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PHYSICAL REVIEW C VOLUME 40, NUMBER 4 OCTOBER 1989

Two-particle rapidity correlations in dC, aC, and CC interactions at 4.2 Gev/c per nucleon

Lj. SimicInstitute of Physics, P.O. Box 57, YU-11001 Belgrade, Yugoslavia

S. Backovic and A. BokanInstitute of Mathematics and Physics, Titograd, Yugoslavia

H. N. Agakishiev, E. N. Kladnitskaya, and A. P. CheplakovJoint Institute for Nuclear Research, Dubna, 141 980, Moscow, U.S.S.R.

(Received 18 April 1989)

Two-particle rapidity correlations are studied in dC, aC, and CC interactions at 4.2 GeV/c pernucleon using 2 m propane bubble chamber. The influence of collision centrality on the short-range correlation strength is also analyzed. Comparison with the Dubna intranuclear cascademodel indicates that the independent nucleon-nucleon collisions may account for the observedcorrelations.

There has been an increasing interest in the study ofhigh-energy nucleus-nucleus collisions in recent years withthe main research goal to extract a new phenomena pecu-liar to highly excited, dense, nuclear matter. Experimen-tal discovery of such a kind of phenomena demands de-tailed study of the multiparticle processes, and in particu-lar, the analysis of various correlation characteristics innucleus-nucleus collisions.

The particle correlations have been extensively studiedin high-energy hadronic interactions. ' One of theprominent results of all these experiments is the presenceof two-particle short-range correlations. In nucleus-nucleus collisions, the correlations have been studied, sofar, in momentum space between identical particlesand in the azimuthal plane, 9'o However, there is practi-cally no data on the two-particle correlations in the longi-tudinal phase space. "'

In this Rapid Communication two-particle rapiditycorrelations are studied in inelastic dC, aC, and CC in-teractions at p 4.2 GeV/c per nucleon. Experimentaldata were obtained in the Joint Institute for NuclearResearch (JINR) 2 m propane bubble chamber exposedon the Dubna synchrophasotron. By using criteria sug-gested in Ref. 13, 70%-80% of all inelastic interactionswere selected. A statistical weight was assigned to the un-selected carbon/hydrogen events, and was determinedfrom the relations between inelastic cross sections fornucleus-nucleus and proton-nucleus interactions. Theprotons were also selected by the statistical method ap-plied on all positive particles with momentum p &500MeV/c.

In nucleus-nucleus collisions, all nucleons may beclassified into participants which interact strongly duringthe collision and spectators which are not actively involvedin the interaction. In our experiment, the projectile spec-tators were protons and heavier fragments with momen-tum of p/Z & 3 GeV/c and emission angle et (4'. Thetarget spectators were protons with momentum p & 300MeV/c. Remaining protons were classified as participant

protons and were used in the following analysis.The experimental results are compared with the calcu-

lations. performed on the basis of the Dubna intranuclearcascade model [(DCM) in the foliowingl. In that modelthe inelastic nucleus-nucleus interactions are treated as asuperposition of successive quasifree two-particle col-lisions described by the relativistic Boltzmann equation.The main assumptions and features of the model arepresented in Ref. 14. It has been shown previously thatthe model reproduces correctly the multiplicities and in-clusive spectra of the secondary particles. ' ' ' The ex-perimental samples consist of 5740 dC, 3260 aC, and5160 CC inelastic interactions. A comparison is madewith 17600 dC, 11560 ttC, and 12900 CC interactionsgenerated according to DCM.

The two-particle correlations are studied as a functionof the rapidity y 0.51n[(E+pL, )/(E —

pL, )], using thenormalized correlation function defined by

R2(y1 y2) - [p2(yi, y2)/pi(yt)pi(y2) jwhere pl(y) cr;„'do/dy and p2(yi, y2) o;„'d o/dyidy2are the single- and two-particle densities, respectively. o;.„is the inelastic cross section. Values of R2(y~, y2) dif-ferent from zero indicate the existence of correlations be-tween particles. Dynamical eff'ects are not the only causefor the positive values of the correlation function. It iswell known that diff'erent production mechanisms, thedependence of the one-particle distributions pi(y) on mul-tiplicity, as well as the trivial correlations due to thekinematical constraints in individual events, all lead tostrong pseudocorrelations, i.e., to R2(yi, y2)a0, even inthe absence of dynamical correlations. This problem canbe avoided by comparison with the DCM. In this way, all"nondynamical" effects are taken into account and anyenhancement of the experimental values of R2(yi, y2),over DCM results, may be considered as a manifestationof dynamical correlations.

Figure 1 shows the correlation function R2(y &,y2-y & )vs y& (short-range correlations) for the proton-proton pair

R1567 O1989 The American Physical Society

R1568 Lj. SIMIC et ul.

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FIG. 1. R2(y&,y2 yi) vsyi, for dC, aC, and CC interactions(filled circles). Open circles represent DCM predictions (a) forpp pairs, (b) for it p pairs.

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-0,1 0, 5 1.1 1.7 2.3 -0.4 0.6 1.6 2.6RAPIOITY y~

production (pp pairs) and proton-x meson pair produc-tion (px pairs) in dC, aC, and CC interactions. Figure1 contains both experimental (filled circles) and DCM(open circles) values. For pp-pairs [Fig. 1(a)] short-rangecorrelations strongly depend on the projectile nucleus. IndC and aC interactions, negative correlations are ob-served on the projectile side (high y) due to energymomentum conservation. Going to lower rapidities, RP'increases and reaches positive values. DCM correctlyreproduces the experimental values, with- exception of thetarget fragmentation region where the model increases thecorrelations due to overestimation of the cascading pro-cesses. In CC collisions the short-range correlations havea maximum value in the central rapidity region and ap-proach zero in the fragmentation regions. In this casedisagreement with the model is appreciable.

There are strong positive short-range correlationsamong tr mesons and protons [Fig. 1(b)] which are inthe backward y region independent on the atomic mass ofthe incident nucleus. For y )y, 1.1, Rg' increaseswith the mass of the projectile. DCM calculations are ingood agreement with experiment in this case.

In order to study the observed correlations in more de-tail we investigated the behavior of the R2 function in CCinteractions for various numbers of participant protons.This number is de6ned via Q n+ n —nt—„s, wherent„sm is the number of spectators from the projectile andtarget and n+(n —) is the number of positive (negative)charged particles. ' The Q value, as a measure of thequantity of nuclear matter involved in the interaction, iscorrelated with the degree of the collision centrality: highQ value corresponds to the event with small impact pa-rameter, i.e., to central or multinucleon interaction. ' Ad-ditionally, the average multiplicity of charged particles in-

FIG. 2. R2(yi, y2 y&) vs yi, for CC interactions with variousnumbers of interacting protons (filled circles). Open circles rep-resent DCM predictions (a) for pp pairs, (b) for it p pairs.

creases with Q.Figure 2 shows R2(yi, y2 yi) vs yi, for pp pairs and

tt p pairs in CC interactions with 6xed number of in-teracting protons. In comparison with inclusive two-particle correlations, at fixed Q, the strength of correla-tions signi6cantly decreases. Figure 2(a) shows that forperipheral (small Q) and central (Q ~ 10) collisions theRP'(y i,y2 y i ) values are, within experimental errors,close to zero. In peripheral interactions (Q 2 —3) parti-cipate in average 2.4 protons from the projectile and tar-get, and there is small probability that both of them areproduced with the close rapidities. In the events withQ~ 10 participate in average 9.9 protons, and short-range correlations should arise as a manifestation of col-lective processes. Their absence indicates that in centralcollisions a superposition of independent nucleon-nucleoncollisions dominates, but it does not exclude that thestrong background of uncorrelated protons could suppressdynamical correlations.

In the case of tt p pairs, [Fig. 2(b)], the situation isquite different. Positive correlations are observed evenwhen the number of interacting protons is fixed, but thestrength of correlations decreases with increasing Q. Atenergies of a few GeV per nucleon almost all pions areproduced through 6 isobars, so that their formation anddecay is responsible for a short-range effect. Weakeningof ir p correlations with increasing Q, points to the in-coherence of processes during the collisions. The agree-ment between DCM and the experimental data is striking.In this model three-particle reaction channels are realized

TWO-PARTICLE RAPIDITY CORRELATIONS IN dC, aC, . . . R1569

mainly through isobar production and, in that way, onlykinematics of isobar formation is taken into account.

In conclusion, the two-particle rapidity correlationsamong the protons and tr mesons from dC, aC, and CCinteractions at 4.2 GeV/c per nucleon are studied. In or-der to discern real dynamical correlations, a comparisonwith the Dubna intranuclear cascade model is performed.It is found that the experimentally observed short-rangecorrelations are not in contradiction with the model of in-dependent nucleon-nucleon collisions (like DCM). We

found no indications for complete coherence in these reac-tions.

The authors are grateful to the team operating theJINR 2 m propane bubble chamber, the technical staff,and our colleagues at the collaborating institutions for as-sistance in 6[m taking and data processing. We wouldalso like to thank V. G. Grishin and Yu. M. Shabelski forvaluable discussions and I. Mendas for carefully readingand preparing the manuscript.

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