... on multiplicity. It can be seen from the data that these problems are ... very highest multiplicity (raw multiplicity in the plastic ball MpB> 300). .... replaced by an identified proton. In the analysis ..... source with z ~> h c/qo = 2 fm/c. In addition, a ...
Z. Phys. C - Particles and Fields 53, 225-237 (1992)
fi3rPhysikC P a r t i d e s
Zeitschrift
and
9 Springer-Verlag 1992
Bose-Einstein correlations in the target fragmentation region in 200A GeV 160 + nucleus collisions WA80 Collaboration R. Albrecht 1, T.C. Awes 5, P. Beckmann 4,a F. Berger 4, M.A. Bloomer 2, D. Bock 4, R. Bock 1, G. Claesson 3, G. Clewing 4, L. Dragon 4'b, A. Eklund 3, R i . Ferguson 5, A. Franz 5.... , S. Garpman a, R. Glasow 4, H.~. Gustafsson 3, H.H. Gutbrod 1, M. Hartig 4, G. H61ker 4, J. Idh 3, P. Jacobs 2, K.H. Kampert 4, B.W. Kolb 1, P. Kristiansson 3, H. L6hner 4, d, I. Lund 1, a, F.E. Obenshain s, A. Oskarsson 3, I. Otterlund 3, T. Peitzmann 4, S. Persson 3, F. Plasil s, A.M. Poskanzer 2, M. Purschke 4'e, H.G. Ritter 2, B. Roters 4, S. Saini s, R. Santo 4, H.R. Schmidt 1, R. Schmidt 4, S.P. Sorensen 5, ~, K. Steffens 4, P. Steinhauser 4' e, E. Stenlund 3, D. Sttiken 4, M.L. Tincknell 5, f, A. Twyhues 4, G.R. Young 5 1 Gesellschaftffir Schwerionenforschung,W-6100 Darmstadt, Federal Republic of Germany 2 Lawrence Berkeley Laboratory, Berkeley,CA 94720, USA 3 University of Lurid, S-22362 Lund, Sweden 4 University of Miinster, W-4400 Miinster, Federal Republic of Germany s Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 29 August 1991
Abstract. Correlations between positive pions are investigated in the target fragmentation region of 200A GeV 160 + nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the C E R N SPS. The target mass dependence of the radii and the correlation strength extracted by interferometry is studied. A new approach to the fit of the correlation function is introduced. The correlation strength and both invariant and transverse radii increase with decreasing target mass. The transverse radius for t60 + C reactions appears to be much larger than the geometrical radius of the nuclei involved. For the Au target only a small fraction of the measured pions contributes to the apparent correlation. Hints for a much larger second component in 1 6 0 + A u reactions are observed. Rescattering phenomena may provide a clue to understand these phenomena.
1 Introduction
Pion interferometry allows a study of the space-time properties of the particle emitting source in high energy collisions [13. An enhancement in the production of " now at: CERN, CH-1211 Geneva 23, Switzerland b now at: Mercedes-Benz, W-7000 Stuttgart, Federal Republic of Germany ~ University of Tennessee, Knoxville, TN 37996, U S A a now at: KVI, University of Groningen, NL-9747 AA Groningen, The Netherlands e now at: Gesellschaft ffir Schwerionenforschung, W-6100 Darmstadt, Federal Rpublic of Germany f now at: Purdue University, Lafayette, IN 47907, USA
identical pions with similar momentum - known as Bose-Einstein correlations, the Hanbury-Brown - Twiss effect (HBT) [2] or the Goldhaber - Goldhaber - Lee - Pais effect (GGLP) [3] - has been observed in various experimental studies. A simple theoretical picture of multi-particle production yields the following prediction for the two-particle correlation function [4]: (n> 2
d 6 N/dp 3dp3
C2 = ( n ( n - 1)>" d 3 N / d p 3. d 3 N / d p 3 = 1 +2.]p(p~-p~)l 2, (1)
where n is the pion multiplicity and d3N/dp~ and d 6 N/dp~ dp~ are the one-pion and two-pion inclusive yields. ~ is the Fourier transform of the distribution of emitters; the most commonly chosen analytic expressions for/5 contain a suitable correlation length R. While this parameter seems to measure dynamical properties of the produced strings in e+e - collisions [-5], it can provide geometrical information in medium energy heavy ion collisions (Ela b ~ 2A GeV) [6]. The parameter 2, a correlation strength, was introduced for technical reasons [7]; it is expected to be = 1 for a completely chaotic source. Theoretically a value of 2 < 1 can be ascribed to a certain amount of coherent production of pions [8], but in the experiment many different effects may reduce the measured value of 2. The existence of a deconfined phase with high energy density but small pressure gradient in nuclear collisions would lead to a considerably larger lifetime of the system. It has been suggested in [9] that such an effect could
226 be observed by pion interferometry and would thus be a possible signal of the quark-gluon plasma. These theoretical investigations as well as the experimental observation of a large source at midrapidity (Rr=7-8fm) by the NA35 collaboration in 160-}-Au collisions at 200A GeV [10] have renewed the interest in interferometric measurements. They can provide an important clue to understand the dynamics of ultrarelativistic nucleus-nucleus collisions. In this paper we will present a study of ~r+~z+-correlations in reactions of 200AGeV 160 + nucleus in the target rapidity region
(yt~= 10 fm. O n the one h a n d these large radii will necessarily be influenced by the detector resolution - they m a y rather provide lower limits on the true values. O n the other h a n d the influence of the C o u l o m b correction and possible systematic errors of it should not be very important, as the cut in opening angle described a b o v e forces the pairs to have finite relative m o m e n t u m in their center of mass Q > 30 MeV.
C2(Qr, QL)=l +[21"exp ( -Q2 R~I-Q~
4 Discussion
+22-exp _--zr--r2 4
~L
.
(7)
Both parametrizations yield better fits than the single Gaussian - (7) gives only a slightly better agreement than (6). The fit results are summarized in Table 5. The projections of these correlation functions for Q L
h c/qo = 2 fm/c. In addition, a two-dimensional fit like the one employed here will provide only average radii, if the two directions do not factorize. A fit to the transverse direction with a very stringent cut on the longitudinal component QL=10 fm). This analysis emphasizes the need to study the dynamics of the so-called "spectator nucleons", which in fact seem
to participate strongly in a later stage of the collision. Care should be taken, if one compares pion interferometry in different rapidity regions, when the presence of target spectators may obscure the interpretation. Acknowledgements. This work was partially supported by the West German BMFT and DFG, the VW-Stiftung, the United States DOE, the Swedish N F R and the Alexander yon Humboldt Foundation. The contributions of K.G.R. Doss to the earlier analysis with the Plastic Ball at the Berkeley Bevalac are gratefully acknowledged.
Appendix A new approach to the one-dimensional correlation function was used in this analysis. A set of "basis functions" pi(r) was chosen for the source distribution. We have used the two choices A:
Pi(r) = exp and B: 0 pi(r)= 1 0
r
