ЯДЕРНАЯ ФИЗИКА, 2012, том 75, № 7, с. 866-872
ЯДРА
MULTIPARTICLE AZIMUTHAL CORRELATIONS OF NEGATIVE PIONS IN NUCLEUS-NUCLEUS COLLISIONS
© 2012 L. V. Chkhaidze1)*, T. D. Djobava1), L. L. Kharkhelauri1), E. N. Kladnitskaya2)
Received August 17,2011
Multiparticle azimuthal correlations of n- mesons have been studied in dC, HeC, CC, CNe, MgMg, (d, He)Ta, CCu, CTa, and OPb collisions at momentum of 4.2, 4.5 GeV/c per nucleon within the standard transverse momentum analysis method of P. Danielewicz and G. Odyniec. The data were obtained by SKM-200-GIBS and Propane Bubble Chamber Collaborations of JINR. The axis has been selected in the phase space and with respect to this axis n- meson correlations were observed. The values of the coefficient of the correlations linearly depend on the mass numbers of projectile (AP) and target (AT) nuclei. The Quark—Gluon String Model satisfactorily describes the experimental results.
INTRODUCTION
Relativistic nucleus—nucleus collisions are very well suited for the investigation of excited nuclear matter properties. The theoretical models predict the formation of exotic states of nucelar matter. The experimental discovery of such states is impossible without understanding the mechanism of collisions and studying the characteristics of multiparticle production and correlations in nucleus—nucleus interactions. The study of multiparticle azimuthal correlations offers unique information about space—time evolution of the collective system.
Pions, being copiously produced in relativistic heavy-ion collisions, can probe the reaction dynamics independently of nucleons. The pattern of pion emission relative to the reaction plane has been first studied at the Bevalac by the Streamer Chamber group [1,2] and later by the EOS Collaboration [3] at Saturne by the Diogene group [4].
Pion production is the most important inelastic channel at our energies. Although not directly related to the nuclear equation of state (EOS) [5], pions are an integral part of the effort to determine properties of nuclear matter. During last several years we have studied multiparticle azimuthal correlations of protons and pions in central and inelastic collisions (4.2, 4.5 GeV/c per nucleon) within two experiments (SKM-200-GIBS, PBC-500) of JINR in order to investigate the mechanism of nucleus—nucleus interactions [6—9]. We have studied the correlations of pions with respect to the relative reaction plane
!)High Energy Physics Institute, Tbilisi State University,
Georgia.
2) Joint Institute for Nuclear Research, Dubna, Russia.
E-mail: ichkhaidze@yahoo.com
(directed flow) [8, 9], as well as with respect to the opening angle between particles emitted in the forward and backward hemispheres [10]. In this paper we present the results of the investigation of the multiparticle azimuthal correlations of negative pions in dC, HeC, CC, CNe, dTa, HeTa, CCu, CTa, and OPb collisions at a momentum of 4.2, 4.5 GeV/c per nucleon. We have employed the transverse analysis method proposed by Danielewicz and Odyniec [11] in determining the azimuthal correlations between n-mesons.
EXPERIMENTAL DATA
The experimental data have been registered in the SKM-200-GIBS setup and 2-m Propane Bubble Chamber of JINR.
The SKM-200-GIBS setup consists of a 2-m streamer chamber placed in a magnetic field of 0.8 T and a triggering system. The streamer chamber was exposed to a beam of C, Mg, O nuclei accelerated in the synchrophasotron up to a momentum of 4.5 GeV/c per nucleon (beam energy E = = 3.7 GeV/nucleon).
The solid targets Mg, Cu, and Pb were shaped as thin discs. The thickness of the Mg was 1.5 and 0.2—0.4 g/cm2 for Cu and Pb, respectively. Neon gas filling of the chamber also served as a nuclear target. A central trigger was used to select events with no charged projectile spectator fragments (with p > 3 GeV/c) within a cone of half angle dch = dn = = 2.4° or 2.9°. Details of data-acquisition techniques of CNe, MgMg, CCu, and OPb interactions and the experimental procedures, such as biases involved and correction procedures utilized in our data analysis have been presented in previous publications [12, 13].
The number of events, n mesons and the parameter F for the experimental and QGSM data
APAT Nsxp . ev -^exp .7r ■^QGSMevi b, fm ^qgsm 7t Fexp, MeV/c; rapid, region Fqgsm, MeV/c
dC 4581 3452 30 000 3.00 24 300 19.0 ±3.0 0.2-1.6 21.7 ± 2.0
HeC 9737 10 700 35 000 2.80 39 870 20.7 ±2.5 0.2-2.1 19.4 ± 1.5
CC 15 962 21891 50 000 2.65 85 030 26.5 ± 1.5 0.1-2.0 28.8 ± 1.0
CNe 902 3882 8400 2.20 31550 31.0 ± 3.0 -0.2-2.4 34.0 ±2.2
MgMg 6239 50 775 30 000 1.35 249 860 34.6 ± 1.5 -0.30-2.15 37.6 ± 1.0
(d, He)Ta 2956 3337 17000 4.20 34 500 37.4 ± 3.8 -0.2-1.4 36.6 ± 1.5
CCu 1866 12 390 8348 2.75 54 490 39.1 ±2.0 -0.2-2.7 41.0 ± 1.5
CTa 2469 6092 10 000 6.55 42 500 47.2 ±3.0 -0.05-2.35 46.0 ±2.5
OPb 732 7023 5000 3.75 54 970 54.5 ±3.0 -0.15-2.00 52.0 ±2.2
The data of dC, HeC, CC, dTa, HeTa, and CTa interactions have been obtained using 2-m Propane Bubble Chamber of JINR (beam energy E = = 3.4 GeV/nucleon). The chamber was placed in a magnetic field of 1.5 T. Three Ta plates 140 x 70 x x 1 mm in size mounted into the fiducial volume of the chamber at a distance of 93 mm from each other served as a nuclear target. The method of separation of HeC and CC collisions in propane, the processing of the data, identification of particles and discussion of corrections is described in detail in [14].
The sub-sample of "semicentral" collisions with the number of n- mesons N > 4 were selected for the analysis from the whole ensemble of dC, HeC, CC, (d, He)Ta, and CTa inelastic collisions (see the table). Due to small statistics, 1424 dTa (1137n- mesons) and 1532 HeTa (2200n- mesons) has been combined for farther analysis.
PION CORRELATIONS
In view of the strong coupling between the nucleons and pions, it's interesting to know the origin of correlations between pions. The n- mesons in our experiment were identified unambiguously. The admixture of electrons and K- mesons is almost negligible [12].
Many different methods were proposed for the study of multiparticle azimuthal correlations. Most of the data below 4 GeV in the literature have been, actually, analyzed following the standard transverse momentum analysis method of Danielewicz and Odyniec [11]. The advantage of this method is that it can be employed even at small event statistics such as typical for film detectors. The method relies on summation over transverse momenta of selected particles in the events.
The reaction plane is spanned by the impact parameter vector b and the beam axis. Within the transverse momentum method, the direction of b is estimated event by event in terms of the vector constructed from particle transverse momenta PTi:
Q = ^P
Ti,
(1)
i=l
where the sum extends over all protons in an event. The summation weight is ui = 1 for yi > yc + 5, ui = = -1 for yi <yc - 5, and u = 0 for yc - 5 < yi < <yc + 5, yi is particle rapidity and yc is system c.m. rapidity. Particles around the c.m. rapidity, with weak correlations with the reaction plane, are not included in the reaction-plane determination. Then the transverse momentum of emitted particles are projected to this reaction plane. The project pTj onto
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(px), MeV/c (Px)> MeV/c
Fig. 1. The dependence of {px(y)) on y for n- mesons in dC and HeC collisions: (•) the experimental data, (*) QGSM generated data. Straight-line stretches represent the slope of data at midrapidity cross-over, obtained by fitting the data with a linear within the rapidity region of 0.2-1.6 for dC, 0.2-2.1 for HeC. Here and in Figs. 2-5 the curves guide the eye over data.
the vector Q is calculated as
p'xj = PTj • Qj/Qj\. (2)
P. Danielewicz and G. Odyniec have proposed to present the collective effects as the dependence of the mean transverse momentum in the reaction plane (px) on the rapidity y. The average transverse momentum (px(y)) is obtained by averaging over all events in the corresponding intervals of rapidity.
Characteristics of transverse collective flow for n-mesons emitted from HeC, CC, CNe, CCu, and CTa reactions at momentum 4.2, 4.5 A GeV/c have been determined in our previous paper [15]. In determining the directed flow of pions, the transverse momentum technique of P. Danielewicz and G. Odyniec has been employed. Pions exhibited directed flow consistent with that for protons in HeC, CC, and CNe collisions, while for the heavier systems, CCu and CTa, pion flow turned into antiflow, with pion average in-plane momenta becoming opposite to those for protons.
In the present paper in order to investigate the multiparticle azimuthal correlations of n- mesons we used the basic ideas of P. Danielewicz technique and constructed the vector Q using solely n- mesons. In our previous paper [16] we constructed the Q vector in MgMg collisions from the transverse momentum vectors of only n- mesons event by event for the events with multiplicity of nn- > 7. This vector in each individual event is defined only by negative pions in the lab. system by Eq. (1), where PTi is the transverse momentum of n- mesons, the weight
(Px), MeV/c
Fig. 2. The dependence of {px(y)) on y for n- mesons in CNe collisions: (•) the experimental data, (*) QGSM generated data. Straight-line stretches represent the slope of data at midrapidity cross-over, obtained by fitting the data with a linear within the rapidity region of -0.2-2.4.
factor ui = yi — yc, where yi is the rapidity of pions i, yc is average rapidity in each systems and n is the number of n- mesons in the event.
When referring a specific pion j to this vector, we estimate the direction of the latter from the vector Q with contribution of pion j removed,
Qj = UiPTi (3)
i = j
to eliminate the correlation of the particle with itself, competing with the dynamic effect we are after.
The transverse momentum of each n- meson was projected onto this Q vect
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