system-size dependence of strangeness production at the sps ( ... and rhic, ags and sis)
DESCRIPTION
System-size dependence of Strangeness Production at the SPS ( ... and RHIC, AGS and SIS). Claudia Höhne, GSI Darmstadt. Introduction. energy. system size. strangeness production sensitive to the phase created in A+A collisions: possible indicator for phase transition. - PowerPoint PPT PresentationTRANSCRIPT
System-size dependence of Strangeness Production at the SPS
( ... and RHIC, AGS and SIS)
Claudia Höhne, GSI Darmstadt
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Introduction
• maximum in energy dependence
observed
• complementary information from
the system-size dependence!
energy
system size
KKEs
2
• strangeness production sensitive to the phase created in A+A collisions:
possible indicator for phase transition
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Outline
• data – system-size dependence of relative strangeness production at SPS:
• central C+C, Si+Si, Pb+Pb collisions at 158 AGeV
• concentrate on s=1
• model – percolation model for quantitative description of data
(hep-ph/0507276)
• conclusion (I)
• energy dependence of system-size dependence of relative strangeness production (RHIC, SPS, AGS, SIS)
• discussion
• conclusion (II) – open questions
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
s-production vs system-size
• fast increase for small systems, saturation from Npart > 60 on!
pp (Lit.)
CC, SiSi 15%, 12%
SS (NA35) 2%
PbPb 5%
2
1
40exp partN
ba
[NA49, PRL 94, 052301 (2005)]
lines are to guide the eye:
158 AGeV
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Statistical model• strangeness enhancement due to release of canonical strangeness suppression
• suppression factor calculated for a certain volume V, common assumption: partN
VV
20
KKEs
2
define V more carefully
[Tounsi and Redlich, J. Phys. G: Nucl. Part. Phys 28 (2002) 2095]
→ qualitatively in agreement with data
→ quantitatively in disagreement: saturation
is reached much too early (Npart ~ 9)
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Redefine hadronization volume V• microscopic model of A+A collisions → high density of collisions/strings
• assign a transverse extension to the individual NN collisions ("string-radius"),
assume that due to the overlap of these strings clusters of highly excited and
strongly interacting matter are formed; strings/collisions no longer independent
• assume independent hadronization of these clusters
• particle compositions (here: relative strangeness production) calculated from
the statistical model (as it is so successful for central AuAu/ PbPb)
• main purpose: calculate system-size dependence of relative strangeness
production in A+A collisions (at 158 AGeV)
percolation model: cluster formation
statistical model: cluster hadronization
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Cluster formation
• strings associated with NN collision are given a transverse size As
• distributed in overlap zone A of a A+A collision
• assumption: overlapping strings form clusters (size AC): percolation model
As = rs2
AC
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Mean area density of NN collisions
• 2 dimensional projection of N+N collisions (SiSi at 158 AGeV < 1fm penetration time)
• VENUS model (calculation for pp, CC, SiSi, SS, centrality dependent PbPb)
• (small) geometry effect
between central (light) A+A
and peripheral Pb+Pb
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Clustersize versus centrality
• combine the two calculations → clustersize for different system sizes!
•small systems ("pp"): basically
one small cluster/ string
• large systems ("central
PbPb"): one large cluster and a
certain probability for small ones
in the outer region of the
overlap zone
• intermediate systems: several
clusters of different size
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Hadronization
• assume: clusters form coherent entity → hadronization volume V
• apply statistical model for calculation of relative strangeness production () of the
hadronization volume V
• here: only goal is relative strangeness production
• here: calculation performed for
strange quarks in a quark phase,
parameters are T, ms
• note: almost same behaviour for
hadron gas with T~161 MeV,
B~260 MeV, V0~7fm3
V0 hadronization volume of pp
[Rafelski, Danos, PLB 97 (1980) 279]
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
AC → hadronization volume V
• in order to apply the statistical hadronization scheme, the clustersizes AC have to
be transformed to hadronization volumes Vh
→ factor that accounts for the (transverse) expansion until hadronization and for
the longitudinal dimension at hadronization
• compare with the situation in a single NN collision: hadronization volume ~ V0
(V0 nucleon volume)
• here: leave V0 as adjustable parameter
20
sCh r
VAV
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Comparison with experiment
• experimentally, total relative s-production is not accessible:
approximate with
• assume
KKEs
2
)()( hwoundS VaNE
parameters:
rs = 0.3fm V0=4.2fm3
ms=280 MeV T=160MeV
a=0.18
V=V0 Npart/2
V from percolation
[hep-ph/0507276]
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Conclusion (I)
• at top SPS energy, the system-size dependence of relative strangeness
production can be quantitatively understood as being due to the release of
canonical strangeness suppression if only the volume is chosen appropriately
(percolation ansatz!):
• in particular for intermediate systems several clusters of different size
• even in central Pb+Pb certain probability of pp-like clusters
• important: formation of collective volumes of increasing size
same shape of increase for partonic or hadronic phase
• multi-strange particles? → future
• other variables? → see application of percolation model to fluctuations etc. from
Pajares et al, Armesto et al,...
• other energies? RHIC, SPS (40 GeV), AGS, SIS
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Comparison to RHIC • PHENIX: K+/+ ratio at midrapidity [PRC 69 (2004) 034909]
• assume K+/+ ratio at midrapidity to be representative for the total relative s-production
BRAHMS: ratio nearly independent on rapidity [JPG 30 (2004) S1129]
• T=164 MeV to adjust for lower total s-enhancement
/K
CuCu 200 GeV
AuAu 200 GeV
PbPb 17.3 GeV
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
SPS 40 AGeV
P. Dinkelaker, NA49, SQM04
[JPG 31 (2005) S1131]
• "geometry" effect comparing central collisions of small nuclei with peripheral Pb+Pb at the same Nwound
• collision densities in central A+A different to peripheral PbPb at same Nwound
40 GeV beam energy
pp NN
semicentral CC
central SiSi
PbPb
4 yields
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
AGS
AGS (E802)
[PRC 60 (1999) 044904]
• strong geometry effect for smaller
systems (however: different energy! –
might change steepness of increase in
addition)
• continuous rise towards central AuAu
4 yields
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
200 Apart
SIS
• SIS: KAOS experiment, Ebeam=1.5 AGeV
[JPG 31 (2005) S693]
open symbols: Ni+Ni
closed symbols Au+Au
• smaller geometry effect compared to AGS?
4 yields
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
E-dependence of size dependence
PHENIX s = 200 GeV
NA49 Ebeam = 40 AGeV
E802 Ebeam = 11.1 AGeV
KAOS Ebeam = 1.5 AGeV
• K+ production taken as representative of total s-production
• normalize to - (available for all; AGS: F.Wang, private communication (1999))
• continuous change with energy?
• later saturation for lower energies
PHENIX yields at midrapidity,
others total yields
all normalized to most central ratio
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
E-dep. of size dependence (II)• ... normalize all to Nwound (central bin of KAOS – two normalizations shown)
• different calculation of Nwound in particular for AGS data
(AGS: Npart from spectator energy, others: Glauber model)
→ (clear) difference for lower/ higher energies?
all normalized to most central ratio ... KAOS yields adjusted to AGS
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Discussion
• saturation of relative strangeness production for all energies – or only for higher??
• role of pions in PbPb/ AuAu?: usage of small systems instead better defined?
• calculation of Nwound?
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Discussion (II)
• statistical model description as discussed for top SPS, RHIC holds for all energies
→ SIS, AGS reach grandcanonical limit (if!) only in central Au+Au collisions
(usage of grand canonical ensemble in statistical model fits justified?)
→ stat. model: lower temperature, higher B slows down increase
→ still rather small clusters needed for SIS, AGS
→ lower collision densities (longer penetration time, lower energy)
→ lower probability for cluster formation ?
→ formation of clusters from hadrons more difficult than from strings
→ geometry effect due to different densities
..... any correlation to the phase of matter?
• purely hadronic rescattering scenario
→ with Nwound the reaction time increases, equilibrium reached for central Au+Au
collisions?
→ geometry effect due to different densities
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Rescattering in RQMD• different scenario at lower energies?
• F. Wang et al. (RQMD), PRC 61 (2000) 064904: continuous rise of K/ ratio due
to rescattering in the hadron gas (effect of ropes negligible for AGS)
• with Npart the reaction time increases, saturation (equilibrium) reached?
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Conclusion (II)
• RHIC data can be easily understood within the same picture introduced
in the first part of this talk
• system-size dependence at lower energies?
• systematic change is visible (20, 30 AGeV data from NA49 to come!)
• unfortunately: data situation unclear (normalization?)
• saturation of relative strangeness production for all energies??
• how strong is the geometry effect for smaller systems?
• all explainable within same picture?
• can the energy dependence of the system-size dependence tell us
something about the scenario?
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
K/ ratio versus rapidity (RHIC)
• BRAHMS collaboration, QM04
[D. Ouerdane, J.Phys.G: Nucl.Part.Phys.30 (2004) S1129]
Claudia Höhne Strangeness in Collisions workshop, BNL, February 16-17, 2006
Discussion (II)
percolation model + statistical
hadronization:
• only changing T to 146 AGeV is not
sufficient to explain the data at 40 AGeV
(small systems? geometry effect!)
simplifying assumptions in model:
• different definition of volumes needed?
e.g. 2-dimensional projection not justified
anymore? usage of 3d-densities and
cluster formation needed?
• centrality dependent parameters for
statistical model?
T=146 MeV
PHENIX data
NA49 40 AGeV