научная статья по теме STUDY OF COLLECTIVE FLOW EFFECTS IN CC COLLISIONS AT A MOMENTUM OF 4.2 GEV/С PER NUCLEON Физика

Текст научной статьи на тему «STUDY OF COLLECTIVE FLOW EFFECTS IN CC COLLISIONS AT A MOMENTUM OF 4.2 GEV/С PER NUCLEON»

= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ

STUDY OF COLLECTIVE FLOW EFFECTS IN CC COLLISIONS AT A MOMENTUM OF 4.2 GeV/c PER NUCLEON

© 2004 L. V. Chkhaidze1)*, T. D. Djobava1), L. L. Kharkhelauri1), E. N. Kladnitskaya2), A. A. Kuznetsov2)

Received November 26, 2002; in final form, April 7, 2003

The directed (in-plane) flows of protons, pions and projectile light fragments (d,t, 3He, 4He) have been observed by investigating the dependence of the mean transverse momentum in the reaction plane (px} on the rapidity y in the c.m. system for CC collisions at a momentum of 4.2 GeV/c per nucleon. The comparison of our in-plane-flow results of protons with flow data for various projectile/target configurations was made using the scaled flow Fs = F/(Ap3 + AT3). Fs demonstrates a common scaling behavior for flow values from different systems. From azimuthal distributions of protons and n- mesons the out-of-plane (squeeze-out) flow effects have been observed and the parameter a2 (the measure of the anisotropic emission strength) have been extracted. The Quark—Gluon String Model reproduces quite well the experimental results.

1. INTRODUCTION

The multiparticle azimuthal correlations are investigated very intensively with the goal to study the dynamics of relativistic nucleus collisions. The study of this effect in terms of the collective flow variables with respect to the reaction plane turned out to be especially fruitful. The collective emission of particles occurs at the expansion stage of the nuclear matter through the short-range repulsion between the nucleons at the expense of the compressional energy concentrated in the high density and temperature overlap region of colliding nuclei. The collective effects lead to characteristic, azimuthally asymmetric sideward emission of the reaction products. The analysis of the main characteristics of the collective flow allows one to obtain the information about the fundamental properties of nuclear matter, connected particularly to the equation of state (EOS) [1].

Two different signatures of the collective flow have been studied:

(a) the bounce-off of compressed matter in the reaction plane (a sideward deflection of the spectator fragments ("bounce-off") as well as directed flow of nucleons from the overlap region between the colliding nuclei (participants) in the reaction plane ("side-splash"), called the sideward or directed flow);

!)High Energy Physics Institute, Tbilisi State University,

Georgia.

2) Joint Institute for Nuclear Research, Dubna, Russia.

E-mail: ida@sun20.hepi.edu.ge, ichkhaidze@yahoo.

com

(b) the squeeze-out of the participant matter out of the reaction plane — the elliptic flow.

The method proposed by Danielewicz and Odyniec [2] turned out to be the most convenient and fruitful for the investigation of collective flow phenomena, which allows one to determine the reaction plane by using the transverse momenta of participating protons. Lately the method of Fourier expansion of azimuthal particle distributions has been widely used [3].

At present the collective flow effects are investigated in the wide range of energies from several hundreds of MeV up to hundreds of GeV. The most part of the experiments are carried out using the electronic technique in the 4n geometry and only in the first experiments at Berkley and lately at Dubna, the streamer chamber served as the detector.

Collective flow of charged particles has been observed experimentally for the first time at BEVALAC by the Plastic-Ball [4—6] and Streamer Chamber [7] Collaborations. It has been studied intensively at Berkeley and GSI [8-13], at AGS [14-17] and CERN/SPS [18-21]. At RHIC (Relativistic Heavy Ion Collider) of BNL the STAR Collaboration recently reported first results on the elliptic flow of charged particles at midrapidity in AuAu collisions at the energy of ^/sW^ = 130 GeV [22].

At Dubna (JINR) in the 2-m Propane Bubble Chamber the shape of the individual events of CTa collisions at a momentum of 4.2 GeV/c per nucleon has been studied in terms of the tensor of kinetic energy (sphericity) [23]. It has been shown that the

dN/dp, (GeV/c)-1

dN/dpT, (GeV/c)-1

pT, GeV/c

Fig. 1. The momentum and transverse-momentum distributions of n- and n + mesons in CC collisions. Points: o — n- mesons, * — n + mesons, identified by ionization, A — n + mesons, additional identification.

angle between the axis of the ellipsoid and the beam direction 6flow is 12° for the high-multiplicity events.

The flows of protons and n- mesons have been observed at Dubna by the SKM-200-GIBS Collaboration [24, 25] in central CNe and CCu collisions at a momentum of 4.5 GeV/c per nucleon. In inelastic CC collisions at a momentum of 4.2 GeV/c per nucleon registered in the 2-m Propane Bubble Chamber, the flow of protons has been obtained only on the part of statistics [26]. The most complete experimental data of collective flow effects are presented in the review article [27].

2. EXPERIMENTAL DATA

In this paper the collective flows of protons, n-mesons, and projectile light fragments (d, t, 3He, 4He) in CC collisions at a momentum of 4.2 GeV/c per nucleon, registered in the 2-m Propane Bubble Chamber of JINR, are studied.

The chamber was placed in a magnetic field of 1.5 T. The method of separation of CC collisions in propane, the processing of the data, identification of particles, and discussion of corrections is described in detail in [28]. The experimental data, apart from the unambiguously identified CC collisions with the probability of ue = 1, contains the sample of CC events with ue = 0.21. When studying the inclusive characteristics of CC collisions the distributions are obtained for the whole of C—propane collisions, taking into account the weight factor ue.

For the analysis of the collective flow of particles, the experimental data contained 15 692 unambiguously identified CC.

The study of collective flow phenomenon needs "event-by-event" analysis, which requires the exclusive analysis of each individual collision. In this connection, there has been a necessity to perform an additional identification of n + mesons, since in the propane chamber the n + mesons have been identified in the narrow interval of momenta (up to 0.5 GeV/c). The weight (probability) is defined statistically for the particles with the momentum p> 0.5 GeV/c with which the particle satisfies the hypothesis of n+ meson or proton for the whole ensemble of CC collisions. However, the group of particles has remained with unseparated hypothesis (p, n +), the most part of which form the protons. The separation of the group of CC collisions with ue = 1 and the necessity of unambiguous separation of protons and n + mesons have led to the difference in the momentum distributions of n- and n + mesons. To remove this difference, the correction of the n + -meson identification has been carried out. The procedure has been performed statistically, based on the well-founded assumption that for symmetric nuclear collisions the distributions of n- and n + mesons are similar.

In Fig. 1 the momentum and transverse-momentum distributions of n- and mesons are presented with the previous and additional identifications. One can see from Fig. 1 that a small difference in the momentum distribution of n+ mesons is removed.

Only participant protons have been selected for the analysis. With this purpose, from the whole ensemble of particles the fragments of the target (p < 0.3 GeV/c), projectile stripping fragments (p > > 3 GeV/c and angle 6 < 4°), and also the light

fragments of the projectile with Z > 1 (3He, 4He) identified by ionization visually, and Z = 1 (d, t) with p > 5 GeV/c have been excluded.

The following restriction — the choice of the events with the number of participant protons Wpart > 4 — is caused by the necessity to obtain reliable results at low multiplicity. In consequence, from the inelastic CC collisions the group of 9490 semicentral collisions with (58 078 participant protons) have been selected.

3. TRANSVERSE-FLOW ANALYSIS METHOD

The method of Danielewicz and Odyniec [2] has been used for study of collective flow of protons, based on the summation of the transverse momenta of selected particles [2]. Most of experimental data at energies below 4 GeV/nucleon have been analyzed by this method. It gives satisfactory results even at small available statistics obtained by the film detectors.

The reaction plane vector Q in each individual event is defined only by the participant protons in the c.m. system:

Q = UiPTi

(1)

i=1

Qj

y] WiPTi i=j

The dependence of the mean transverse momentum of each particle in the reaction plane (px) on the rapidity y is constructed. The average transverse momentum (p'x(y)) is obtained by averaging over all events in the corresponding intervals of rapidity.

It is known [4] that the estimated reaction plane differs from the true one, due to the finite number of particles in each event. The component px in the true reaction plane is systematically larger than the component p'x in the estimated plane, hence,

(Px ) = (p'x )/(cOS 0),

(4)

where pTi is the transverse momentum of particle i; the weight factor ui is taken as 1 for yi > 0 and —1 for yi < 0, where yi is the rapidity of particle i, and n is the number of participant protons in the event. This choice leads to the result that the forward and backward moving particles, which are azimuthally anticorrelated if there is a collective transverse flow, will contribute equally to Q.

The reaction plane is the plane containing the impact parameter b and beam axis. Taking into account that the definition of b experimentally is not possible, in the transverse momentum analysis method of Danielewicz and Odyniec [2] the vector b is replaced by Q. If one projects the transverse momentum of each particle pTi onto the summary momentum, the autocorrelations will arise, from which it will be very difficult to extract true dynamic correlations. To remove the auto

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