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Energy and system size
dependence of strangeness production,
from SPS to RHIC
Jun Takahashi & Marcelo Munhoz for the STAR collaboration
1
Motivation
In A+A,
With a QGP scenario strangeness production is expected to be enhanced.
Strangeness enhancement has been observed from various data from
SPS and RHIC.
In p+p,
Strangeness production may be limited due to canonical suppression.
This suppression should scale with the strange quark content of the
particles.
Energy dependence of the yields:
Can we see any dramatic variation in the excitation function of different
parameters?
Chemical Equilibrium:
Is strangeness equilibrated?
Baryon to Meson enhancement and RCP behavior in the
intermediate pT region.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 2/34
The STAR experiment
STAR is a large acceptance collider detector.
We have accumulated a large amount of data
that allows a systematic study of the
strangeness production, both as a function of
energy and as a function of system size.
Strange particles are identified through various
analysis methods, such as: V0 and Cascade
reconstruction, event mixing and dE/dx.
In this presentation, I will mainly concentrate on
the weak decaying strange particles: K0s, Λ, Ξ,
Ω.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 3/34
Data
RHIC data: STAR
p+p
200 GeV
d+Au
200 GeV
Au+Au
200 GeV
130 GeV
62.4 GeV
19.6 GeV
Cu+Cu
200 GeV
62.4 GeV (some very preliminary results)
SPS data: NA57 & NA49
Pb+Pb
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 4/34
System Size:
Event centrality classes are defined based on the measured charge
particle multiplicities.
The equivalent number of particles that participate in the reaction
Npart is calculated using Glauber Model, that also provides the
equivalent number of binary collisions NColl or NBin.
Phys. Rev.C 70, 054907 (2004)
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 5/34
System Size:
Au+Au Cu+Cu
Cent Npart NBin Cent Npart NBin
0-5 351.0 ± 3.0 1039 ± 79
5-10 293 ± 7. 810 ± 58
10-20 231 ± 3.2 574 ± 42
20-40 139 ± 5. 278 ± 30 0-10 98.4 ± 1.0 185.7 ±5.9
10-20 74.8 ± 2.5 126.7 ±6.7
40-60 59.0 ± 5. 82 ± 12
20-30 54.4 ± 2.8 81.5 ± 6.0
30-40 38.5 ± 2.5 51.0 ± 4.8
60-80 19.0 ± 3.5 19 ± 5
40-60 21.9 ± 2.6 24.3 ± 3.9
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 6/34
CuCu200 spectra: K0s and Λ´s
From A. Timmins
55M min-bias events analyzed.
High statistic data, with spectra extended to high pT coverage.
Fully corrected spectra, with feed-down corrections.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 7/34
CuCu200 spectra: Ξ´s and Ω´s
STAR Preliminary
STAR Preliminary
Statistics is high enough to separate even the Ω spectra in different
event centrality classes.
After efficiency, acceptance and feed-down correction, total yield is
obtained from a fit using a Boltzmann function.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 8/34
Λ Spectra Comparison between Au+Au and Cu+Cu
From A. Timmins
Talk on sunday
STAR Preliminary
Λ spectra measured from Cu+Cu(0-10%) has same shape as spectra
measured in Au+Au(20-40%), with equivalent Npart.
Shows deviation from Maxwell-Boltzmann behavior at high pT.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 9/34
Ξ Spectra Comparison between Au+Au and Cu+Cu
STAR Preliminary
Ξ spectra shows same shape and slope parameter for Cu+Cu(0-10%) and
Au+Au (20-40%).
Ξ error bars are larger but, similar trend as seen in the Lambda spectra can
be observed with a deviation from exponential at the high pt region.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 10/34
Mean Transverse momentum of Ξs and Ωs.
Spectra measured from Au+Au
seems to be harder than p+p.
For Au+Au collisions, <pT> does
not seem to vary with Npart, in the
measured range.
Ω spectra shows higher <pT>
than Ξ´s.
New Cu+Cu data seems to be
STAR Preliminary consistent with Au+Au values.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 11/34
CuCu200 & AuAu200 Yields: K0s, and Λ´s
Au+Au: particle yields increase with
system size.
Cu+Cu: On a first look, data seems
to be consistent with Au+Au yields.
Central Λ yields from Cu+Cu data
seems to be slightly higher than the
equivalent centrality region in
Au+Au.
• Red symbols are for Au+Au 200GeV
• Black symbols are for Cu+Cu 200 GeV
• Solid symbols are for the particles
• Open symbols are for anti-particles.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 12/34
Strangeness Enhancement:
Au+Au 200 GeV
nucl-ex/0705.2511
STAR Preliminary Clear enhancement of hyperon
production in Au+Au compared to
pp.
Strangeness enhancement over
p+p data is already observed at
most peripheral Au+Au bin.
Difference between baryon and
Anti-baryon is expected due to
non-zero net baryon number.
Clear increase of strangeness
enhancement with strange quark
content, indicating enhancement
hierarchy that is in accordance to
GC Thermal production.
Strangeness enhancement show
dependence with volume (Npart)
which disagrees with GC thermal
production.
A. Tounsi, A. Mischke and K. Redlich,
Nucl. Phys. A 715, 565 (2003).
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 13/34
Strangeness Enhancement:
AuAu200 & CuCu200
STAR Preliminary Still see clear hyperon
enhancement in Cu+Cu
data.
On a first look, Cu+Cu
data seems to be
consistent with
enhancement observed
at Au+Au.
Λ enhancement from
most central Cu+Cu
data seems to be higher
than equivalent Au+Au
data, should try different
scaling.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 14/34
Statistical Thermal Model: Cu+Cu 200 GeV
Cu Cu, S NN 200 GeV Statistical Thermal Model
(THERMUS)* was used fitting Tch,
μB, μS, and γS (strangeness
saturation factor).
Particles used in the fit:
π, K, p, Λ, Ξ, Ω.
Particles were corrected for weak
decays.
Measured particle ratios are
reasonably well fit with statistical
thermal model.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 15/34
Statistical Thermal Model: Au+Au 200 GeV
Au Au, S NN 200 GeV
Statistical Thermal Model (THERMUS)*
was used fitting Tch, μB, μS, and γS
(strangeness saturation factor).
Particles used in the fit:
π, K, p, Λ, Ξ, Ω.
Particles were corrected for weak
decays.
Measured particle ratios are reasonably
well fit with statistical thermal model.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 16/34
Statistical Thermal Model: Au+Au 62.4 GeV
Au Au, S NN 62.4 GeV Statistical Thermal Model (THERMUS)*
was used fitting Tch, μB, μS, and γS
(strangeness saturation factor).
Particles used in the fit:
π, K, p, Λ, Ξ, Ω.
Particles were corrected for weak
decays.
Measured particle ratios are reasonably
well fit with statistical thermal model.
Ratio K0/π was excluded to improve chi2.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 17/34
Statistical Thermal Model:
Fit parameters vs. system size
Baryon chemical potential μB is small for
Au+Au 200 GeV.
Cu+Cu 200 GeV Small variation with system size.
μS is consistent with zero.
Au+Au 62 GeV data shows higher value
of baryon chemical potential.
Au+Au 62 GeV shows larger variation
with system size when compared to
Au+Au 200 GeV.
Cu+Cu 200 GeV baryon chemical
potential seems to be in good agreement
with Au+Au 200 GeV.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 18/34
Statistical Thermal Model:
Fit parameters vs. system size
Temperature seems constant with system
size for Au+Au 200 GeV.
Au+Au 62 GeV data shows same
temperature values of Au+Au 200 GeV
and also no system size dependence can
be observed within error bars.
Cu+Cu 200 GeV temperature shows a
Cu+Cu 200 GeV
smaller value then compared to Au+Au
data, but is in agreement within error
bars.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 19/34
Statistical Thermal Model:
Fit parameters vs. system size
Strangeness saturation constant, shows
an increase with system size, reaching
saturation around Npart~150.
Cu+Cu 200 GeV
Au+Au 62 GeV data shows same values
and behavior of Au+Au 200 GeV.
Cu+Cu 200 GeV data fits seems to yield
a strangeness saturation constant
consistent with 1.
* Thermus, A thermal Model Package for Root
S. Wheaton & Cleymans, hep-ph/0407174
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 20/34
Statistical Thermal Model:
Excitation function
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 21/34
Particle ratios
Smooth rise of ratio with
energy from SPS to RHIC,
indicating an evolution from
baryon transport regime to
pair production dominated
regime.
Ratio approaching baryon free
environment at RHIC
energies.
The higher the strangeness
content, ratio is closer to unity.
STAR preliminary
New Cu+Cu data seems to
follow systematic.
Ratio seems to be the same,
SPS data from NA49 independent of the system
AGS data from E896 & E802 size.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 22/34
Baryon Excitation function
Above AGS energies baryon
yield is relatively constant
with energy.
Λ yield variation with energy
seems to follow proton
dependence.
For LHC: Λ ~ 10-30
Ξ ~ 3-6
Ω ~ 0.4-0.7
STAR Preliminary
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 23/34
Anti-Baryon Excitation function
Anti-baryon yield increase
continuously and smoothly
with energy, indicating
smooth increase of pair-
production.
STAR Preliminary
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 24/34
Baryon to Meson ratio:
Au+Au 62 GeV & Au+Au 200 GeV
Baryon enhancement in the intermediate pT region observed in p/π ratio, consistent
with recombination model.
Λ/K0S ratio shows that strange baryon production is also enhanced over strange
meson production in the intermediate pT region.
Strange baryon enhancement is higher than enhancement observed with p/π ratio.
Au+Au 62 GeV also shows baryon to meson enhancement at intermediate pT.
Values seems to be higher than Au+Au 200 GeV data.
Lambdas are not feed-down corrected
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 25/34
Baryon to meson ratio:
Cu+Cu 200 GeV & Au+Au 200 GeV
Baryon to meson enhancement is also observed in Cu+Cu data, but
seems to be higher then in Au+Au data when comparing equivalent
centrality classes.
STAR preliminary
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 26/34
Baryon to meson enhancement
Higher enhancement of Λ to K0s in the intermediate pT region in
Cu+Cu 200 GeV when comparing to Au+Au 200 GeV.
These seems to be also a energy dependence of the baryon to
meson enhancement.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 27/34
Nuclear Modification factor: Au+Au 200 GeV
Baryon meson difference in the intermediate pT region is also observed in Rcp and
RAA plots for strange particles.
RAA of protons seems to be different from the RAA of strange particles, where
higher values of RAA are observed for strange particles.
Maybe canonical suppression of strangeness production in p+p collisions
observed in low pt region extends to the intermediate pT region causing the
increase of RAA ratio, something that cannot be observed with Rcp plots. This is
different from Cronin effect.
STAR preliminary
nucl-ex/0705.2511
0 1 2 3 4
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 28/34
Rcp - Nuclear Modification factor:
Cu+Cu 200 GeV & Au+Au 200 GeV
Similar suppression observed in Cu+Cu at high pT for same centrality
ratios.
Similar baryon meson suppression difference seen in the intermediate
pT region.
Cu Cu, S NN 200 GeV Au Au, S NN 200 GeV
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 29/34
Rcp - Nuclear modification factor:
Au+Au 62 GeV & Au+Au 200 GeV
Similar suppression observed in Au+Au 62.4 GeV at high pT.
Similar baryon meson suppression difference seen in the intermediate
pT region.
Au Au, S NN 62.4 GeV Au Au, S NN 200 GeV
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 30/34
Difference between mesons & baryons Rcp
This ratio describes how different is the baryon Rcp from the meson
Rcp in the intermediate pT region.
Remarkable consistency is seen between different systems and
different energies.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 31/34
Summary / Conclusions
New Cu+Cu data was compared with Au+Au data
scaled by Npart to study system size dependence.
Overall yields and spectra shape seems to be consistent with the
equivalent peripheral Au+Au collision.
Lambdas produced at most central event centrality classes
seems to show higher yield compared to the equivalent Au+Au
peripheral collision. Perhaps a higher thermalization degree is
achieved for Cu+Cu central collisions?
Ξ and Ω are consistent within error with Au+Au data, but, error
bars are large and may not be sensitive to an enhancement over
Au+Au data.
We are currently analyzing Cu+Cu 62.4 GeV data that should
enrich this systematic study.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 32/34
Summary / Conclusions
Statistical Thermal model fits reasonably well the particle ratios measured
in STAR, indicating that data is consistent with a thermalized system.
Au+Au 200 and 62 GeV: Centrality dependence of thermal fits show increase of
the strangeness saturation parameter, and it reaches 1 only after Npart ~150.
Cu+Cu200 GeV: yields the same temperature and baryon chemical potential
values obtained from the fit to Au+Au data, but, the strangeness saturation
parameter is already at 1, even in the peripheral events.
Excitation functions of strange baryon production don’t show any
surprises.
At intermediate pt, baryon to meson enhancement is observed.
Λ/π ratio shows an enhancement higher than the enhancement observed with
p/π.
The enhancement increases with centrality and are different for Au+Au 62 and
200 GeV and Cu+Cu 200 GeV data.
Nuclear modification factor Rcp data also consistent with baryon to meson
enhancement in the intermediate pT.
Even in a small system like Cu+Cu, we still see a suppression at high p T region
and the baryon to meson difference in the intermediate pT region.
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 33/34
The STAR Collaboration
University of Illinois at Chicago - Argonne National Laboratory Institute of High Energy Physics - University of
Birmingham Brookhaven National Laboratory - California Institute of Technology - University of California,
Berkeley - University of California, Davis - University of California, Los Angeles - Carnegie Mellon University -
Creighton University – Nuclear Physics Inst., Academy of Sciences - Laboratory of High Energy Physics -
Particle Physics Laboratory - University of Frankfurt - Institute of Physics, Bhubaneswar - Indian Institute of
Technology, Mumbai - Indiana University Cyclotron Facility - Institut de Recherches Subatomiques de
Strasbourg - University of Jammu - Kent State University - Institute of Modern Physics - Lawrence Berkeley
National Laboratory - Massachusetts Institute of Technology - Max-Planck-Institut fuer Physics - Michigan
State University - Moscow Engineering Physics Institute - City College of New York - NIKHEF and Utrecht
University - Ohio State University - Panjab University - Pennsylvania State University - Institute of High
Energy Physics - Purdue University – Pusan National University - University of Rajasthan - Rice University -
Instituto de Fisica da Universidade de Sao Paulo - University of Science and Technology of China - Shanghai
Institue of Applied Physics - SUBATECH - Texas A&M University - University of Texas, Austin - Tsinghua
University - Valparaiso University – Variable Energy Cyclotron Centre, Kolkata - Warsaw University of
Technology - University of Washington - Wayne State University - Institute of Particle Physics - Yale University
- University of Zagreb -UNICAMP
Jun Takahashi for the STAR collaboration, Levoca, Slovakia 34/34
