ЯДЕРНАЯ ФИЗИКА, 2014, том 77, № 5, с. 648-657
ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
CENTRALITY DEPENDENCE OF RAPIDITY SPECTRA OF NEGATIVE PIONS IN 12C + 12C AND 12C + 181Ta COLLISIONS AT 4.2 GeV/c
PER NUCLEON
© 2014 Kh. K. Olimov1),2)*, Sayyed A. Hadi3), Mahnaz Q. Haseeb1)**
Received July 19,2013
The centrality dependences of the experimental rapidity as well as transverse momentum versus rapidity spectra of negative pions were analyzed quantitatively in 12 C + 12 C and 12 C + 181 Ta collisions at 4.2 GeV/c per nucleon using fitting the pion spectra by Gaussian distribution function. The experimental results were compared systematically with the predictions of the Quark—Gluon—String Model (QGSM) adapted to intermediate energies.
DOI: 10.7868/S0044002714050158
INTRODUCTION
A large number of pions are produced in relativists hadron—nucleus and nucleus—nucleus collisions. Therefore these pions may carry important information on dynamics of a nuclear collision. It should be mentioned that the negatively charged pions can be unambiguously separated from the other particles produced in nuclear collisions. The pions are the particles produced predominantly at the energies of Dubna synchrophasotron. Production of a large fraction of pions in hadron—nucleus and nucleus— nucleus collisions at the energies of the order of few GeV/nucleon was shown to be due to excitation of baryon resonances, which finally decay into nucleons and pions. In [1—9] it was estimated that the significant fraction of pions produced in bubble chamber experiments of Joint Institute for Nuclear Research (JINR, Dubna, Russia) came from decay of A resonances.
The rapidity distributions of pions in relativistic nuclear collisions were studied earlier in [10—13]. The momentum, transverse momentum, and rapidity distributions of negative pions produced in Mg + + Mg collisions at 4.3 GeV/c per nucleon were analyzed in [13]. It was observed [13] that the rapidity distributions of pions followed a Gaussian shape.
'■'Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan.
2)Physical—Technical Institute of SPA "Physics—Sun" of Uzbek Academy of Sciences, Tashkent.
3)Karakoram International University, Gilgit-Baltistan, Pakistan.
E-mail: olimov@comsats.edu.pk E-mail: mahnazhaseeb@comsats.edu.pk
Analysis of pion rapidity distribution showed that the central rapidity region was occupied with pions of larger transverse momentum as compared to the fragmentation region of interacting nuclei. The experimental results could be described satisfactorily by the Quark-Gluon-String Model (QGSM) [1417]. The rapidity distributions of negative pions in (p, d, a, C)C and (d, a, C)Ta collisions at 4.2 GeV/c per nucleon were analyzed in various intervals of transverse momentum of n- mesons in [11, 12]. With increasing the transverse momentum of negative pions, the fraction of n- mesons in the central rapidity region increased, whereas the corresponding fraction in the fragmentation region of colliding nuclei decreased [11, 12]. The centrality dependence of pion rapidity spectra in minimum bias 12 C +12 C and 12C + 181 Ta collisions at a momentum of 4.2 GeV/c per nucleon was partly studied in early work [10] on statistics of 7900 12C + 12C and 2000 12C + 181 Ta collision events. It was shown [10] that the pion rapidity spectrum was Gaussian in shape regardless of the target mass number and collision centrality. The QGSM could reproduce quite satisfactorily the main features of pion rapidity as well as transverse momentum spectra [10].
This work is a continuation of our recent papers [18, 19] devoted to analysis of various characteristics of negative pions produced in nucleus-nucleus collisions at 4.2 GeV/c per nucleon. The aim of this work is to study the dependences of experimental rapidity distributions of negative pions produced in 12C + 12C and 12C + 181Ta collisions at a momentum of 4.2 GeV/c per nucleon on the mass of target nucleus and collision centrality. Also the dependence
Table 1. Mean multiplicities per event of negative pions and participant protons and the average values of rapidity and transverse momentum of n- mesons in 12C + 12C and 12C + 181Ta collisions at 4.2 GeV/c per nucleon (The mean rapidities are calculated in cms of nucleon—nucleon collisions at 4.2 GeV/c.)
Type (n(7T-)> {^part.prot ) (2/cm) (pt(7r-)>, GeV/c
12C + 12C Exper. 1.45 ±0.01 4.35 ±0.02 —0.016 ± 0.005 0.242 ± 0.001
QGSM 1.59 ±0.01 4.00 ±0.02 0.007 ±0.005 0.219 ±0.001
12 C + 181 Ta Exper. 3.50 ±0.10 13.3 ±0.2 -0.34 ±0.01 0.217 ±0.002
QGSM 5.16 ±0.09 14.4 ±0.2 -0.38 ±0.01 0.191 ±0.001
of thetransverse-momentum-versus-rapidity spectra of negative pions on the mass of target nucleus and collision centrality will be investigated. In order to perform quantitative analysis, the widths and centers of rapidity as well as the (pt) versus ycm spectra of negative pions in 12C + 12C and 12C + 181 Ta collisions will be extracted from fitting these spectra with Gaussian distribution function. The experimental results will be compared systematically with the corresponding results calculated using QGSM [14— 17].
In the present work, we use QGSM developed to describe hadron—nucleus and nucleus—nucleus collisions at intermediate and relativistic energies [14—17]. In the QGSM, hadron production takes place via formation and decay of quark gluon strings. In the present analysis, we use the version of QGSM [15] adapted to the range of intermediate energies (y/snn ^ 4 GeV). The incident momentum of 4.2 GeV/c per nucleon for the collisions analyzed in the present paper corresponds to incident kinetic energy 3.37 GeV per nucleon and nucleon—nucleon center-of-mass system (cms) energy yjsnn = = 3.14 GeV. The QGSM is based on the Regge and string phenomenology of particle production in inelastic binary hadron collisions. To describe the evolution of the hadron and quark—gluon phases, a coupled system of Boltzmann-like kinetic equations was used in the model. The model accounted for the rescattering of hadrons as well as resonance production and decay processes. The time of hadron formation was taken into account in the QGSM. At nucleon—nucleon c.m. energy yjsnn = 3.14 GeV the masses of strings are smaller than 2 GeV, and these strings fragment predominantly (~90%) through two-particle decay channel.
EXPERIMENTAL PROCEDURES AND ANALYSIS
The experimental data analyzed in the present work were obtained using 2-m propane (C3H8) bub-
ble chamber of Laboratory of High Energies of JINR (Dubna, Russia). The 2-m propane bubble chamber was placed in a magnetic field of strength 1.5 T [10, 20—26]. Three tantalum 181 Ta foils were also placed inside the propane bubble chamber. Thickness of each tantalum foil was 1 mm, and the separation distance between foils was 93 mm. The bubble chamber was then irradiated with beams of 12 C nuclei accelerated to a momentum of 4.2 GeV/c per nucleon at Dubna synchrophasotron. The 12C nuclei were made to interact with the carbon nuclei and protons of the propane molecules and the tantalum 181 Ta foils placed in the propane bubble chamber [10]. Methods of selection of inelastic 12C + 12C and 12C + 181 Ta collision events in this experiment were explained in detail in [10, 22,24-26].
Threshold for detection of negative pions was 70 MeV/c for12C + 12C collisions, whereas for12C + + 181 Ta collisions it was 80 MeV/c. In some momentum and angular intervals, the particles could not be detected with 100% efficiency. To account for the loss of particles emitted under large angles to object plane of the camera as well as for the particles absorbed by tantalum foils, the relevant corrections were introduced [10, 24-26]. The average uncertainty in measurement of emission angle of negative pions was 0.8°. The mean relative uncertainty of momentum measurement of n- mesons from the curvature of their tracks in propane bubble chamber was about 6%. All the negatively charged particles, except identified electrons, were considered to be n- mesons. Admixtures of unidentified electrons and negative strange particles among n- mesons did not exceed 5% and 1%, respectively. Statistics of the experimental data consist of 20528 and 2420 12C + 12C and 12C + 181 Ta minimum-bias collision events, respectively, with practically all the secondary particles detected under 4n solid angle. For sake of comparison with the experimental data, 30000 and 6000 minimum-bias 12C + 12C and 12C + 181 Ta
Fig. 1. The experimental rapidity distributions of negative pions in 12 C + 12 C (•) and 12 C + 181 Ta (o) collisions at 4.2 A GeV/c. The corresponding QGSM spectra (a) and fits by Gaussian function (b) are given by the solid curves. The spectra are obtained in cms of nucleon—nucleon collisions at 4.2 GeV/c. The distributions are normalized by the total number Nev of corresponding inelastic events.
collision events at 4.2 A GeV/c, respectively, were simulated using the QGSM.
In our experiment, the spectator protons are the protons with momenta p > 3 GeV/c and emission angle d < 4° (projectile spectators), and protons with momenta p < 0.3 GeV/c (target spectators) [10, 24— 26]. The participant protons are the protons remaining after elimination of spectator protons. Comparison of the mean multiplicities per event of negative pions and participant protons and of the average values of rapidity and transverse momentum of n-mesons in 12 C + 12 C and 12 C + 181 Ta collisions at 4.2 GeV/c per nucleon both in the experiment and QGSM is shown in Table 1.
Comparison of the experimental and QGSM rapidity distributions of negative pions in 12 C + 12 C and 12C + 181 Ta collisions at a momentum of 4.2 GeV/c per nucleon is presented in Fig. 1a. All the spectra in Fig. 1 and the figures that follow are obtained in cms of nucleon—nucleon collisions at 4.2 GeV/ c (Vcm ~ « 1.1 at this incident momentum). As observed from Fig. 1a, the rapidity distribution of negative pions in 12 C + 12 C collisions
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