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Recent results from NA35
~-~ EI~SEVIER
NUCLEAR PHYSICSA Nuclear PhysicsA590 (1995) 197c-214c
Recent Results from NA35 Marek Ga~dzicki for the NA35 Collaboration: T. Alber 9, H. Appelsh£user s, J. B~hler e, J. Bartke4, H. Bialkowska12, M.A. Bloomer3, R. Bock5, W.J. Braithwaite 1°, D. Brinkmanne, R. Brockmanns, P. Bun~i65, P. Chan 1°, J.G. Cramer 1°, P.B. Cramer l°, I. Deradog, V. Eckardt 9, J. Eschkes, C. Favuzzi2, D. Ferenc 13, B. Fleischmanns, P. Fokas, M. Fuchss, M. Ga~dzicki s, E. Gtadysz4, J. G/inthers , J.W. Harris 3, M. Hoffmann7, P. Jacobs 3, S. Kabanas, K. Kadija 9'13, J. Kosiec1' , M. Kowaiski 4, A. Kfihmichels, J.Y. Lees, A. Ljubi~i6 jr. 13, S. Margetiss, J.T. Mitchell3, R. Morse 3, E. Nappi 2, G. Odyniec3, G. pai613 A.D. Panagiotou 1, A. Petridis 1, A. Piper s, F. Posa2, A.M. Poskanzer3, F. Pfihlhofers, W. Rauch 9, R. Renfordts, W. Retyk It, D. RShrich e, G. Rolande, H. Rothard 8, K. R.unger, A. Sandovals, N. Schmitz 9, E. Schmoetten r, R. Sendelbachs, P. Seyboth 9, J. Seyerlein9, E. Skrzypczak 11, P. Spinelli 2, P. Stefasiski 4, R. Stocks, H. StrSbeles, T.A. Trainor 1°, G. Vasileiadis 1, M. Vassiliou l, D. Vranic 13, S. Wenigs, B. Wosiek4'9, X. Zhu m ~Department of Physics, University of Athens, Athens, Greece, 2Dipartimento di Fisica, Universit5. di Bari and INFN Bari, Bari, Italy, 3Lawrence Berkeley Laboratory, Berkeley, CA, USA, 4Institute of Nuclear Physics, Cracow, Poland, SGesellschaft ffir Schwerionenforschung (GSI), Darmstadt, Germany, SFachbereich Physik der Universit£t, Frankfurt, Germany, rFakult/it f/ir Physik der Universit~t, Freiburg, Germany, SFachbereich Physik der Universit/i.t, Marburg, Germany, 9Max-Planck-Institut ffir Physik, Mfinchen, Germany, mUniversity of Washington, Seattle, USA, ~Institute for Experimental Physics, University of Warsaw, Warsaw, Poland, 12Institute for Nuclear Studies, Warsaw, Poland, 13Rudjer Bo§kovi6 Institute, Zagreb, Croatia. Recent results on hadron production in central nucleus-nucleus collisions obtained by the experiment NA35 at the CERN SPS are reviewed. The first preliminary results on central Pb+Pb collisions obtained by the NA49 experiment, the successor of NA35, are also discussed. Their impact on our understanding of the properties of strongly interacting matter at different stages of the collision is underlined. 1. I N T R O D U C T I O N The aim of the heavy ion program carried out at the CERN SPS is to study the properties of strongly interacting matter. We hope that for sufficiently large interaction energies and for sufficiently large colliding nuclei partonic degrees of freedom play a dominant role in the interaction dynamics and a new state of matter - the Quark Gluon Plasma - is created [1]. In this presentation we review recent results of the NA35 experiment on hadron production in central sulphur-nucleus (S+AT) collisions at 200 GeV/c per nucleon. We show also some preliminary results on central Pb+Pb collision at 158 GeV/c 0375-9474/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved. SSD! 0375-9474(95)00236-7
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per nucleon obtained by the NA49 experiment [2], the successor of NA35 in the lead era at the CERN SPS. The data presentation is organized in a way which allows us to deduce properties of strongly interacting matter at subsequent stages of the evolution of central nucleus-nucleus collision: - the early stage of the collision, - the expansion stage and - the freeze--out stage. 1.1. T h e N A 3 5 E x p e r i m e n t The experiment NA35 studied collisions of p, 180 and 32S projectiles of 200 GeV/c per nucleon incident momentum with various nuclear targets (C-Pb). The schematic set-up of the experiment is shown in Fig. 1. The charged particles are detected in two large
~
CAMERAS
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!jJJ...
_
tZ × '. 19 1
~
g
TARGETS STREAMER CHAMBER
TIME PROJECTION CHAMBER
RING VETO CALORIMETER CALORIMETER
Figure 1. Schematic side view of the NA35 experiment. The magnetic field in the sweeper magnet (SM) and in the VTM magnet is parallel to the z axis.
volume tracking devices: the NA35 Streamer Chamber placed inside a 1.5 Tesla dipole magnet (VTM) and the NA35 Time Projection Chamber positioned downstream of the magnet. In order to cover almost the full rapidity-transverse momentum (Y-PT) range the data were collected in two different target-magnetic field configurations (see Fig. 1). Central S+AT collisions were selected by the requirement that the energy in the projectile fragmentation region (OLAB < 0.3 °) measured by the Veto Calorimeter was lower than a given threshold energy. The central collision fractions of the inelastic cross sections are about 3% for the S+S, S+Cu and S+Ag systems and about 6% for the S+Au system. The Ring Calorimeter was used to measure the transverse energy distributions.
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The NA35 collaboration has previously published results on single particle spectra and multiplicities of negatively and positively charged hadrons [3-8], strange particles [9-12], particle correlations [13-19] and forward and transverse energy spectra [20,21]. Based on the experience of the NA35 experiment in studying high multiplicity sulphurnucleus collisions the experiment NA49 was designed and built. The aim of NA49 is to investigate central Pb+Pb collisions at CERN SPS energies in an inclusive as well as an event-by-event way [22]. The experimental set-up and preliminary results from the first lead run at the CERN SPS (November/December 1994) are presented in detail elsewhere [2]. Some preliminary results on central Pb+Pb collisions (about 5% of total inelastic cross section) are however also discussed in this presentation.
80
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.
1
2
.
.
.
,
3
,
,
,
'
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5
,
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Y Figure 2. The rapidity distributions (normalized to the mean multiplicities) for negatively charged hadrons treated as n--mesons produced in central S+S, S+Ag and S+Au collisions at 200 GeV/c per nucleon. The solid line indicates the shape of the rapidity distribution (arbitrary normalization) of relativistic pions emitted isotropically in the N+N c.m. system.
2. M A T T E R AT F R E E Z E - O U T We define the freeze-out stage to be the last stage in the history of strongly interacting matter produced in high energy nuclear collisions. At this stage all strong interactions have stopped and the final number of particles and their momenta, observed later in the experiment, are defined. The prevailing scenario of the evolution of strongly interacting
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M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
300 C
200
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0
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Figure 3. The preliminary rapidity distribution (normalized to the mean multiplicity) obtained by NA49 for negatively charged hadrons produced in central Pb+Pb collisions at 158 GeV/c per nucleon (circles: the NA49 Vertex TPC data, squares: the NA49 Main TPC data). The measured distribution (full symbols) is reflected (open symbols) with respect to the c.m. rapidity. The solid line indicates the shape of the rapidity distribution (arbitrary normalization) of relativistic pions emitted isotropically in the N+N c.m. system. The dashed line indicates the gaussian fit to the data. The distribution for central S+S collisions at 200 GeV/nucleon is indicated by stars.
matter requires first chemical freeze--out, from which point onwards particle abundances are preserved, followed by the kinetic freeze--out after which the final momenta of particles are established. 2.1. K i n e t i c F r e e z e - o u t
The rapidity distributions of negatively charged hadrons (more than 90% of them are r--mesons) calculated assuming the pion mass are shown in Fig. 2 for central S+S, S+Ag and S+Au collisions. For relativistic particles (the rest mass can be neglected) the shape of the rapidity distribution is determined by the angular distribution. It depends only very weakly on the shape of the momentum distribution. The solid line in Fig. 2 indicates the rapidity distribution of relativistic pions emitted isotropically in the nucleonnucleon (N+N) c.m. system. This distribution is significantly narrower than the measured distributions of negatively charged hadrons. This indicates a strong angular anisotropy of the hadron distribution at the kinetic freeze-out which is caused (as will be shown below) by a strong longitudinal flow of the freezing-out matter. A similar conclusion is
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
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obtained from the analysis of preliminary data on central P b + P b collisions at 158 GeV/c per nucleon. The rapidity distribution of negatively charged hadrons produced in these collisions is shown in Fig. 3. It is, as in the case of S+AT collisions, about two times broader than the distribution for isotropic emission. Shapes of distributions for minimum bias N + N interactions, central S+S collisions [6] and central P b + P b collisions are similar. The transverse momentum or transverse mass (ra T = ~ o + P~', where ra0 is the rest mass of the particle) spectra of hadrons are sensitive to the transverse flow of the freezingout matter [23-25]. In Fig. 4 the transverse mass distribution of negatively charged hadrons (the pion mass is assumed) produced in central S + A g collisions is presented as an example. The distribution shows the well established concave shape which deviates from the shape expected for a purely thermal distribution of pions (approximately a straight line in Fig. 4). A careful analysis of the transverse momentum spectra [24,25] leads to the conclusion I that they can be explained taking into account the influence of resonance decays and assuming weak (~f ~ 0.3) transverse flow of the matter freezing-out at a temperature 7"/,~ 150 MeV.
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I
I
1 1.5 2 m-r-m0 [ G e V / c ]
Figure 4. The transverse mass distribution for negatively charged hadrons treated as 7r--mesons produced in central S + A g collisions at 200 GeV/c per nucleon.
IThe requirement of the consistency of the freezing-out system with the freeze-out conditions given by hadronic cross-sections and its size is needed for unique extraction of transverse flow and freeze-out temperature.
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8 '~
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6 5 4
4
5
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2
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Y Figure 5. The dependence of the longitudinal parameter of the two-pion correlation function on the pion rapidity for central S+Ag collisions at 200 GeV/c per nucleon. The solid line shows the fit result obtained using Eq. 2.
0
0.4
0.2
O.6
kr [GeV/c] Figure 6. The dependence of the longitudinal parameter of the two-pion correlation function on the average transverse momentum of the pion pair for central S+Ag collisions at 200 GeV/c per nucleon. The solid line shows the fit result obtained using Eq. 2.
The fact that the shape of the transverse momentum spectrum only weakly depends on rapidity and the extracted transverse flow is weak suggests that the expansion of matter at the kinetic freeze-out can be approximated by a boost invariant longitudinal expansion 2 (see discussion below). The properties of matter at the kinetic freeze-out can be further studied by the analysis of the two-pion correlations as a function of rapidity and transverse momentum [13,16, 18,19,26]. In order to perform this analysis the two pion correlation function expressed as a function of three components of the momentum difference (QL, Qs, Qo - for definition see [18]) was parametrized as:
C(QL, Qs, Qo) = 1 + ~. exp[-0.5(Q~ • ~ + Q~. R~ + Q~. R~)],
(1)
where the parameters RL, Rs, Ro, and A were fitted to the experimental data in Y-PT intervals. 2Note that the system as a whole is not boost invariant, e.g. pion and baryon rapidity density changes significantly with y, but its local velocityspectrum (the expansion of the system) at freeze-out is approximately boost invariant.
M. Ga2dzickiet al. / Nuclear PhysicsA590 (1995) 197c-214c
S+Cu
S+Ag
203c
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0
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I,,
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kr [GeV/c] Figure 7. The dependence of the side parameter of the two-pion correlation function on the average transverse momentum of the pion pair for central S+Cu, S-FAg and S+Au collisions at 200 GeV/c per nucleon. The dashed lines show the approximate transverse size of the sulphur nucleus.
The longitudinal parameter of the correlation function, RL, is plotted against the rapidity and the average transverse momentum of the pion pair (fcT = 0.5. (p-4~)+ p--(~))) for central S+AT collisions in Figs. 5 and 6, respectively. On the basis of a boost invariant longitudinal expansion Makhlin and Sinyukov [27] derived the dependence of R5 on y and kT: RL(y, kr)
2. T:
T: + k S cosh( - y0)'
(2)
where rn, is the pion mass, yo is the rapidity of the observer (y0 = 3 in the NA35 analysis) and r! is the proper expansion time of the system. The solid lines in Figs. 5 and 6 show the result of the fit of r/in Eq. 2 to the data assuming 7"/= 150 MeV. The obtained proper expansion time of the system is about 5 fm/c. The fact that the data are well described by the Makhlin-Sinyukov approach supports the assumption of a boost invariant expansion and indicates that the emission anisotropy observed in the analysis of the rapidity spectra can be treated as a consequence of longitudinal flow of freezing-out matter. A transverse expansion is expected to cause a decrease of the side (Rs) parameter of the correlation function with increasing transverse momentum [28]. The dependence of Rs on kT is shown in Fig. 7 for central S+Cu, S+Ag and S+Au collisions. In fact a slight decrease of Rs is observed in the experimental data. A unique interpretation of this decrease as due to transverse flow is, however, difficult since an effect of a similar magnitude can be caused by the resonance decays [18]. In the early fifties Pomeranchuk suggested [29] that pions produced in high energy collisions should decouple at approximately constant density 3. In order to test experimentally 3The freeze-out pion density was estimated by Pomeranchuk to be of the order of the cube of the strong
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I 50
O0
50
°6
20
40
60
80
dn /dy Figure 8. The dependence of the R~ • R~, on the rapidity density of negatively charged hadrons for central nucleus-nucleus collisions. The results for 4°Ar+AT collisions at LBL energies are indicated by triangles, the results for 2sSi+AT collisions at AGS are indicated by squares and the results for 32S+AT collisions at CERN SPS (the NA35 data) are indicated by circles. The lines are drawn to guide the eye.
this hypothesis the dependence of R~ • R~ (the R~, is the longitudinal parameter of the correlation function fitted in the local frame) on the negative charged hadron rapidity density is plotted in Fig. 8 for central nucleus-nucleus collisions at 1.9 - 200 GeV/c per nucleon. Only data on collisions of 4°Ar, 2ssi and 32S projectiles were compiled [30]. In addition the R~ • R~, values for 4°Ar collisions were scaled by a factor 3 Thus the data presented in Fig. 8 should be treated as obtained for collisions with approximately fixed initial geometry. The results from AGS and CERN (the NA35 experiment) concern the central rapidity region, whereas the results from LBL are for 47r and forward rapidity measurements. At high energies for which the boost invariant longitudinal expansion may be used as an approximation and the baryon density can be neglected in comparison to the pion density, the pion rapidity density is expected to be proportional to the number of particles freezingout from a volume which is proportional to R~. R~. 4 Therefore in this case R~- R~ should be proportional to dn/dy providing that the freeze-out occurs at a constant density. This interaction length scale, (m./h) a .~ 0.3 fm-a. 4Note that in this case R~. is proportional to the longitudinal homogenity scale of the system and it cannot be treated as a measure of the total longitudinal dimension of the freeze-out volume. Thus the R~ • R~ is proportional to the homogenity volume.
M. Ga~dzicki et ai. / Nuclear Physics A590 (1995) 197c-214c
E
300
I
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n~
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200
100
0
50 l
100 I
150 I dn
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/dy
Figure 9. Same as in Fig. 8 but with the preliminary result for central P b + P b collisions at 158 GeV/c per nucleon (star, the NA49 data) and non-midrapidity results for central 32S+AT collisions (the NA35 data).
behaviour is observed for the NA35 data at 200 GeV/c per nucleon. In the case of low energy collisions the particle density is dominated by the density of baryons and thus the freeze--out volume should be proportional to the initial volume of the colliding system and independent of the multiplicity or dn/dy of pions. The approximate independence of the R~ • R~. of dn/dy is in fact observed for the low energy data (LBL, AGS). Due to the fact that for nucleus-nucleus collisionsat C E R N SPS energies the volume of the freezing-out system is determined by the pion multiplicity,the resultsfor collisions with different initialgeometries can be compared in the same plot. In Fig. 9, together with the data presented in Fig. 8, we plot the preliminary result obtained by the NA49 experiment for central P b + P b collisionsin the rapidity interval 4-5 [18]. The results for non-midrapidity intervals obtained by the NA35 experiment for central S + A T collisions are also plotted in Fig. 9. 2.2. C h e m i c a l F r e e z e - o u t The NA35 experiment measured spectra and multiplicities of rr-, p, ~, K +, K - , A, A and /(~ for central sulphur-nucleus collisions. If the evolution of the strongly interacting matter is slow enough one can expect that the system freezes out as an equilibrated hadronic gas. In this case all possible particle ratios should be described by two thermody-
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S+Ag
> A+ X
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5
Figure 10. The rapidity distribution of A hyperons produced in central S+S collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of A hyperons produced in N+N interactions at 200 GeV/c (squares). The open circles indicate the data reflected with respect to y~m.
6
0
i
0
r
7[~r -'0-
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Figure 11. The rapidity distribution of A hyperons produced in central S+Ag collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of A hyperons produced in p+S interactions at 200 GeV/c (squares). The open circles indicate the data obtained by an interpolation procedure.
namical parametersS: chemical freeze--out temperature and baryonic chemical potential. The NA35 multiplicity data allow a test of the ha&on gas hypothesis. It was found [31] that the freezing-out system is out of chemical equilibrium. The measured hadron ratios can be described in a hadron gas approach only if pions are excluded from the anMysis. In this case the chemical freeze--out temperature is about 200 MeV and the baryonic chemical potential is about 300 MeV. The number of pions in the hadron gas with the above parameters is underestimated by a factor of about 1.6. 3. H O T E X P A N D I N G M A T T E R The properties of the hot expanding matter can be studied by the analysis of strange particle production. The number of strange quark-antiquark pairs in elementary high energy processes (e.g. p+p, e++e -) is strongly suppressed (relative to the number of nonstrange quark-antiquark pairs) [32] and significantly below the equilibrium level [31,33]. As the corresponding abundance may be treated as an estimation of strangeness population in the early stage of a nucleus-nucleus collision [33] one expects that any secondary SThis is for the case of strangeness neutral and charge symmetric systems.
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Figure 12. The rapidity distribution of h'~ mesons produced in central S+S collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of K~ mesons produced in N+N interactions at 200 GeV/c (squares). The open circles indicate the data reflected with respect to ycm.
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Figure 13. The rapidity distribution of I(~ mesons produced in central S+Ag collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of I(~ mesons produced in p+S interactions at 200 GeV/c (squares). The open circles indicate the data obtained by an interpolation procedure.
processes should increase the relative strangeness yield, s Furthermore it was estimated that the strangeness equilibration time in deconfined matter is similar to the measured time of the system expansion (5-10 fm/c), whereas the strangeness equilibration time in hadronic matter is several times longer [34]. Therefore a substantial increase in the relative strangeness yield can be treated as an indication that the expanding matter was in the deconfined phase. The experiment NA35 observes significantly enhanced production of strange particles. In order to illustrate this effect we show in Figs. 10 and 11 the rapidity distribution of A hyperons produced in central S+S and S+Ag collisions, respectively. The rapidity distributions of I(~ mesons are shown in Figs. 12 and 13. These distributions are compared with the corresponding distributions obtained for N+N interactions (reference data for S+S collisions) [35] and p+S interactions (reference data for S+Ag collisions) [12] at 200 GeV/c. The spectra for N+N interactions are multiplied by the ratio (~r-)ss/(Tr-)N g and the spectra for p+S interactions are multiplied by the ratio (lr-)SAg/(Tr-)ps, where (~r-) is the mean multiplicity of 7r--mesons produced in the collisions indicated by the index. The observed large difference (factor 2-3) between the strange particle yields in central S+AT collisions and the scaled yields in the reference 6In nuclear collisions the strangeness production is studied relative to the production of non-strange particles in order to remove a trivial dependence of the number of produced sg-pairs on the volume of the colliding systems. The relative strangeness production reflects therefore the space density of sg-pairs.
M. Ga3zicki et al. /Nuclear Physics A590 (1995) 197c-214~
208~
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$
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t LUCIAE ____MCSg; __________-.-----
0
_______________________.______~ I _________________________-_---FRITIOF
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I
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I
3
4
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6 l/3
AT
Figure 14. The difference between Es values (see Eq. 3) determined for central S+S and S+Ag collisions and the Es value for N+N interactions is plotted (circles) as a function of the target nucleus mass number. The corresponding differences for PSAT interactions are indicated by squares. The solid line shows the level for N+N interactions, by definition equal to zero. The dashed lines show the results of calculations performed within stringhadronic models: FRITIOF, QGSM, MCSFM and LUCIAE.
interactions (N+N and p+S) illustrates the enhanced production of strangeness to 7r--mesons) in central sulphur-nucleus collisions. It amounts to the absolute about 20 ST-pairs in central sulphur-nucleus collisions. In order to quantify the total enhancement of strangeness production in central nucleus collisions we define the ratio [12]:
(relative excess of nucleus-
(3) where (A) and (li!j!) are the mean multiplicities of A and Ki charge symmetric collisions this ratio is equal to the ratio:
particles,
respectively.
(A) + (K + R)
For
(4)
(r) The Es ratio is approximately [12] usually used in elementary
two times smaller particle physics.
than
the strangeness
suppression
factor
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
209c
2 A ,,,t
Z
V A V m
m-{[.... }+ ..................................... -1
,
0
I
1
,
I
2
,
I
3
,
I
4
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,
5
6
F,,N [GeV '/=] Figure 15. The difference of the mean multiplicity of pions per participant nucleon for central collisions of identical nuclei and for N+N interactions at the same collision energy per nucleon as a function of the modified Fermi-Landau variable (see Eq. 6). The low energy data (LBL, Dubna, AGS) are indicated by squares, the result for central S+S collisions at 200 GeV/c per nucleon (NA35)isindicated by the circle and the preliminary result for central Pb+Pb collisions at 158 GeV/c is indicated by the star (NA49).
The difference between Es for central S+AT collisions (and p+AT interactions [36]) and Es for N+N interactions [33] is plotted in Fig. 14. No strangeness enhancement is observed for p+AT interactions, all corresponding Es differences are close to zero. The previously discussed strong strangeness enhancement is observed for central S+AT collisions. We conclude that a fast strangeness production, characteristic for the expansion of hot deconfined matter, is observed in the experiment. The strangeness enhancement effect for CERN SPS energies was for the first time reported in 1988 [37]. Since that time there were numerous attempts to modify stringhadronic approaches in order to reproduce the data. The published model results for central S+Ar collisions (FRITIOF [38], LUCIAE [38], QGSM [39] and MCSFM [40]) are indicated in Fig. 14 by dashed lines. In addition to the standard string approach [41] the models include secondary hadronic interactions fusion of strings [40] and collective gluon emission [38]. They approximately reproduce the absolute strangeness yield in central nucleus-nucleus collisions, however they fail to reproduce the strangeness enhancement effect as quantified in Fig. 14 because they cannot simultaneously describe
[38,39],
M. Ga2dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
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strange and non-strange particle production in N+N interactions and central nucleusnucleus collisions. 4. M A T T E R AT T H E E A R L Y S T A G E It is argued [42] that the dominant fraction of the entropy is produced in the early stage of the collision - the stage during which all incoming matter is excited. Therefore the amount of entropy created is dependent on the effective number of degrees of freedom of matter at this stage [43]. The creation of deconfined matter, the matter with high number of degrees of freedom, should cause a high entropy production in comparison to the entropy production in the hadronic matter. For high energy collisions the entropy of the system is approximately proportional to the final state pion multiplicity [44]. Therefore a systematic study of pion multiplicity in nuclear collisions was undertaken [45]. The difference between the mean pion multiplicity per participant nucleon for central collisions of identical nuclei and N+N interactions at the same collision energy per nucleon:
A ( ~ / _ (~/AAAa is plotted in
Fig.
(~)NN (Np)NIV
(5)
15 as a function of the modified Fermi-Landau variable [43,45]:
FNN = ( V q N N -- 2 - r a n -- m . ) s / ' v~l/4
(6)
NN
where V~NN is the c.m. energy per N+N pair and mN is the nucleon mass. In addition to the data presented in [43,45] a preliminary result for central P b + P b collisions at 158 GeV/c per nucleon obtained by the NA49 experiment is shown. The results for central S+S collisions (the NA35 experiment) and central P b + P b collisions (the NA49 experiment) suggest an increase of the pion production at CERN SPS energies in comparison to lower energy collisions r (LBL, JINR, AGS). The effect of the pion (entropy) enhancement could be interpreted, in the generalized Landau model [43], as a manifestation of the increase (by a factor of about 3) of the effective number of degrees of freedom in the early stage of the collision. A change in the collision dynamics seems to occur between AGS and CERN SPS energies. 5. S U M M A R Y The NA35 experiment at the CERN SPS studied hadron (n-, p, ~, K+, K - , A, and I(~) production in central sulphur-nucleus collisions at 200 GeV/c per nucleon. The results obtained by the experiment concern particle multiplicities, spectra, correlations and transverse and forward energy flow. The first preliminary results on ha&on production in central P b + P b collisions at 158 GeV/c per nucleon obtained by the NA49 experiment [2], the successor of the NA35 experiment for the lead age at the CERN SPS, are now available. rThe suppression of the pion production for low energy nucleus-nucleus collisions in comparison to N+N interactions may be interpreted as due to entropy transfer (e.g. by pion absorption) to the baryoaic sector [43].
M. Ga2dzicki et al. /Nuclear Physics A590 (1995) 197c-214c
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The analysis of the results of NA35 and the preliminary results of NA49 allows one to establish properties of strongly interacting matter created in central nucleus-nucleus collisions in subsequent stages of the evolution: the early stage of the collisions, the expansion stage and the freeze--out stage. The analysis of the properties of matter at freeze-out indicates: a strong longitudinal flow, a possible weak transverse flow, - a kinetic freeze-out temperature of about 150 MeV, a proper expansion time of about 5 fro/c, - a constant freeze-out particle density, a hadronic system out of chemical equilibrium (an 'excess' of pions is observed). The analysis of the properties of hot expanding matter indicates: - a fast production of strangeness as expected for deconfined matter. The analysis of the properties of matter at the early stage of the collision indicates: - an enhanced entropy content as expected for deconfined matter, - a possible change of the collision dynamics between AGS and CERN SPS energies. -
-
-
-
Acknowledgements This work was supported by the Bundesministerium f/ir Forschung und Technologie and by the Leibniz Grant of Deutsche Forschungsgemeinschaft, Germany, by the US Department of Energy (DE-AC03-76SF00098), by the Polish State Committee for Scientific Research (204369101) and by the Commission of European Communities (CI-0250YU(A)).
REFERENCES 1. E. V. Shuryak, Phys. Rep. 61 (1980) 71 and 115 (1984) 151. 2. S. Margetis for the NA49 Collaboration, in these proceedings. 3. A. Bamberger et al., Phys. Lett. B184 (1987) 271. 4. A. Bamberger et al., Phys. Lett. B205 (1988) 583. 5. J. B£chler et al., Z. Phys. C51 (1991) 157. 6. J. B£chler et al., Phys. Rev. Lett. 72 (1994) 1419. 7. D. RShrich et al., Nucl. Phys. A566 (1994) 35c. 8. J. G/inther et al., in these proceedings. 9. A. Bamberger et al., Z. Phys. C43 (1989) 25. 10. J. Bartke et al., Z. Phys. C48 (1990) 191. l l . J . B£chler et al., Z. Phys. C58 (1993) 367. 12. T. Alber et al., Z. Phys. C64 (1994) 195. 13. D. Ferenc et al., Nucl. Phys. A544 (1992) 293c. 14. J. B£chler et al., Z. Phys. C56 (1992) 347. 15. J. B£chler et al., Z. Phys. C57 (1993) 541. 16. G. Roland et al., Nucl. Phys. A566 (1994) 527c. 17. J. B£chler et al., Z. Phys. C61 (1994) 551. 18. T. Alber et al., to be published in Z. Phys. C. 19. T. Alber et al., Phys. Rev. Lett. 74 (1995) 1303.
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20. W. Heck et al., 2. Phys. C38 (1988) 19. 21. J. Bachler et al., 2. Phys. C52 (1991) 239. 22. R. Stock, Event by Event Analysis of Ultrarelativistic Nuclear Collisions: a new Method to Search /OF Critical Fluctuations, Proceeding of the NATO Advanced Re search Workshop, Hot Hadronic Matter: Theory and Experiment, Divonne-les-Bains, June 1994. 23. P. J. Siemens and J. 0. Rasmussen, Phys. Rev. Lett. 42 (1979) 880. 24. E. Schnedermann, J. Sollfrank and U. Heinz, Phys. Rev. C48 (1993) 2462. 25. U. Heinz, Nucl. Phys. A566 (1994) 205~. 26. Th. Alber for the NA35 and NA49 Collaborations, in these proceedings. 27. 28. 29. 30.
31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
42.
43. 44. 45.
A. N. Makhlin and Yu. M. Sinyukov, Z. Phys. C39 (1988) 69. S. Pratt, Phys. Rev. D33 (1986) 1314. I. Ya. Pomeranchuk, Dokl. Akad. Nauk SSSR 78 (1951) 884. D. Brinkmann, Ph. D. Thesis, Frankfurt University (1995), W. B. Christie Jr., Ph. D. Thesis, University of California (1990), G. S. F. Stephans et al., Nucl. Phys. A566 (1994) 269c, T. Abbott et al., Phys. Rev. Lett. 69 (1992) 1030, 0. E. Vossnack et al., Nucl. Phys. A566 (1994) 535~. J. Sollfrank, M. Gaidzicki, U. Heinz and J. Rafelski, Z. Phys. C61 (1994) 659. A. K. Wroblewski, Acta Phys. Pol. B16 (1985) 379. M. Gajdzicki, St. Mrowczynski, Z. Phys. C49 (1991) 569. P. Koch, B. Miiller and J. Rafelski, Phys. Rep. 142 (1986) 167. M. Gaidzicki and 0. Hansen, Nucl. Phys. A528 (1991) 754. H. BiaIkowska, M. Gaidzicki, W. Retyk and E. Skrzypczak, Z. Phys. C55 (1992) 491. M. Gaidzicki et al., Nucl. Phys. A498 (1989) 375~. A. Tai, B. Andersson and Ben-Hao Sa, Proceedings of International Conference Strangeness’95, January 4-7, 1995, Tucson, Arizona. N. S. Amelin et al., Phys. Rev. C47 (1993) 2299. N. Armesto, M. A. Braun, E. G. Ferreiro and C. Pajares, Report of Universidade de Santiago de Compostela, US-FT/16-94 (1994). B. Andersson, G. Gustafson and B. Nilsson-Almqvist, Lund Report LU-TP-87-6 (1987), B. Andersson, G. Gustafson and H. Pi, Z. Phys. C57 (1993) 485. B. Miiller, Proceeding of the NATO Advanced Research Workshop, Hot Hadronic Matter: Theory and Experiment, Divonne-les-Bains, June 1994, Hot Hadronic MatTh. Elze, Proceeding of the NATO Ad vanced Research Workshop, ter: Theory and Experiment, Divonne-les-Bains, June 1994, J. Letessier, J. Rafelski and A. Tounsi, Phys. Rev. C50 (1994) 406. M. Gaidzicki, Frankfurt University Report, IKF-HENPG/S-94, to be published in Z. Phys. C. L. D. Landau, Izv. Akad. Nauk SSSR 78 (1951) 884. M. Gaidzicki, D. Riihrich, Z. Phys. C65 (1995) 215.
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
Marek Gazdzicki (second from right) T.D. Lee and Nick Samios Tom Ludlam
213c
Bill Willis Nick Samios, Satoshi Ozaki and Peter Bond Felix Obenshain and Frank Plasil
NUCLEAR PHYSICSA Nuclear PhysicsA590 (1995) 197c-214c
Recent Results from NA35 Marek Ga~dzicki for the NA35 Collaboration: T. Alber 9, H. Appelsh£user s, J. B~hler e, J. Bartke4, H. Bialkowska12, M.A. Bloomer3, R. Bock5, W.J. Braithwaite 1°, D. Brinkmanne, R. Brockmanns, P. Bun~i65, P. Chan 1°, J.G. Cramer 1°, P.B. Cramer l°, I. Deradog, V. Eckardt 9, J. Eschkes, C. Favuzzi2, D. Ferenc 13, B. Fleischmanns, P. Fokas, M. Fuchss, M. Ga~dzicki s, E. Gtadysz4, J. G/inthers , J.W. Harris 3, M. Hoffmann7, P. Jacobs 3, S. Kabanas, K. Kadija 9'13, J. Kosiec1' , M. Kowaiski 4, A. Kfihmichels, J.Y. Lees, A. Ljubi~i6 jr. 13, S. Margetiss, J.T. Mitchell3, R. Morse 3, E. Nappi 2, G. Odyniec3, G. pai613 A.D. Panagiotou 1, A. Petridis 1, A. Piper s, F. Posa2, A.M. Poskanzer3, F. Pfihlhofers, W. Rauch 9, R. Renfordts, W. Retyk It, D. RShrich e, G. Rolande, H. Rothard 8, K. R.unger, A. Sandovals, N. Schmitz 9, E. Schmoetten r, R. Sendelbachs, P. Seyboth 9, J. Seyerlein9, E. Skrzypczak 11, P. Spinelli 2, P. Stefasiski 4, R. Stocks, H. StrSbeles, T.A. Trainor 1°, G. Vasileiadis 1, M. Vassiliou l, D. Vranic 13, S. Wenigs, B. Wosiek4'9, X. Zhu m ~Department of Physics, University of Athens, Athens, Greece, 2Dipartimento di Fisica, Universit5. di Bari and INFN Bari, Bari, Italy, 3Lawrence Berkeley Laboratory, Berkeley, CA, USA, 4Institute of Nuclear Physics, Cracow, Poland, SGesellschaft ffir Schwerionenforschung (GSI), Darmstadt, Germany, SFachbereich Physik der Universit£t, Frankfurt, Germany, rFakult/it f/ir Physik der Universit~t, Freiburg, Germany, SFachbereich Physik der Universit/i.t, Marburg, Germany, 9Max-Planck-Institut ffir Physik, Mfinchen, Germany, mUniversity of Washington, Seattle, USA, ~Institute for Experimental Physics, University of Warsaw, Warsaw, Poland, 12Institute for Nuclear Studies, Warsaw, Poland, 13Rudjer Bo§kovi6 Institute, Zagreb, Croatia. Recent results on hadron production in central nucleus-nucleus collisions obtained by the experiment NA35 at the CERN SPS are reviewed. The first preliminary results on central Pb+Pb collisions obtained by the NA49 experiment, the successor of NA35, are also discussed. Their impact on our understanding of the properties of strongly interacting matter at different stages of the collision is underlined. 1. I N T R O D U C T I O N The aim of the heavy ion program carried out at the CERN SPS is to study the properties of strongly interacting matter. We hope that for sufficiently large interaction energies and for sufficiently large colliding nuclei partonic degrees of freedom play a dominant role in the interaction dynamics and a new state of matter - the Quark Gluon Plasma - is created [1]. In this presentation we review recent results of the NA35 experiment on hadron production in central sulphur-nucleus (S+AT) collisions at 200 GeV/c per nucleon. We show also some preliminary results on central Pb+Pb collision at 158 GeV/c 0375-9474/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved. SSD! 0375-9474(95)00236-7
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per nucleon obtained by the NA49 experiment [2], the successor of NA35 in the lead era at the CERN SPS. The data presentation is organized in a way which allows us to deduce properties of strongly interacting matter at subsequent stages of the evolution of central nucleus-nucleus collision: - the early stage of the collision, - the expansion stage and - the freeze--out stage. 1.1. T h e N A 3 5 E x p e r i m e n t The experiment NA35 studied collisions of p, 180 and 32S projectiles of 200 GeV/c per nucleon incident momentum with various nuclear targets (C-Pb). The schematic set-up of the experiment is shown in Fig. 1. The charged particles are detected in two large
~
CAMERAS
lrn
VI'M SM
!jJJ...
_
tZ × '. 19 1
~
g
TARGETS STREAMER CHAMBER
TIME PROJECTION CHAMBER
RING VETO CALORIMETER CALORIMETER
Figure 1. Schematic side view of the NA35 experiment. The magnetic field in the sweeper magnet (SM) and in the VTM magnet is parallel to the z axis.
volume tracking devices: the NA35 Streamer Chamber placed inside a 1.5 Tesla dipole magnet (VTM) and the NA35 Time Projection Chamber positioned downstream of the magnet. In order to cover almost the full rapidity-transverse momentum (Y-PT) range the data were collected in two different target-magnetic field configurations (see Fig. 1). Central S+AT collisions were selected by the requirement that the energy in the projectile fragmentation region (OLAB < 0.3 °) measured by the Veto Calorimeter was lower than a given threshold energy. The central collision fractions of the inelastic cross sections are about 3% for the S+S, S+Cu and S+Ag systems and about 6% for the S+Au system. The Ring Calorimeter was used to measure the transverse energy distributions.
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
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The NA35 collaboration has previously published results on single particle spectra and multiplicities of negatively and positively charged hadrons [3-8], strange particles [9-12], particle correlations [13-19] and forward and transverse energy spectra [20,21]. Based on the experience of the NA35 experiment in studying high multiplicity sulphurnucleus collisions the experiment NA49 was designed and built. The aim of NA49 is to investigate central Pb+Pb collisions at CERN SPS energies in an inclusive as well as an event-by-event way [22]. The experimental set-up and preliminary results from the first lead run at the CERN SPS (November/December 1994) are presented in detail elsewhere [2]. Some preliminary results on central Pb+Pb collisions (about 5% of total inelastic cross section) are however also discussed in this presentation.
80
I •
-(3 C "0
60
S+Au S+Ag
"A- S+S
40 20
O0
'
T
.
1
2
.
.
.
,
3
,
,
,
'
4
5
,
6
Y Figure 2. The rapidity distributions (normalized to the mean multiplicities) for negatively charged hadrons treated as n--mesons produced in central S+S, S+Ag and S+Au collisions at 200 GeV/c per nucleon. The solid line indicates the shape of the rapidity distribution (arbitrary normalization) of relativistic pions emitted isotropically in the N+N c.m. system.
2. M A T T E R AT F R E E Z E - O U T We define the freeze-out stage to be the last stage in the history of strongly interacting matter produced in high energy nuclear collisions. At this stage all strong interactions have stopped and the final number of particles and their momenta, observed later in the experiment, are defined. The prevailing scenario of the evolution of strongly interacting
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M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
300 C
200
1 O0
0
0
2
4
6 Y
Figure 3. The preliminary rapidity distribution (normalized to the mean multiplicity) obtained by NA49 for negatively charged hadrons produced in central Pb+Pb collisions at 158 GeV/c per nucleon (circles: the NA49 Vertex TPC data, squares: the NA49 Main TPC data). The measured distribution (full symbols) is reflected (open symbols) with respect to the c.m. rapidity. The solid line indicates the shape of the rapidity distribution (arbitrary normalization) of relativistic pions emitted isotropically in the N+N c.m. system. The dashed line indicates the gaussian fit to the data. The distribution for central S+S collisions at 200 GeV/nucleon is indicated by stars.
matter requires first chemical freeze--out, from which point onwards particle abundances are preserved, followed by the kinetic freeze--out after which the final momenta of particles are established. 2.1. K i n e t i c F r e e z e - o u t
The rapidity distributions of negatively charged hadrons (more than 90% of them are r--mesons) calculated assuming the pion mass are shown in Fig. 2 for central S+S, S+Ag and S+Au collisions. For relativistic particles (the rest mass can be neglected) the shape of the rapidity distribution is determined by the angular distribution. It depends only very weakly on the shape of the momentum distribution. The solid line in Fig. 2 indicates the rapidity distribution of relativistic pions emitted isotropically in the nucleonnucleon (N+N) c.m. system. This distribution is significantly narrower than the measured distributions of negatively charged hadrons. This indicates a strong angular anisotropy of the hadron distribution at the kinetic freeze-out which is caused (as will be shown below) by a strong longitudinal flow of the freezing-out matter. A similar conclusion is
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
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obtained from the analysis of preliminary data on central P b + P b collisions at 158 GeV/c per nucleon. The rapidity distribution of negatively charged hadrons produced in these collisions is shown in Fig. 3. It is, as in the case of S+AT collisions, about two times broader than the distribution for isotropic emission. Shapes of distributions for minimum bias N + N interactions, central S+S collisions [6] and central P b + P b collisions are similar. The transverse momentum or transverse mass (ra T = ~ o + P~', where ra0 is the rest mass of the particle) spectra of hadrons are sensitive to the transverse flow of the freezingout matter [23-25]. In Fig. 4 the transverse mass distribution of negatively charged hadrons (the pion mass is assumed) produced in central S + A g collisions is presented as an example. The distribution shows the well established concave shape which deviates from the shape expected for a purely thermal distribution of pions (approximately a straight line in Fig. 4). A careful analysis of the transverse momentum spectra [24,25] leads to the conclusion I that they can be explained taking into account the influence of resonance decays and assuming weak (~f ~ 0.3) transverse flow of the matter freezing-out at a temperature 7"/,~ 150 MeV.
"o
10 3
,-10
2
%
2.0
O00oo00000 O000Q o~
1
°Ooo
IOoooll°
-1
%,,
10
I
0
0.5
I
I
1 1.5 2 m-r-m0 [ G e V / c ]
Figure 4. The transverse mass distribution for negatively charged hadrons treated as 7r--mesons produced in central S + A g collisions at 200 GeV/c per nucleon.
IThe requirement of the consistency of the freezing-out system with the freeze-out conditions given by hadronic cross-sections and its size is needed for unique extraction of transverse flow and freeze-out temperature.
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M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
8 '~
E
7
6 5 4
4
5
5
2
2
1
1
0
2
6
4
Y Figure 5. The dependence of the longitudinal parameter of the two-pion correlation function on the pion rapidity for central S+Ag collisions at 200 GeV/c per nucleon. The solid line shows the fit result obtained using Eq. 2.
0
0.4
0.2
O.6
kr [GeV/c] Figure 6. The dependence of the longitudinal parameter of the two-pion correlation function on the average transverse momentum of the pion pair for central S+Ag collisions at 200 GeV/c per nucleon. The solid line shows the fit result obtained using Eq. 2.
The fact that the shape of the transverse momentum spectrum only weakly depends on rapidity and the extracted transverse flow is weak suggests that the expansion of matter at the kinetic freeze-out can be approximated by a boost invariant longitudinal expansion 2 (see discussion below). The properties of matter at the kinetic freeze-out can be further studied by the analysis of the two-pion correlations as a function of rapidity and transverse momentum [13,16, 18,19,26]. In order to perform this analysis the two pion correlation function expressed as a function of three components of the momentum difference (QL, Qs, Qo - for definition see [18]) was parametrized as:
C(QL, Qs, Qo) = 1 + ~. exp[-0.5(Q~ • ~ + Q~. R~ + Q~. R~)],
(1)
where the parameters RL, Rs, Ro, and A were fitted to the experimental data in Y-PT intervals. 2Note that the system as a whole is not boost invariant, e.g. pion and baryon rapidity density changes significantly with y, but its local velocityspectrum (the expansion of the system) at freeze-out is approximately boost invariant.
M. Ga2dzickiet al. / Nuclear PhysicsA590 (1995) 197c-214c
S+Cu
S+Ag
203c
S+Au
,-.8
E .-,6 rY 4
{
÷÷ +
,I, ÷
+ +
2 0
, ,,I,
0
0.2
,,I
0.4
,,,I,,,I,,
,,,
0
0.2
0.4
....
0
I,,,
0.2
I,,
0.4
~
0,6
kr [GeV/c] Figure 7. The dependence of the side parameter of the two-pion correlation function on the average transverse momentum of the pion pair for central S+Cu, S-FAg and S+Au collisions at 200 GeV/c per nucleon. The dashed lines show the approximate transverse size of the sulphur nucleus.
The longitudinal parameter of the correlation function, RL, is plotted against the rapidity and the average transverse momentum of the pion pair (fcT = 0.5. (p-4~)+ p--(~))) for central S+AT collisions in Figs. 5 and 6, respectively. On the basis of a boost invariant longitudinal expansion Makhlin and Sinyukov [27] derived the dependence of R5 on y and kT: RL(y, kr)
2. T:
T: + k S cosh( - y0)'
(2)
where rn, is the pion mass, yo is the rapidity of the observer (y0 = 3 in the NA35 analysis) and r! is the proper expansion time of the system. The solid lines in Figs. 5 and 6 show the result of the fit of r/in Eq. 2 to the data assuming 7"/= 150 MeV. The obtained proper expansion time of the system is about 5 fm/c. The fact that the data are well described by the Makhlin-Sinyukov approach supports the assumption of a boost invariant expansion and indicates that the emission anisotropy observed in the analysis of the rapidity spectra can be treated as a consequence of longitudinal flow of freezing-out matter. A transverse expansion is expected to cause a decrease of the side (Rs) parameter of the correlation function with increasing transverse momentum [28]. The dependence of Rs on kT is shown in Fig. 7 for central S+Cu, S+Ag and S+Au collisions. In fact a slight decrease of Rs is observed in the experimental data. A unique interpretation of this decrease as due to transverse flow is, however, difficult since an effect of a similar magnitude can be caused by the resonance decays [18]. In the early fifties Pomeranchuk suggested [29] that pions produced in high energy collisions should decouple at approximately constant density 3. In order to test experimentally 3The freeze-out pion density was estimated by Pomeranchuk to be of the order of the cube of the strong
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M. Ga2dzicki a al. / Nuclear Physics A590 (1995) 197c-214c
I 50
O0
50
°6
20
40
60
80
dn /dy Figure 8. The dependence of the R~ • R~, on the rapidity density of negatively charged hadrons for central nucleus-nucleus collisions. The results for 4°Ar+AT collisions at LBL energies are indicated by triangles, the results for 2sSi+AT collisions at AGS are indicated by squares and the results for 32S+AT collisions at CERN SPS (the NA35 data) are indicated by circles. The lines are drawn to guide the eye.
this hypothesis the dependence of R~ • R~ (the R~, is the longitudinal parameter of the correlation function fitted in the local frame) on the negative charged hadron rapidity density is plotted in Fig. 8 for central nucleus-nucleus collisions at 1.9 - 200 GeV/c per nucleon. Only data on collisions of 4°Ar, 2ssi and 32S projectiles were compiled [30]. In addition the R~ • R~, values for 4°Ar collisions were scaled by a factor 3 Thus the data presented in Fig. 8 should be treated as obtained for collisions with approximately fixed initial geometry. The results from AGS and CERN (the NA35 experiment) concern the central rapidity region, whereas the results from LBL are for 47r and forward rapidity measurements. At high energies for which the boost invariant longitudinal expansion may be used as an approximation and the baryon density can be neglected in comparison to the pion density, the pion rapidity density is expected to be proportional to the number of particles freezingout from a volume which is proportional to R~. R~. 4 Therefore in this case R~- R~ should be proportional to dn/dy providing that the freeze-out occurs at a constant density. This interaction length scale, (m./h) a .~ 0.3 fm-a. 4Note that in this case R~. is proportional to the longitudinal homogenity scale of the system and it cannot be treated as a measure of the total longitudinal dimension of the freeze-out volume. Thus the R~ • R~ is proportional to the homogenity volume.
M. Ga~dzicki et ai. / Nuclear Physics A590 (1995) 197c-214c
E
300
I
fY U')
n~
205c
200
100
0
50 l
100 I
150 I dn
200
/dy
Figure 9. Same as in Fig. 8 but with the preliminary result for central P b + P b collisions at 158 GeV/c per nucleon (star, the NA49 data) and non-midrapidity results for central 32S+AT collisions (the NA35 data).
behaviour is observed for the NA35 data at 200 GeV/c per nucleon. In the case of low energy collisions the particle density is dominated by the density of baryons and thus the freeze--out volume should be proportional to the initial volume of the colliding system and independent of the multiplicity or dn/dy of pions. The approximate independence of the R~ • R~. of dn/dy is in fact observed for the low energy data (LBL, AGS). Due to the fact that for nucleus-nucleus collisionsat C E R N SPS energies the volume of the freezing-out system is determined by the pion multiplicity,the resultsfor collisions with different initialgeometries can be compared in the same plot. In Fig. 9, together with the data presented in Fig. 8, we plot the preliminary result obtained by the NA49 experiment for central P b + P b collisionsin the rapidity interval 4-5 [18]. The results for non-midrapidity intervals obtained by the NA35 experiment for central S + A T collisions are also plotted in Fig. 9. 2.2. C h e m i c a l F r e e z e - o u t The NA35 experiment measured spectra and multiplicities of rr-, p, ~, K +, K - , A, A and /(~ for central sulphur-nucleus collisions. If the evolution of the strongly interacting matter is slow enough one can expect that the system freezes out as an equilibrated hadronic gas. In this case all possible particle ratios should be described by two thermody-
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M. Ga2dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
4
S+S
"o
8
---> A + X
¢-
g
3.3
++++%++
2
6 4
S+Ag
> A+ X
+ +++
2 ~~.C>_D_~
o,6
I
2
~
5
4
4D-Ct-
.
5
Figure 10. The rapidity distribution of A hyperons produced in central S+S collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of A hyperons produced in N+N interactions at 200 GeV/c (squares). The open circles indicate the data reflected with respect to y~m.
6
0
i
0
r
7[~r -'0-
6 Y
Figure 11. The rapidity distribution of A hyperons produced in central S+Ag collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of A hyperons produced in p+S interactions at 200 GeV/c (squares). The open circles indicate the data obtained by an interpolation procedure.
namical parametersS: chemical freeze--out temperature and baryonic chemical potential. The NA35 multiplicity data allow a test of the ha&on gas hypothesis. It was found [31] that the freezing-out system is out of chemical equilibrium. The measured hadron ratios can be described in a hadron gas approach only if pions are excluded from the anMysis. In this case the chemical freeze--out temperature is about 200 MeV and the baryonic chemical potential is about 300 MeV. The number of pions in the hadron gas with the above parameters is underestimated by a factor of about 1.6. 3. H O T E X P A N D I N G M A T T E R The properties of the hot expanding matter can be studied by the analysis of strange particle production. The number of strange quark-antiquark pairs in elementary high energy processes (e.g. p+p, e++e -) is strongly suppressed (relative to the number of nonstrange quark-antiquark pairs) [32] and significantly below the equilibrium level [31,33]. As the corresponding abundance may be treated as an estimation of strangeness population in the early stage of a nucleus-nucleus collision [33] one expects that any secondary SThis is for the case of strangeness neutral and charge symmetric systems.
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M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
>., "o
6
10
S + S ---> K°s + X
¢..
S+Ag - ~ K°s + X
8 4
+.
6 4 °
O0 _ : i 1
_t~_-C~- or
2
5
f
~
4
5
6 Y
Figure 12. The rapidity distribution of h'~ mesons produced in central S+S collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of K~ mesons produced in N+N interactions at 200 GeV/c (squares). The open circles indicate the data reflected with respect to ycm.
-O-
,.¢_.
0o
1
2
5
4
5
6 Y
Figure 13. The rapidity distribution of I(~ mesons produced in central S+Ag collisions at 200 GeV/c per nucleon (circles) and the scaled (see text) rapidity distribution of I(~ mesons produced in p+S interactions at 200 GeV/c (squares). The open circles indicate the data obtained by an interpolation procedure.
processes should increase the relative strangeness yield, s Furthermore it was estimated that the strangeness equilibration time in deconfined matter is similar to the measured time of the system expansion (5-10 fm/c), whereas the strangeness equilibration time in hadronic matter is several times longer [34]. Therefore a substantial increase in the relative strangeness yield can be treated as an indication that the expanding matter was in the deconfined phase. The experiment NA35 observes significantly enhanced production of strange particles. In order to illustrate this effect we show in Figs. 10 and 11 the rapidity distribution of A hyperons produced in central S+S and S+Ag collisions, respectively. The rapidity distributions of I(~ mesons are shown in Figs. 12 and 13. These distributions are compared with the corresponding distributions obtained for N+N interactions (reference data for S+S collisions) [35] and p+S interactions (reference data for S+Ag collisions) [12] at 200 GeV/c. The spectra for N+N interactions are multiplied by the ratio (~r-)ss/(Tr-)N g and the spectra for p+S interactions are multiplied by the ratio (lr-)SAg/(Tr-)ps, where (~r-) is the mean multiplicity of 7r--mesons produced in the collisions indicated by the index. The observed large difference (factor 2-3) between the strange particle yields in central S+AT collisions and the scaled yields in the reference 6In nuclear collisions the strangeness production is studied relative to the production of non-strange particles in order to remove a trivial dependence of the number of produced sg-pairs on the volume of the colliding systems. The relative strangeness production reflects therefore the space density of sg-pairs.
M. Ga3zicki et al. /Nuclear Physics A590 (1995) 197c-214~
208~
I T
$
t
0.05 -
2 xW
t LUCIAE ____MCSg; __________-.-----
0
_______________________.______~ I _________________________-_---FRITIOF
-0.05 2
I
I
I
3
4
5
6 l/3
AT
Figure 14. The difference between Es values (see Eq. 3) determined for central S+S and S+Ag collisions and the Es value for N+N interactions is plotted (circles) as a function of the target nucleus mass number. The corresponding differences for PSAT interactions are indicated by squares. The solid line shows the level for N+N interactions, by definition equal to zero. The dashed lines show the results of calculations performed within stringhadronic models: FRITIOF, QGSM, MCSFM and LUCIAE.
interactions (N+N and p+S) illustrates the enhanced production of strangeness to 7r--mesons) in central sulphur-nucleus collisions. It amounts to the absolute about 20 ST-pairs in central sulphur-nucleus collisions. In order to quantify the total enhancement of strangeness production in central nucleus collisions we define the ratio [12]:
(relative excess of nucleus-
(3) where (A) and (li!j!) are the mean multiplicities of A and Ki charge symmetric collisions this ratio is equal to the ratio:
particles,
respectively.
(A) + (K + R)
For
(4)
(r) The Es ratio is approximately [12] usually used in elementary
two times smaller particle physics.
than
the strangeness
suppression
factor
M. Ga~dzicki et al. / Nuclear Physics A590 (1995) 197c-214c
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2 A ,,,t
Z
V A V m
m-{[.... }+ ..................................... -1
,
0
I
1
,
I
2
,
I
3
,
I
4
~
I
,
5
6
F,,N [GeV '/=] Figure 15. The difference of the mean multiplicity of pions per participant nucleon for central collisions of identical nuclei and for N+N interactions at the same collision energy per nucleon as a function of the modified Fermi-Landau variable (see Eq. 6). The low energy data (LBL, Dubna, AGS) are indicated by squares, the result for central S+S collisions at 200 GeV/c per nucleon (NA35)isindicated by the circle and the preliminary result for central Pb+Pb collisions at 158 GeV/c is indicated by the star (NA49).
The difference between Es for central S+AT collisions (and p+AT interactions [36]) and Es for N+N interactions [33] is plotted in Fig. 14. No strangeness enhancement is observed for p+AT interactions, all corresponding Es differences are close to zero. The previously discussed strong strangeness enhancement is observed for central S+AT collisions. We conclude that a fast strangeness production, characteristic for the expansion of hot deconfined matter, is observed in the experiment. The strangeness enhancement effect for CERN SPS energies was for the first time reported in 1988 [37]. Since that time there were numerous attempts to modify stringhadronic approaches in order to reproduce the data. The published model results for central S+Ar collisions (FRITIOF [38], LUCIAE [38], QGSM [39] and MCSFM [40]) are indicated in Fig. 14 by dashed lines. In addition to the standard string approach [41] the models include secondary hadronic interactions fusion of strings [40] and collective gluon emission [38]. They approximately reproduce the absolute strangeness yield in central nucleus-nucleus collisions, however they fail to reproduce the strangeness enhancement effect as quantified in Fig. 14 because they cannot simultaneously describe
[38,39],
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strange and non-strange particle production in N+N interactions and central nucleusnucleus collisions. 4. M A T T E R AT T H E E A R L Y S T A G E It is argued [42] that the dominant fraction of the entropy is produced in the early stage of the collision - the stage during which all incoming matter is excited. Therefore the amount of entropy created is dependent on the effective number of degrees of freedom of matter at this stage [43]. The creation of deconfined matter, the matter with high number of degrees of freedom, should cause a high entropy production in comparison to the entropy production in the hadronic matter. For high energy collisions the entropy of the system is approximately proportional to the final state pion multiplicity [44]. Therefore a systematic study of pion multiplicity in nuclear collisions was undertaken [45]. The difference between the mean pion multiplicity per participant nucleon for central collisions of identical nuclei and N+N interactions at the same collision energy per nucleon:
A ( ~ / _ (~/AA
Fig.
(~)NN (Np)NIV
(5)
15 as a function of the modified Fermi-Landau variable [43,45]:
FNN = ( V q N N -- 2 - r a n -- m . ) s / ' v~l/4
(6)
NN
where V~NN is the c.m. energy per N+N pair and mN is the nucleon mass. In addition to the data presented in [43,45] a preliminary result for central P b + P b collisions at 158 GeV/c per nucleon obtained by the NA49 experiment is shown. The results for central S+S collisions (the NA35 experiment) and central P b + P b collisions (the NA49 experiment) suggest an increase of the pion production at CERN SPS energies in comparison to lower energy collisions r (LBL, JINR, AGS). The effect of the pion (entropy) enhancement could be interpreted, in the generalized Landau model [43], as a manifestation of the increase (by a factor of about 3) of the effective number of degrees of freedom in the early stage of the collision. A change in the collision dynamics seems to occur between AGS and CERN SPS energies. 5. S U M M A R Y The NA35 experiment at the CERN SPS studied hadron (n-, p, ~, K+, K - , A, and I(~) production in central sulphur-nucleus collisions at 200 GeV/c per nucleon. The results obtained by the experiment concern particle multiplicities, spectra, correlations and transverse and forward energy flow. The first preliminary results on ha&on production in central P b + P b collisions at 158 GeV/c per nucleon obtained by the NA49 experiment [2], the successor of the NA35 experiment for the lead age at the CERN SPS, are now available. rThe suppression of the pion production for low energy nucleus-nucleus collisions in comparison to N+N interactions may be interpreted as due to entropy transfer (e.g. by pion absorption) to the baryoaic sector [43].
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The analysis of the results of NA35 and the preliminary results of NA49 allows one to establish properties of strongly interacting matter created in central nucleus-nucleus collisions in subsequent stages of the evolution: the early stage of the collisions, the expansion stage and the freeze--out stage. The analysis of the properties of matter at freeze-out indicates: a strong longitudinal flow, a possible weak transverse flow, - a kinetic freeze-out temperature of about 150 MeV, a proper expansion time of about 5 fro/c, - a constant freeze-out particle density, a hadronic system out of chemical equilibrium (an 'excess' of pions is observed). The analysis of the properties of hot expanding matter indicates: - a fast production of strangeness as expected for deconfined matter. The analysis of the properties of matter at the early stage of the collision indicates: - an enhanced entropy content as expected for deconfined matter, - a possible change of the collision dynamics between AGS and CERN SPS energies. -
-
-
-
Acknowledgements This work was supported by the Bundesministerium f/ir Forschung und Technologie and by the Leibniz Grant of Deutsche Forschungsgemeinschaft, Germany, by the US Department of Energy (DE-AC03-76SF00098), by the Polish State Committee for Scientific Research (204369101) and by the Commission of European Communities (CI-0250YU(A)).
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Marek Gazdzicki (second from right) T.D. Lee and Nick Samios Tom Ludlam
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Bill Willis Nick Samios, Satoshi Ozaki and Peter Bond Felix Obenshain and Frank Plasil