Polynomial fit of the background
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K0
Minimum bias0.5 < pt < 4.5
STAR Preliminary
Cu+Cu, 62.4 GeV
Λ
Minimum bias0.5 < pt < 4.5
STAR Preliminary
Cu+Cu, 62.4 GeV
Multi-strange Hadrons
• Particle identification through the topology of the weak decay
• ~10M events analyzed
• Clear peak at the invariant mass spectra for |y| < 0.5 and 0.75 < pt < 4.0 GeV/c
• Good statistics (less than 1% statistical error for Ξ and less than 7% for Ω)
• Polynomial fit of the background
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Cu+Cu, 62.4 GeV
Cu+Cu, 62.4 GeV
Centrality definition
• The data sample was divided in 5 centrality bins according to measured charged particle multiplicities
• Each centrality bin is associated to a number of participant nucleons (Npart) using a geometrical Glauber approach
0 – 10%
40 –
60%
30 –
40%
20 –
30%
10 –
20%
Npart
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Transverse Mass Spectra
ΛK0
STAR Preliminary
STAR Preliminary
• Good statistics for |y| < 0.5 and 0.5 < pt < 4.5
• Corrected for detector efficiency and acceptance• Statistical error only• Λ spectra corrected for feed-down from Ξ weak decay.
Feed-down from Ω is negligible
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Cu+Cu, 62.4 GeV Cu+Cu, 62.4 GeV
Transverse Mass Spectra• Good statistics for |y| < 0.5 and 0.75 < pt <3.5
• Corrected for detector efficiency and acceptance• Statistical error only• For Ω, studied 3 centrality bins: 0-10%, 10-20% and 20-40%
Ξ-
STAR Preliminary
Ω-
STAR Preliminary
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Cu+Cu, 62.4 GeV Cu+Cu, 62.4 GeV
Bulk strangeness production• Strangeness
enhancement:▫strange hadrons are
enhanced relative to p+p▫more strangeness
production or canonical suppression in p+p?
▫relative enhancement seems to be slightly lower than in SPS
▫dependence with Npart – production volume not proportional to Npart
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F. Antinori et. al. (NA57 Collab.), J. Phys. G, 32 (2006)
J. Takahashi for the STAR Collaboration, nucl-ex/0809.0823
STAR Preliminary
J. Takahashi for the STAR Collaboration, nucl-ex/0809.0823
Bulk strangeness production• Previous observation
for strange hadrons production at Cu+Cu 200 GeV: ▫yield does not follow
the same Npart dependence as in Au+Au collisions
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STAR Preliminary
M.A.C Lamont for the STAR Collaboration, J. Phys.: Conf. Ser. 110 032011
Bulk strangeness production• Previous observation
for strange hadrons production at Cu+Cu 200 GeV: ▫yield does not follow the
same Npart dependence as in Au+Au collisions
▫ this result contradicts the suggested volume dependence with Npart:
V = AαV0, where A = Npart/2, V0 = 4/3.πR3 and α = 1
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, 2/3 or 1/3
STAR Preliminary
Bulk strangeness production• Previous observation
for strange hadrons production at Cu+Cu 200 GeV: ▫yield does not follow
the same Npart dependence as in Au+Au collisions
▫in addition, the meson Φ seems to be an exception, for both Cu+Cu 200 and 62.4 GeV
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Bulk strangeness production• Similar behavior was
already observed for lower energies (SPS - √sNN=17.2 GeV), where the K/π ratio was higher in lighter systems for the same Npart
• What about the strangeness production in lighter systems (Cu+Cu) at √sNN=62.4 GeV?
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C. Höhne for the NA49 Collaboration, Nucl. Phys. A 715 (2003) 474
Yield at mid-rapidity × centrality• Integrated pt spectra using an exponential function up to 2.0 GeV/c
• Statistical error only• Cu+Cu yield higher than Au+Au for the same Npart
• Same behavior for 200 and 62.4 GeV
ΛK0
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Yield at mid-rapidity × centrality• Integrate the pt spectra using a boltzman function
• Statistical error only• Cu+Cu yield slightly higher than Au+Au for the same
Npart
• Same behavior for 200 and 62.4 GeV
Ξ- Ω- + Ω+
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Bulk strangeness production• Strangeness production
does not have the same Npart dependence in Cu+Cu and Au+Au collisions at 200 and 62.4 GeV
• Is it a unique feature from strange hadrons?
• For a quick answer, one can compare strange hadron production with pion yields
STAR Preliminary
A. Iordanova for the STAR Collaboration, nucl-ex/0806.0286
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Bulk strangeness production• Strangeness production
does not have the same Npart dependence in Cu+Cu and Au+Au collisions at 200 and 62.4 GeV
• Is it a unique feature from strange hadrons?
• Npart is not a good parameter to account for geometry differences from lighter (Cu+Cu) to heavier (Au+Au) collision systems
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Summary• First results on K0 short, Λ, Ξ and Ω production from
Cu+Cu collisions at √sNN = 62.4 GeV
• Bulk strangeness production:
▫Cu+Cu yield higher than Au+Au for the same Npart, as observed for other hadrons
▫Similar behavior in Cu+Cu 62.4 GeV and 200 GeV
▫Npart is not a good parameter to account for geometry differences in Au+Au and Cu+Cu collisions
▫Work in progress: once we have the data analyzed, we need to develop a deeper understanding of strangeness production mechanisms comparing all systems and energies available…
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