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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