The system size and centrality dependence of multiplicity fluctuations in
nuclear collisions at $158A$ GeV as well as the energy dependence for the most
central $Pb+Pb$ collisions were studied by the NA49 experiment at CERN SPS. A
strong increase of fluctuations was observed with decreasing centrality in
$C+C$, $Si+Si$ and $Pb+Pb$ collisions. The string hadronic models (UrQMD,
Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate
a strong mixing of target and projectile contribution in a broad rapidity
range. For the most central collisions at all SPS energies multiplicity
distributions are significantly narrower than a corresponding Poisson one both
for negatively and positively charged hadrons. The UrQMD model seems to
reproduce the measured values on scaled variance. Statistical model
calculations overpredict results when conservation laws are not taken into
account.Comment: Proceedings Quark Confinement and the Hadron Spectrum VI

Benjamin Lungwitz

null null

English

arXiv

The system size and centrality dependence of multiplicity fluctuations in nuclear collisions at 158A GeV as well as the energy dependence for the most central Pb + Pb collisions were studied by the NA49 experiment at CERN SPS. A strong increase of fluctuations was observed with decreasing centrality in C + C, Si + Si and Pb + Pb collisions. The string hadronic models (UrQMD, Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate a strong mixing of target and projectile contribution in a broad rapidity range. For the most central collisions at all SPS energies multiplicity distributions are significantly narrower than a corresponding Poisson one both for negatively and positively charged hadrons. The UrQMD model seems to reproduce the measured values on scaled variance. Statistical model calculations overpredict results when conservation laws are not taken into account.The system size and centrality dependence of multiplicity fluctuations in nuclear collisions at $158A$ GeV as well as the energy dependence for the most central $Pb+Pb$ collisions were studied by the NA49 experiment at CERN SPS. A strong increase of fluctuations was observed with decreasing centrality in $C+C$, $Si+Si$ and $Pb+Pb$ collisions. The string hadronic models (UrQMD, Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate a strong mixing of target and projectile contribution in a broad rapidity range. For the most central collisions at all SPS energies multiplicity distributions are significantly narrower than a corresponding Poisson one both for negatively and positively charged hadrons. The UrQMD model seems to reproduce the measured values on scaled variance. Statistical model calculations overpredict results when conservation laws are not taken into account

Lungwitz, Benjamin

CERN Document Server

arXiv:nucl-ex/0610047v1  31 Oct 2006Multiplicity Fluctuations in Heavy Ion Collisionsat CERN SPSBenjamin Lungwitzfor the NA49 CollaborationFachbereich Physik der Universität, Frankfurt, Germany.Abstract. The system size and centrality dependence of multiplicity fluctuations in nuclear colli-sions at 158A GeV as well as the energy dependence for the most central Pb + Pb collisions werestudied by the NA49 experiment at CERN SPS. A strong increase of fluctuations was observed withdecreasing centrality in C+C, Si+Si and Pb+Pb collisions. The string hadronic models (UrQMD,Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate a strong mixingof target and projectile contribution in a broad rapidity range. For the most central collisions at allSPS energies multiplicity distributions are significantly narrower than a corresponding Poisson oneboth for negatively and positively charged hadrons. The UrQMD model seems to reproduce themeasured values on scaled variance. Statistical model calculations overpredict results when conser-vation laws are not taken into account.Keywords: Multiplicity Fluctuations, NA49, CERN SPSPACS: 24.60.KyINTRODUCTIONAt high energy densities (≈ 1GeV/ f m3) a phase transition from hadron gas to quark-gluon-plasma (QGP) is expected to occur. There are indications that at top SPS energiesquark-gluon-plasma is created at the early stage of heavy ion collisions [1]. Lattice QCDcalculations suggest furthermore the existence of a critical point in the phase diagramof strongly interacting matter which separates the line of the first order phase transitionfrom a crossover. Models predict an increase of multiplicity fluctuations near the onsetof deconfinement [2] or the critical point [3]. Statistical model calculations [4] showed anon-trivial decrease of fluctuations due to conservation laws. These reduced fluctuationswould then serve as a “background” in the search for the critical point.THE NA49 EXPERIMENTThe NA49 detector is a large acceptance fixed target hadron spectrometer describedin [5]. The centrality of a collision is determined using a downstream Veto calorimeterwhich measures the energy in the projectile spectator domain [6]. This allows a deter-mination of the number of projectile participant nucleons N pro jP . The number of targetparticipants is not fixed in the experiment, model calculations show large fluctuations oftheir number in peripheral collisions [7].Analysis ProcedureSince detector effects like track reconstruction efficiency might have a large influenceon multiplicity fluctuations, it is important to select a very clean track sample for theanalysis. Therefore the acceptance for this analysis is limited to a part of the forwardhemisphere, where the NA49 detector has the highest tracking efficiency. This was doneby restricting the analysis to the rapidity interval 1 < y(pi) < ybeam 1 for 20A to 80AGeV and 1.08 < y(pi) < 2.57 for 158A GeV. In addition a cut on transverse momentumaccording to [8] was applied, which is dependent both on rapidity and azimuthal angle.For such tracks the reconstruction efficiency is larger than 98%.The basic measure of multiplicity fluctuations used in this analysis is the scaledvariance: ω = Var(n)<n> , where Var(n) and < n > are variance and mean of the multiplicitydistribution, respectively. In this paper only the results for negatively charged hadronsare shown.The data is corrected for the finite width of centrality bins. Data on centrality and sys-tem size dependence at 158A GeV is corrected for the resolution of the Veto calorimeter,for most central Pb + Pb data this correction is small (it would decrease the results byless than 5%) and was neglected.The total systematic error is estimated to be +2% respectively −5% [6] for the mostcentral Pb+Pb collisions, the systematic error for p+ p, C +C, Si+Si and non-centralPb+Pb collisions is shown in figure 1.CENTRALITY AND SYSTEM SIZE DEPENDENCEThe centrality dependence of the scaled variance for negatively charged hadrons (h−)at 158A GeV is shown in figure 1. Scaled variance increases with decreasing centralityof a collision. For very peripheral collisions there is a hint that it decreases again, butthe systematic errors are large in this region. Data is compared to predictions of variousstring hadronic models for Pb +Pb collisions; they all predict, in contradiction to data,a flat centrality dependence. Scaled variance behaves similar in p + p, C +C, Si + Siand Pb + Pb collisions if plotted against centrality defined as N pro jP /A, where A is thenumber of nucleons in nuclei. This is a hint that not the size of the collision systemis the correct scaling parameter for the observed increase of ω but the fraction of thecolliding nucleons in the projectile nuclei. In [9] it is suggested that target participantscontribute to particle production in the projectile hemisphere and their fluctuationstherefore result in an increase of multiplicity fluctuations. The predictions of this effectcalled mixing is in approximate agreement with data as shown in figure 1 (right). Inter-particle correlations [10] and percolation models [11] provide alternative explanationsof the results.1 Rapidity is calculated in the center of mass system assuming pion mass.P R OJPN0 50 100 150123mixingreflec tiontrans parency>)/<nVar(nVar(n)/<n>0.511.52 p+pC +CS i+S iP b+P b/AP R OJPN0 0.5 1Var(n)/<n>0.511.52P R OJPN0 50 100 150FIGURE 1. Left: Comparison of scaled variance of h− in Pb + Pb collisions (circles) with stringhadronic models: HIJING [12], UrQMD, HSD [7] and Venus [13]. Middle: Centrality dependence ofscaled variance of h− in p+ p, C +C, Si+Si and Pb+Pb collisions at 158A GeV. Right: Scaled varianceof h− in comparison to model calculations for transparency, mixing and reflection of matter in the earlystage [9]. The outer errors correspond to a sum of statistical and systematical uncertainties.NNs6 8 10 12 14 16 18 20)- (hω0.70.80.911.11.21.31.4data Pb+PbVenus Pb+PbUrQMD Pb+PbNNs6 8 10 12 14 16 18 20)- (hω0.70.80.911.11.21.31.4data Pb+Pbcanonical modelgrand canonical modelFIGURE 2. Energy dependence of multiplicity fluctuations of h− in Pb + Pb collisions in comparisonto string-hadronic models Venus [13] and UrQMD [14] (left) and in comparison to canonical and grand-canonical statistical hadron-resonance gas models [15] (right). Only statistical errors are shown, thesystematical errors (not shown) are σsys ≈+2−5 %.ENERGY DEPENDENCE IN CENTRAL COLLISIONSAt all energies the scaled variance of h− is significantly smaller than one, the value fora corresponding Poisson distribution (see figure 2) [6]. A direct quantitative comparisonof ω at different energies is not possible due to different experimental acceptance. TheUrQMD model is in approximate agreement with data. The Venus model overpredictsω at higher energies. No significant increase of multiplicity fluctuations due to criticalpoint or onset of deconfinement is observed.Predictions of a statistical model [15] for canonical and grand-canonical ensemblesare compared to data. The measured scaled variance is much lower than predicted by thegrand-canonical ensemble. The canonical model predicts ω smaller than 1, in qualitativeagreement with data. Energy momentum conservation and the finite volume of hadronsare expected to cause an additional suppression of fluctuations. Thus the observed smallfluctuations seem to be a non-trivial effect of conservation laws in a relativistic hadrongas [4].REFERENCES1. U. W. Heinz, and M. Jacob (2000), nucl-th/0002042.2. M. Gazdzicki, M. I. Gorenstein, and S. Mrowczynski, Phys. Lett. B585, 115–121 (2004),hep-ph/0304052.3. M. A. Stephanov, K. Rajagopal, and E. V. Shuryak, Phys. Rev. D60, 114028 (1999),hep-ph/9903292.4. V. V. Begun, M. Gazdzicki, M. I. Gorenstein, and O. S. Zozulya, Phys. Rev. C70, 034901 (2004),nucl-th/0404056.5. S. Afanasev, et al., Nucl. Instrum. Meth. A430, 210–244 (1999).6. B. Lungwitz (2006), nucl-ex/0610046.7. V. P. Konchakovski, et al., Phys. Rev. C73, 034902 (2006), nucl-th/0511083.8. C. Alt, et al., Phys. Rev. C70, 064903 (2004), nucl-ex/0406013.9. M. Gazdzicki, and M. Gorenstein, Phys. Lett. B640, 155–161 (2006), hep-ph/0511058.10. M. Rybczynski, and Z. Wlodarczyk, J. Phys. Conf. Ser. 5, 238–245 (2005), nucl-th/0408023.11. L. Cunqueiro, E. G. Ferreiro, F. del Moral, and C. Pajares, Phys. Rev. C72, 024907 (2005),hep-ph/0505197.12. M. Gyulassy, and X.-N. Wang, Comput. Phys. Commun. 83, 307 (1994), nucl-th/9502021.13. K. Werner, Phys. Rept. 232, 87–299 (1993).14. S. A. Bass, et al., Prog. Part. Nucl. Phys. 41, 225–370 (1998), nucl-th/9803035.15. V. V. Begun, M. I. Gorenstein, M. Hauer, V. P. Konchakovski, and O. S. Zozulya, Phys. Rev. C74,044903 (2006), nucl-th/0606036.

Multiplicity Fluctuations in Heavy Ion Collisions at CERN SPS

Crossref

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Multiplicity Fluctuations in Heavy Ion Collisions
at CERN SPS
Benjamin Lungwitz
for the NA49 Collaboration
Fachbereich Physik der Universität, Frankfurt, Germany.
Abstract. The system size and centrality dependence of multiplicity fluctuations in nuclear colli-
sions at 158A GeV as well as the energy dependence for the most centralPb+Pb collisions were
studied by the NA49 experiment at CERN SPS. A strong increaseof fluctuations was observed with
decreasing centrality inC+C, Si+SiandPb+Pbcollisions. The string hadronic models (UrQMD,
Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate a strong mixing
of target and projectile contribution in a broad rapidity range. For the most central collisions at all
SPS energies multiplicity distributions are significantlynarrower than a corresponding Poisson one
both for negatively and positively charged hadrons. The UrQMD model seems to reproduce the
measured values on scaled variance. Statistical model calculations overpredict results when conser-
vation laws are not taken into account.
Keywords: Multiplicity Fluctuations, NA49, CERN SPS
PACS: 24.60.Ky
INTRODUCTION
At high energy densities (≈ 1GeV/ f m3) a phase transition from hadron gas to quark-
gluon-plasma (QGP) is expected to occur. There are indications hat at top SPS energies
quark-gluon-plasma is created at the early stage of heavy ion collisions [1]. Lattice QCD
calculations suggest furthermore the existence of a critical point in the phase diagram
of strongly interacting matter which separates the line of the first order phase transition
from a crossover. Models predict an increase of multiplicity fluctuations near the onset
of deconfinement [2] or the critical point [3]. Statistical model calculations [4] showed a
non-trivial decrease of fluctuations due to conservation laws. These reduced fluctuations
would then serve as a “background” in the search for the critical point.
THE NA49 EXPERIMENT
The NA49 detector is a large acceptance fixed target hadron spectrometer described
in [5]. The centrality of a collision is determined using a downstream Veto calorimeter
which measures the energy in the projectile spectator domain [6]. This allows a deter-
mination of the number of projectile participant nucleonsNpro jP . The number of target
participants is not fixed in the experiment, model calculations show large fluctuations of
their number in peripheral collisions [7].
Analysis Procedure
Since detector effects like track reconstruction efficiency might have a large influence
on multiplicity fluctuations, it is important to select a very clean track sample for the
analysis. Therefore the acceptance for this analysis is limited to a part of the forward
hemisphere, where the NA49 detector has the highest tracking efficiency. This was done
by restricting the analysis to the rapidity interval 1< y(π) < ybeam 1 for 20A to 80A
GeV and 1.08< y(π)< 2.57 for 158A GeV. In addition a cut on transverse momentum
according to [8] was applied, which is dependent both on rapidity and azimuthal angle.
For such tracks the reconstruction efficiency is larger than98%.
The basic measure of multiplicity fluctuations used in this analysis is the scaled
variance:ω = Var(n)<n> , whereVar(n) and< n> are variance and mean of the multiplicity
distribution, respectively. In this paper only the resultsfor negatively charged hadrons
are shown.
The data is corrected for the finite width of centrality bins.Data on centrality and sys-
tem size dependence at 158A GeV is corrected for the resolution of the Veto calorimeter,
for most centralPb+Pb data this correction is small (it would decrease the resultsby
less than 5%) and was neglected.
The total systematic error is estimated to be+2% respectively−5% [6] for the most
centralPb+Pbcollisions, the systematic error forp+ p, C+C, Si+Siand non-central
Pb+Pbcollisions is shown in figure 1.
CENTRALITY AND SYSTEM SIZE DEPENDENCE
The centrality dependence of the scaled variance for negatively charged hadrons (h−)
at 158A GeV is shown in figure 1. Scaled variance increases with decreasing centrality
of a collision. For very peripheral collisions there is a hint that it decreases again, but
the systematic errors are large in this region. Data is compared to predictions of various
string hadronic models forPb+Pb collisions; they all predict, in contradiction to data,
a flat centrality dependence. Scaled variance behaves similar in p+ p, C+C, Si+Si
andPb+Pb collisions if plotted against centrality defined asNpro jP /A, whereA is the
number of nucleons in nuclei. This is a hint that not the size of the collision system
is the correct scaling parameter for the observed increase of ω but the fraction of the
colliding nucleons in the projectile nuclei. In [9] it is suggested that target participants
contribute to particle production in the projectile hemisphere and their fluctuations
therefore result in an increase of multiplicity fluctuations. The predictions of this effect
called mixing is in approximate agreement with data as shownin figure 1 (right). Inter-
particle correlations [10] and percolation models [11] provide alternative explanations
of the results.
1 Rapidity is calculated in the center of mass system assumingpion mass.
P R OJ
PN
0 50 100 150
1
2
3
mixing
reflec tion
trans parency
>
)/
<
n
V
a
r(
n
V
a
r(
n
)/
<
n
>
0.5
1
1.5
2 p+p
C +C
S i+S i
P b+P b
/AP R OJPN
0 0.5 1
V
a
r(
n
)/
<
n
>
0.5
1
1.5
2
P R OJ
PN
0 50 100 150
FIGURE 1. Left: Comparison of scaled variance ofh− in Pb+ Pb collisions (circles) with string
hadronic models: HIJING [12], UrQMD, HSD [7] and Venus [13].Middle: Centrality dependence of
scaled variance ofh− in p+ p, C+C, Si+SiandPb+Pbcollisions at 158A GeV. Right: Scaled variance
of h− in comparison to model calculations for transparency, mixing and reflection of matter in the early
stage [9]. The outer errors correspond to a sum of statistical and systematical uncertainties.
NNs
6 8 10 12 14 16 18 20
)-
(h
ω
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
data Pb+Pb
Venus Pb+Pb
UrQMD Pb+Pb
NNs
6 8 10 12 14 16 18 20
)-
(h
ω
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
data Pb+Pb
canonical model
grand canonical model
FIGURE 2. Energy dependence of multiplicity fluctuations ofh− in Pb+Pbcollisions in comparison
to string-hadronic models Venus [13] and UrQMD [14] (left) and in comparison to canonical and grand-
canonical statistical hadron-resonance gas models [15] (right). Only statistical errors are shown, the
systematical errors (not shown) areσsys≈+2
−5 %.
ENERGY DEPENDENCE IN CENTRAL COLLISIONS
At all energies the scaled variance ofh− is significantly smaller than one, the value for
a corresponding Poisson distribution (see figure 2) [6]. A direct quantitative comparison
of ω at different energies is not possible due to different experim ntal acceptance. The
UrQMD model is in approximate agreement with data. The Venusmodel overpredicts
ω at higher energies. No significant increase of multiplicityfluctuations due to critical
point or onset of deconfinement is observed.
Predictions of a statistical model [15] for canonical and grand-canonical ensembles
are compared to data. The measured scaled variance is much lower than predicted by the
grand-canonical ensemble. The canonical model predictsω smaller than 1, in qualitative
agreement with data. Energy momentum conservation and the finit volume of hadrons
are expected to cause an additional suppression of fluctuations. Thus the observed small
fluctuations seem to be a non-trivial effect of conservationlaws in a relativistic hadron
gas [4].
REFERENCES
1. U. W. Heinz, and M. Jacob (2000),nucl-th/0002042.
2. M. Gazdzicki, M. I. Gorenstein, and S. Mrowczynski,Phys. Lett. B585, 115–121 (2004),
hep-ph/0304052.
3. M. A. Stephanov, K. Rajagopal, and E. V. Shuryak,Phys. Rev.D60, 114028 (1999),
hep-ph/9903292.
4. V. V. Begun, M. Gazdzicki, M. I. Gorenstein, and O. S. Zozulya, Phys. Rev.C70, 034901 (2004),
nucl-th/0404056.
5. S. Afanasev, et al.,Nucl. Instrum. Meth.A430, 210–244 (1999).
6. B. Lungwitz (2006),nucl-ex/0610046.
7. V. P. Konchakovski, et al.,Phys. Rev.C73, 034902 (2006),nucl-th/0511083.
8. C. Alt, et al.,Phys. Rev.C70, 064903 (2004),nucl-ex/0406013.
9. M. Gazdzicki, and M. Gorenstein,Phys. Lett.B640, 155–161 (2006),hep-ph/0511058.
10. M. Rybczynski, and Z. Wlodarczyk,J. Phys. Conf. Ser.5, 238–245 (2005),nucl-th/0408023.
11. L. Cunqueiro, E. G. Ferreiro, F. del Moral, and C. Pajares, Phys. Rev.C72, 024907 (2005),
hep-ph/0505197.
12. M. Gyulassy, and X.-N. Wang,Comput. Phys. Commun.83, 307 (1994),nucl-th/9502021.
13. K. Werner,Phys. Rept.232, 87–299 (1993).
14. S. A. Bass, et al.,Prog. Part. Nucl. Phys.41, 225–370 (1998),nucl-th/9803035.
15. V. V. Begun, M. I. Gorenstein, M. Hauer, V. P. Konchakovski, and O. S. Zozulya,Phys. Rev.C74,
044903 (2006),nucl-th/0606036.


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Multiplicity Fluctuations in Heavy Ion Collisions at CERN SPS

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