Energy and system size dependence of strangeness production, from SPS to RHIC

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Energy and system sizedependence of strangeness production, from SPS to RHIC

Jun Takahashi & Marcelo Munhoz for the STAR collaboration

2/34Jun Takahashi for the STAR collaboration, Levoca, Slovakia

MotivationMotivation 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.


The STAR experimentThe 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: K0

s, Λ, Ξ, Ω.


DataDataRHIC 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


System Size: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)


System Size:System Size:

Au+AuAu+Au Cu+CuCu+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

40-60 59.0 ± 5. 82 ± 1210-20 74.8 ± 2.5 126.7 ±6.7

20-30 54.4 ± 2.8 81.5 ± 6.0

60-80 19.0 ± 3.5 19 ± 530-40 38.5 ± 2.5 51.0 ± 4.8

40-60 21.9 ± 2.6 24.3 ± 3.9


CuCu200 spectra: K0s and Λ´sCuCu200 spectra: K0s and Λ´s

55M min-bias events analyzed.High statistic data, with spectra extended to high pT coverage.Fully corrected spectra, with feed-down corrections.

From A. Timmins


CuCu200 spectra: Ξ´s and Ω´sCuCu200 spectra: Ξ´s and Ω´s

STAR PreliminaryStatistics 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.

STAR Preliminary


Λ Spectra Comparison between Au+Au and Cu+CuΛ Spectra Comparison between Au+Au and Cu+Cu

From A. TimminsTalk on sunday

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.

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.

STAR Preliminary

Ξ Spectra Comparison between Au+Au and Cu+CuΞ Spectra Comparison between Au+Au and Cu+Cu


Mean Transverse momentum of Ξs and Ωs.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 consistent with Au+Au values.STAR Preliminary


CuCu200 & AuAu200 Yields: K0s, and Λ´sCuCu200 & 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.


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.

STAR Preliminary

Strangeness Enhancement: Au+Au 200 GeVStrangeness Enhancement: Au+Au 200 GeV

nucl-ex/0705.2511

A. Tounsi, A. Mischke and K. Redlich,Nucl. Phys. A 715, 565 (2003).


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.

STAR Preliminary

Strangeness Enhancement: AuAu200 & CuCu200 Strangeness Enhancement: AuAu200 & CuCu200


Statistical Thermal Model: Cu+Cu 200 GeVStatistical Thermal Model: Cu+Cu 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.

GeVSCuCu NN 200,

* Thermus, A thermal Model Package for Root S. Wheaton & Cleymans, hep-ph/0407174


Statistical Thermal Model: Au+Au 200 GeVStatistical Thermal Model: Au+Au 200 GeV

Statistical Thermal Model (THERMUS)* was used fitting Tch, μB, μS, and γS (strangeness saturation factor).


π, K, p, Λ, Ξ, Ω.



GeVSAuAu NN 200,



Statistical Thermal Model: Au+Au 62.4 GeVStatistical Thermal Model: Au+Au 62.4 GeV

Statistical Thermal Model (THERMUS)* was used fitting Tch, μB, μS, and γS (strangeness saturation factor).


π, K, p, Λ, Ξ, Ω.



Ratio K0/π was excluded to improve chi2.

GeVSAuAu NN 4.62,



Statistical Thermal Model: Fit parameters vs. system sizeStatistical Thermal Model: Fit parameters vs. system size

Baryon chemical potential μB is small for Au+Au 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.


Cu+Cu 200 GeV



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 smaller value then compared to Au+Au data, but is in agreement within error bars.


Cu+Cu 200 GeV



Strangeness saturation constant, shows an increase with system size, reaching saturation around Npart~150.

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.


Cu+Cu 200 GeV


Statistical Thermal Model: Excitation functionStatistical Thermal Model: Excitation function


Particle ratiosParticle ratios

SPS data from NA49AGS data from E896 & E802

STAR preliminary

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.New Cu+Cu data seems to follow systematic.Ratio seems to be the same, independent of the system size.


Baryon Excitation functionBaryon 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 ~ 10-30Ξ ~ 3-6Ω ~ 0.4-0.7

STAR Preliminary


Anti-Baryon Excitation functionAnti-Baryon Excitation function

Anti-baryon yield increase continuously and smoothly with energy, indicating smooth increase of pair-production.

STAR Preliminary


Baryon to Meson ratio: Au+Au 62 GeV & Au+Au 200 GeVBaryon to Meson ratio: Au+Au 62 GeV & Au+Au 200 GeVBaryon enhancement in the intermediate pT region observed in p/π ratio, consistent with recombination model.Λ/K0

S 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


Baryon to meson ratio: Cu+Cu 200 GeV & Au+Au 200 GeVBaryon to meson ratio: Cu+Cu 200 GeV & Au+Au 200 GeV

STAR preliminary

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.


Baryon to meson enhancementBaryon 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.


Nuclear Modification factor: Au+Au 200 GeVNuclear 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 preliminarynucl-ex/0705.2511

0 1 2 3 4


Rcp - Nuclear Modification factor: Cu+Cu 200 GeV & Au+Au 200 GeV

Rcp - Nuclear Modification factor: Cu+Cu 200 GeV & Au+Au 200 GeV

GeVSAuAu NN 200, GeVSCuCu NN 200,

Similar suppression observed in Cu+Cu at high pT for same centrality ratios.Similar baryon meson suppression difference seen in the intermediate pT region.


Rcp - Nuclear modification factor: Au+Au 62 GeV & Au+Au 200 GeV

Rcp - Nuclear modification factor: Au+Au 62 GeV & Au+Au 200 GeV

GeVSAuAu NN 200, GeVSAuAu NN 4.62,

Similar suppression observed in Au+Au 62.4 GeV at high pT.Similar baryon meson suppression difference seen in the intermediate pT region.


Difference between mesons & baryons Rcp 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.


Summary / ConclusionsSummary / 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.


Summary / ConclusionsSummary / 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 pT region

and the baryon to meson difference in the intermediate pT region.


The STAR CollaborationUniversity 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


p/π - Baryon to meson ratio (PHENIX)p/π - Baryon to meson ratio (PHENIX)

PHENIX Preliminary

Cu+CuD. Morrison, SQM2006

Au+Au 200 GeVM. Konno, JPS2007

Baryon to meson ratio show baryon enhancement in the intermediate pt region.


Rcp - Nuclear modification factor: AuAu62 & AuAu200

Rcp - Nuclear modification factor: AuAu62 & AuAu200

GeVSAuAu NN 200, GeVSAuAu NN 4.62,


Scaling with charge particle production (dNch/dy)Scaling with charge particle production (dNch/dy)


SPS NA57 data also shows clear enhancement behavior.

Enhancement rate at RHIC seems to be lower than the enhancement observed at SPS data.

Need to check if the enhancement reduction is in agreement with GC Thermal model predictions.

NA57 data fromnucl-ex/0601021

Strangeness Enhancement: RHIC & SPSStrangeness Enhancement: RHIC & SPS


Total YieldTotal Yield

STAR Preliminary

After efficiency, acceptance and feed-down correction, total yield is obtained from a fit using a Boltzmann function.


Scaling with NbinScaling with Nbin


Lambda/K0s from SPSLambda/K0s from SPS

Energy and system size dependence of strangeness production, from SPS to RHIC

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