Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://theory.sinp.msu.ru/~qfthep04/2004/Proceedings2004/Feofilov_QFTHEP04_412_418.ps Äàòà èçìåíåíèÿ: Thu Sep 17 21:33:22 2009 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 08:28:58 2012 Êîäèðîâêà: IBM-866 Ïîèñêîâûå ñëîâà: color

Parton String Model analysis of strange particles yields and
slopes for PbPb collisions at 158A*GeV in NA57 experiment at
CERN SPS
Presented by G.A.Feofilov (for the NA57 collaboration)
F Antinori 1 , P A Bacon 2 , A BadalÑa 3 , R Barbera 3 , A Belogianni 4 , I J Bloodworth 2 , M Bombara 5 ,
G E Bruno 6 , S A Bull 2 , R Caliandro 6 , M Campbell 7 , W Carena 7 , N Carrer 7 , R F Clarke 2 , A Dainese 1 ,
D Di Bari 6 , S Di Liberto 8 , R Divia 7 , D Elia 6 , D Evans 2 , G A Feofilov 9 , R A Fini 6 , P Ganoti 4 ,
B Ghidini 6 , G Grella 10 , H Helstrup 11 , K F Hetland 11 , A K Holme 12 , A Jacholkowski 3 , G T Jones 2 ,
P Jovanovic 2 , A Jusko 2 , R Kamermans 13 , J B Kinson 2 , K Knudson 7 , V Kondratiev 9 , I KrÒalik 5 ,
A KravßcÒakovÒa 14 , P Kuijer 13 , V Lenti 6 , R Lietava 2 , G Lœvhœiden 12 , V Manzari 6 , M A Mazzoni 8 ,
F Meddi 8 , A Michalon 15 , M Morando 1 , P I Norman 2 , A Palmeri 3 , G S Pappalardo 3 , B PastirßcÒak 5 ,
R J Platt 2 , E Quercigh 1 , F Riggi 3 , D RØohrich 16 , G Romano 10 , K ß
SafaßrÒÐk 7 , L ß SÒandor 5 , E Schillings 13 ,
G Segato 1 , M SenÒe 17 , R SenÒe 17 , W Snoeys 7 , F Soramel 1# , M SpyropoulouíStassinaki 4 , P Staroba 18 ,
R Turrisi 1 , T S Tveter 12 , J UrbÒan 14 , P van de Ven 13 , P Vande Vyvre 7 , A Vascotto 7 , T Vik 12 ,
O Villalobos Baillie 2 , L Vinogradov 9 , T Virgili 10 , M F Votruba 2 , J VrlÒakovÒa 14 and P ZÒavada 18
1 Dipartimento di Fisica dell'UniversitÑa and Sezione INFN, Padua, Italy
2 School of Physics and Astronomy, University of Birmingham, Birmingham, UK
3 Dipartimento di Fisica dell'UniversitÑa and Sezione INFN, Catania, Italy
4 Physics Department, University of Athens, Athens, Greece
5 Institute of Experimental Physics, Slovak Academy of Science, Koßsice, Slovakia
6 Dipartimento I.A. di Fisica dell'UniversitÑa e del Politecnico di Bari and Sezione INFN, Bari, Italy
7 CERN, European Laboratory for Particle Physics, Geneva, Switzerland
8 Dipartimento di Fisica dell'UniversitÑa ``La Sapienza'' and Sezione INFN, Rome, Italy
9 State University of St. Petersburg, St. Petersburg, Russia
10 Dipartimento di Scienze Fisiche ``E.R. Caianiello'' dell'UniversitÑa and INFN, Salerno, Italy
11 Hœgskolen i Bergen, Bergen, Norway
12 Fysisk institutt, Universitetet i Oslo, Oslo, Norway
13 Utrecht University and NIKHEF, Utrecht, The Netherlands
14 P.J. ß
SafÒarik University, Koßsice, Slovakia
15 Institut de Recherches Subatomiques, IN2P3/ULP, Strasbourg, France
16 Fysisk institutt, Universitetet i Bergen, Bergen, Norway
17 CollÑege de France and IN2P3, Paris, France
18 Institute of Physics, Academy of Science of the Czech Republic, Prague, Czech Republic
# Permanent address: University of Udine, Udine, Italy
and N.Armesto,
CERN, Theory Division, Department of Physics,
1211,CERNíCH,Geneve,Switzerland.
Eímail address for correspondence: feof ilov@hiex.niif.spb.su
Abstract
In this work we analyze the recent experimental data on strange and multistrange particle yields
and transverse mass spectra obtained in píPb and PbíPb collisions at 158A*GeV energies by the
WA97 and NA57 experiments at CERN SPS within the Parton String Model. This model assumes
nucleusínucleus collisions at the partonic level and can be applied to describe a possible noníQGP
scenario that may play an essential role in earlier stages of nucleusínucleus collisions, before the onset
of the QGP. With default parameters and a simplified account of rescattering, PSM results are found
in a qualitative agreement with the majority of the latest PbíPb data at 158A*GeV on strangeness
yields, transverse mass inverse slopes and particle ratios. The deviations of the model predictions
from the experiment were observed in some cases, in particular, it was obtained that the yields for
# +
are overestimated by the PSM by a factor of about 2 for all classes of centrality, while
for# +# they are underestimated by a factor of about 3 for the most central collisions. For # - yields in case
of píPb collisions results are also underestimated by the PSM (a factor of about 2). An increase of
the inverse mass slopes with the collision centrality (towards central collisions) observed for # and
# +
is also not reproduced. The mentioned general agreement and the particular di#erences in some
412

model predictions and data could be stimulating for further theoretical studies. The results obtained
in the framework of the PSM indicate also that future experimental studies of the eventíbyíevent
longírange correlations for strange particles could be proposed as a new method to quantify collective
phenomena.
1 Introduction. Motivation of this study
The recent experimental data on strange and multistrange particle yields and transverse mass spectra
obtained in píPb and PbíPb collisions at 158A*GeV energies by the WA97 [1],[2] and NA57 [3],[4]
experiments at CERN SPS manifest a number of intriguing features:
. the increase of enhancements of hyperon and antihyperon production at 158 AGeV/c with the
strangeness content of the particle. Strange particle yields per wounded nucleon increase with
strangenes content of the particle up to factor 20
for# ++# - as compared to píBe data[1],[3]
. inverse exponential slopes of the m T distributions at midrapidity of di#erent particles vs. the mass
of the particles demonstrate a deviation from the linear increase [2],[4]. An increase of the inverse
mass slopes with collision centrality is observed for # and # +
.
. the energy dependence of the hyperon production is much weaker than for the antihyperons [5],[6].
The antihyperon to hyperon ratios increase with the energy, with a stronger dependence for particles
with smaller strangeness content.
These experimentally observed features could be used as a discriminating probe to validate a number
of detailed Monte Carlo models describing nucleusínucleus interactions at relativistic energies.
In this work we analyze the first two of these peculiarities within the Parton String Model (PSM) [7]
that assumes nucleusínucleus collisions at the partonic level. Thus we investigate a possible noníQGP
scenario that may play an essential role in earlier stages of nucleusínucleus collisions, before the onset of
the QGP. It may be useful in understanding a possible role of a string fusion mechanism in the formation
of the QGP.
2 PSM results for strange particles yields and slopes
We apply the Monte Carlo PSM event generator [7] to describe the latest detailed data [3],[4] on strange
particles yields, inverse slopes and their centrality dependence obtained for PbPb collsions at 158 A
GeV/c in the NA57 experiment at CERN SPS.
The details of the NA57 experimental setup and data treatment procedures could be found elsewhere
[11],[12].
The description of the Parton String Model event generator can be found in [7]. It is based on the
modern theory of the multiparticle production in high energy hadron collisions that describes it in a
twoístage scenario [8, 9]. At the first stage a certain number of colour strings are formed, stretched
between the incoming partons. At the second stage these strings decay into the observed secondary
hadrons. In the simplest version the strings emit particles independently. However, due to a high density
of strings in the transverse plane in the case of nuclear collision one has also to take into account the
interaction between strings in the form of their fusion and/or percolation [13]í[18]. Color strings that
are formed as a result of elementary partoníparton collisions are allowed to overlap producing strings
in higher color representations. Thus the collective e#ects in nucleusínucleus collisions are taken into
account. Among the main predictions of the string fusion phenomenon are the following: (i) a strong
reduction of multipicity of charged particles [13], (ii) growth of < p 2
t > [13], and (iii) an increase of baryon
and strangeness production [13],[19].
We started the calculations from a conventional set of PSM parameters successfully applied previously
[7],[20] to the description of the experimental data on the rapidity and transverse momentum distributions
of charged particles in heavy ion collisions at SPS and RHIC. Various options of model calculations were
tried including cases with and without fusion and rescattering, some variations of the PSM parameters
were also investigated.
413

Wed Jan 12 14:11:56 2005 Mean number of wounded nucleons
10 2
10
Yield/event
í2
10
í1
10
1
10
Strange particle yields in A+A at 158 AGeV
Lambda and Xií yiels
Model: PSM (F+R)
Experiment: NA57
Pb+Pb
p+Pb
Lambda
Xií
Figure 1: Strange particles yields per event vs. collision centrality:
Circles í experiment (statistical errors are within the data points);
Squares í PSM calculations
As a final result we obtained that with the default parameters and a simplified model for rescattering
of secondaries, PSM shows a good qualitative agreement with the majority of data:
(i) it was found that the general trends of the centrality dependence of strange particles yields at
midírapidity as well as the inverse slopes parameters of spectra of charged particles are reasonably well
reproduced only if both string fusion and rescattering are considered. (This confirms previuos results [20]
where the inclusion of string fusion was also found to be crucial for the agreement with the RHIC data);
(ii) good agreement is obtained (within 1í2 error bars) for strange particles yields at 158 A GeV/c
calculated for PbPb collisions with the standard set of parameters for # and # - yields/event vs. centrality
of collision (see Fig.1) and within 3í4 error bars for # (see Fig.2).
(iii) a qualitative agreement is achieved for # +
( a factor of about 2)
and# +# (a factor about 3 for
the most central data) yields/event vs. centrality of collision), see Fig.2.
It was found in the framework of the PSM that the rescattering of string fragmentation products in
the final state (i.e. secondaries) in PbPb collisions brings at CERN SPS energy a factor of about 2 for
# + # adds about 30% to # +
yields and decreases at the same time the # yields also by about 30%.
The last fact mentioned for # is due to annihilation, (it was also found that for antiprotons annihilation
decreases the yield by more than a factor 2).
The importance of rescattering was also stressed by our observation that the variations of the PSM
parameters near the default values failed to account for the experimental yields and slopes unless the
rescattering is included.
At the same time, we see from the Fig.2 that the yields for # +
are overestimated ( a factor of about
2) and
for# +# are still underestimeted ( a factor of about 3 for the most central collisions) by the
PSM. We can guess here, that this discrepancy could be either a result of the simplified rescattering used
in the model [7] or an indication on a possible contribution of some other mechanisms of # +
and# +# production, e.g. fusion of more than two strings. This suggestion is based on the reasonable agreement
414

Wed Jan 12 15:28:31 2005 Mean number of wounded nucleons
10 2
10
Yield/event
í4
10
í3
10
í2
10
í1
10
1
Strange particle yields in A+A at 158 AGeV
Lambda and Xií yiels
Model: PSM (F+R)
Experiment: NA57
Pb+Pb
p+Pb
AntiLambda
AntiXi+
Omega + AntiOmega
Figure 2: Strange particles yields/event vs. collision centrality:
Circles í experiment (statisyical errors are within the data points);
Squares í PSM calculations
of the previous PSM results to the available data on total multiplicties, transverse momentum spectra
and strange particle yields in a wide energy range from SPS to TeVatron obtained earlier [7],[20]. The
presently observed particular discrepancies could point to the necessity of further theoretical studies.
In our PSM calculations the string fusion phenomenon was found to be the most influential on the
strange particles yields:
. String fusion phenomenon gives about 3 fold increase (as compared to the calculations without
fusion) for # yields.
. It appeares to be the most important (bringing about 10 fold increase) for #, # +
and# + # and
about 15 fold increase for # - .
One has to comment that the previuos results of our VENUS and RQMD analysis of NA57 data
[10] showed that both models predicted the enhancement increasing with the strangeness content of the
particles but they were not able to reproduce the measured values of strange particles yields at central
rapidity. This fact is also confirmed by the results of the present work: it was found that only the
inclusion of the string fusion process has provided the needed increase of yields mentioned above.
The overal agreement of the PSM results obtained with the account of string fusion and rescattering
processes was achieved with the experimental data on inverse m T slopes (temperatures) (see Tables 1
and 2). In particular, the deviation from the linear increase of inverse exponential slopes of the m T
distributions at midrapidity vs. the mass of the particles is obtained (a decrease of the corresponding
temperatures is obtained for multistrange hyperons in a qualitative agreement with the experiment). It
was found that in case of the most central collisions (40%) the inverse m T slope parameters for all strange
hyperons (except #) in PSM are not very sensitive to the string fusion (a slight decrease at the level of
10% from the values obtained without string fusion was observed in PSM calculations bringing a better
agreement with the experiment).
415

However, the centrality dependence of the inverse m T slope parameters shows that the PSM is draí
matically overestimating the inverse slopes for # and for # - for the peripheral case (classes 0 and 1,
see Table2). This e#ect was observed previously also in our calculations using the VENUS model, so it
looks like that the inverse slopes description for peripheral events (i.e. those that should be free from
any nuclear matter involvement) are, probably, the common drawbacks for both models (VENUS and
the PSM).
The following general conclusion could be obtained here from the comparison of PSM calculations
to the experimental results: in the framework of the Parton String Model both fusion and rescattering
processes are found to play a crucial role in the description of strange particles yields and slopes at the
SPS energies.
Table 1: Experimental inverse m T slopes (MeV) parameters of strange particles in PbPb ( # s=17.3 GeV)
at midírapidity ( |#y # | 0.5 ), see [4]. (Definition of the NA57 event centrality classes for PbPb at 158 A
GeV/c could be found elsewhere [12]. 0 í refers to the most peripheral collisions, class 4 í to the most
central).
Centrality
class
0 1 2 3 4
# 237‘19 274‘13 282‘12 315‘14 305‘15
# 277‘19 264‘11 283‘10 313‘14 295‘14
# 290‘20 290‘11 295‘9 304‘11 299‘12
# 232‘29 311‘23 294‘18 346‘28 356‘31
# +# 274‘34 274‘28 268‘23
Table 2: PSM calculated m T inverse slopes (MeV) parameters of strange particles in PbPb ( # s=17.3
GeV) at midírapidity ( |#y # | 0.5 ).
Centrality
class
0 1 2 3 4
# 522 438 378 349 334
# 298 325 321 319 308
# 391 343 311 297 283
# 278 280 300 308 261
# +# 271 225 217 245 234
3 Longírange correlations for strange particles as a method to
quantify collective phenomena
Results of the present study indicate a new possibility to quantify the collective phenomena within a
string fusion mechanism.
As it was already mentioned, color strings that are formed as a result of elementary partoníparton
collisions could overlap (or fuse), producing strings of higher color representations. At the same time,
the hypothesis of color string fusion has clear predictions which could be tested experimentally. It is
the so called longírange correlations, i.e. eventíbyíevent study of observables obtained in a su#ciently
separated di#erent rapidity windows (see [21]) and references therein as well as [22], [23], [24]), that could
be a signature of the basic new phenomenon, i.e. of the formation of emitters with higher color density.
Such phenomenon is expected to occur at large string density values reached in the high energy nucleusí
nucleus collisions [14, 23, 21]. The search of longírange correlations was started using the available SPS
416

experimental data[25]. Noticeable longírange correlations are found between event mean p t and charged
particle multiplicty in NA49 experimental data obtained in two separated rapidity intervals in 158 A
GeV/c PbPb collisions at the CERN SPS[25].
The results of the present study confirm the substantial increase of strange particles yields from
the overlapped strings. Therefore, this could be an indication that the eventíbyíevent studies of longí
range correlations for strange particles produced in heavyíion collisions could be capable to bring a
new experimental insight into the heavyíion collison process. The main benefits expected from the new
experimental investigations based on the string fusion hypothesis with involvment of strange particles
are:
. to obtain a new experimental evidence of collectivity at SPS energies
. to get a clear indications on the directions of the further upgrade of the PSM and of the other
models of multiparticle production,
. to predict and to search for experimental longírange correlation e#ects at other energies and for
other colliding nuclei
. to provide new observables to quantify collective phenomena in heavyíion nuclear collisions
4 Conclusions
The recent experimental data on strange and multistrange particle yields and transverse mass spectra
obtained in píPb and PbíPb collisions at 158A GeV/c energies by the WA97 and NA57 experiments at
CERN SPS are analysed within the Parton String Model.
With default parameters and a simplified model for rescattering of secondaries, PSM shows a qualí
itative agreement with the majority of the latest PbíPb data at 158A GeV/c including the increase of
hyperon and antihyperon production at 158 AGeV/c with the strangeness content of the particle and
with the collision centrality.
Our calculations confirm that in the framwork of the PSM both fusion and rescattering play a crucial
role in description of strange particles yields and slopes at the SPS energies. We found that inclusion
of a string fusion phenomenon in the PSM gives about 3 fold increase (as compared to the calculations
without fusion) for # yields, although it appeares to be the most pronounced (bringing about 10 fold
increase) for #, # +
and# + # and about 15 fold increase for # - yields. Data on yields are spanning
about 3 orders in magnitude vs. centrality of collision in all cases mentioned above.
At the same time, it is important to stress that some dicreapancies are observed in comparison of
the model predictions and experiment. It was obtained that the yields for # +
are overestimated by the
PSM by a factor of about 2 for all classes of centrality, while
for# +# they are still underestimated by a
factor of about 3 for the most central collisions. For # - yields in case of píPb collisions results are also
underestimated by the PSM (a factor of about 2). The mentioned particular di#erences in the model
prediction and data could point to the necessity of further theoretical studies.
The experimentally observed deviation from the linear increase of inverse exponential slopes of the
m T distributions at midrapidity of di#erent particles vs. the mass of the particles (lower temperatures
for# +# as compared to the # +
) is qualitatevly described by the PSM for the central collisions. The
overal agreement of the PSM with the experimental data on the inverse slopes centrality dependence is
achieved for all classes of collision centrality and particles, with the exception of # and # - for the most
peripheral cases. An increase of the inverse mass slopes with the collision centrality (towards central
collisions) observed for # and # +
is also not reproduced, pointing at the necessity of more studies for a
possible modification of the PSM.
The future studies may involve the investigations of the eventíbyíevent longírange correlations for
strange particles in heavyíion collisions, and the NA57 experimental data could be an important test
for the PSM event generator with the purpose to develop a sensitive method to quantify the collective
phenomena.
Acknowledgements: This work was partially supported for St.Petersburg State University participants
by the grants No.1649 of the Ministry of High Education, RF.
417

References
[1] E.Andersen, et al.,Phys.Lett.B449(1999)401
[2] F.Antinori, et al., Eur.J.Phys.C14(2000)633; CERNíEPí200í001, 03 January 2000
[3] V.Manzari, et al., Nucl.Phys.A715(2003)140í150
[4] F.Antinori, et al, J.Phys.G:Nucl.Part.Phys.30(2004) 823í840
[5] F.Antinori et al., Phys.Lett.,B595(2004)68í74
[6] D.Elia, J.Phys.G:Nucl.Part.Phys.30(2004)S1329íS1332
[7] N.S.Amelin, N.Armesto, C.Pajares, D.Sousa, Eur.Phys.J. C22(2001)149í163
[8] A.Capella, U.P.Sukhatme, C.--I.Tan and J.Tran Thanh Van,
Phys. Lett. B81(1979)68; Phys. Rep.236(1994) 225.
[9] A.B.Kaidalov, Phys. Lett.B116(1982)459;
A.B.Kaidalov K.A.TeríMartirosyan, Phys. Lett.B117(1982)247.
[10] F.Antinori et al. (NA57 collaboration), Eur.Phys.J.C11(1999)79í88.
[11] R.Caliandro, et al., NA57 Proposal, CERN/SPSLC 96í40, SPLC/P300
[12] V.Manzari et al.,J.Phys.G:Nucl.Part.Phys.25(1999)473; V.Manzari et al.,Nucl.Phys.A661(1999)716c;
[13] M.A.Braun and C.Pajares, Phys. Lett.B287(1992)154;
Nucl. Phys.B390(1993) 542, 549;
N.S.Amelin, M.A.Braun and C.Pajares, Phys. Lett. B306 (1993) 312;
Z.Phys.C63(1994)507.
[14] N.S.Amelin, N.Armesto, M.A.Braun, E.G.Ferreiro and C.Pajares, Phys. Rev. Lett.73(1994)2813.
[15] N.Armesto, M.A.Braun, E.G.Ferreiro and C.Pajares,
Phys. Rev. Lett.77(1996)3736.
[16] M.Nardi and H.Satz, Phys. Lett.B442(1998)14;
H.Satz, Nucl. Phys.A661(2000)104c.
[17] M.A.Braun, C.Pajares and J. Ranft.
Int. J. of Mod. Phys.A14(1999)2689.
[18] M.A.Braun and C.Pajares, Eur. Phys. J.C16(2000)349.
[19] N.Armesto,M.A.Braun,E.G.Ferreiro and C.Pajares,
Phys.Lett.B334(1995)301;E.G.Ferreiro,C.Pajares and D.Sousa, Phys.Lett.B422(1998)314.
[20] N.Armesto,C.Pajares, D.Sousa, Phys.Lett.B527(2002)92í98
[21] M.A.Braun,R.S.Kolevatov,C.Pajares.V.V.Vechernin, Eur.Phys.J.C.32.535í546(2004)
[22] N.Armesto,M.A.Braun.E.C.Ferreiro.C.Pajares, Z.Phys.C.67(1995)489í493
[23] M.A.Braun,C.Pajares, Phys.Rev.Let,85(2000)4864í4867
[24] M.A.Braun,C.Pajares, Eur.Phys.J.C16(2000)349í359
[25] NA49 Collaboration at CERN and G.Feofilov, R.Kolevatov, V.Kondratiev, P.Naumenko,
V.Vechernin,''LongíRange Correlations in PbPb Collisions at 158 A GeV/c'', Abstracts of the XVII
International Baldin Seminar on High Energy Physics Problems, Dubna, Russia, September 27í
October 2 (2004), p.59, (to be published).
418