ЯДЕРНАЯ ФИЗИКА, 2012, том 75, № 4, с. 515-523
ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
COMPARISON OF CHARACTERISTICS OF A°(1232) PRODUCED IN p12C AND d12C COLLISIONS AT 4.2 A GeV/c
©2012 Kh. K. Olimov1),2)*, Mahnaz Q. Haseeb1)**, Imran Khan1)
Received July 11, 2011; in final form, October 6, 2011
Reconstructed momentum, transverse momentum, kinetic energy, rapidity, and emission angle distributions along with their mean values were compared for Д0(1232) resonances produced in p12C and d12C collisions at 4.2 A GeV/c. Mean momentum, transverse momentum, and rapidity of protons and negative pions coming from Д0(1232) decay were extracted and compared with the corresponding mean values for protons and n- mesons in experiment and the relevant model calculations.
INTRODUCTION
A(1232) Resonances are unique tools to probe various properties of the medium created in relativistic nuclear collisions due to their very short lifetime and their coupling to the medium. These resonances dominate pion-production phenomena at beam kinetic energies of the order of a few GeV/nucleon, decaying in 99% of cases into nucleons and pions (A — Nn). The mass and width of A(1232) generated in nuclear matter formed in relativistic hadron-nucleus and nucleus—nucleus collisions were found to decrease significantly [1—6] compared to those (MAnn = 1232 MeV/c2, r = 115-120 MeV/c2) [7] of A(1232) produced in nucleon-nucleon collisions. For example, in near-central NiNi and AuAu [1] and NiCu [5] collisions at incident energies between 1 and 2 A GeV the masses of A(1232) were found to be shifted by an average value -60 MeV/c2 relative to the mass of free nucleon resonance and the obtained widths were of the order of 50 MeV/c2. The reduction in mass and width of A(1232) produced in dense hadron matter in heavy-ion collisions was interpreted in terms of thermal and isobar models [1, 8]. This modification was also related to the values of hadronic density, temperature, and non-nucleon degrees of freedom in nuclear matter [1, 8-12]. However, still many experimental results on A(1232) production are nontrivial and not a single theoretical model can account for all the observed properties of A(1232) produced in hadron-nucleus and nucleus-nucleus
^Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan.
2)Physical-Technical Institute of SPA "Physics-Sun" of Uzbek Academy of Sciences, Tashkent. E-mail: olimov@comsats.edu.pk E-mail: mahnazhaseeb@comsats.edu.pk
collisions in a wide range of incident energies [1315]. The possibility of a phase transition of nuclear matter into state of quark-gluon plasma has been widely discussed and researched over the last few decades. The spectrum of nuclear matter on the border of the phase transition should be very complex, but it seems undoubted that the lower states of this spectrum are related to excitation of A(1232) and other resonances [13]. Thus, information on the properties of A(1232) resonances produced in nuclear matter is important for an in-depth understanding and interpretation of nucleus-nucleus collisions at ultrarelativistic energies [13, 14].
Experimental analysis of A(1232) production in relativistic hadron-nucleus and nucleus-nucleus collisions was done in a series of our papers [1622]. The present paper is a continuation of our recent works [20-22] in which analyses of A(1232) production and its various properties were carried out separately for p12C and d12C collisions at 4.2 A GeV/c. Comparison of the parameters and various characteristics of A0(1232) produced in d12C collisions with those of A0(1232) in p12C collisions at the same initial momentum can give us information about the role of the weakly bound neutron of projectile deuterons in A0(1232) production. The main aim of the present analysis is to extract further experimental information on A0(1232) production by comparing the momentum, transverse momentum, kinetic energy, rapidity, and angular spectra and their mean values for A0(1232) in p12C and d12C collisions at 4.2 GeV/c per nucleon. In the present work we also extract the mean momentum, transverse momentum, and rapidity of protons and n- mesons coming from A0(1232) decay in p12C and d12C collisions at 4.2 A GeV/c. The extracted mean characteristics of protons and n- mesons from A0(1232) decay
Table 1. The experimental mean multiplicities of participant protons and n mesons in p12C and d12 C collisions at 4.2 A GeV/c in comparison with those from model calculations [23—27]
Collision type P12 C d12 C
Particle type protons 7T mesons protons n mesons
Experiment DCM FRITIOF QGSM 1.83 ±0.04 1.79 ±0.01 1.99 ±0.02 1.77 ±0.03 0.36 ±0.01 0.42 ±0.01 0.41 ±0.01 0.35 ±0.01 1.94 ±0.06 1.97 ±0.01 2.05 ±0.03 1.86 ±0.02 0.66 ±0.01 0.68 ± 0.02 0.70 ±0.01 0.63 ±0.01
are also compared with the corresponding mean values for participant protons and n- mesons [23—27] calculated using Dubna Cascade Model (DCM) [28, 29], FRITIOF model [30-34], and Quark-Gluon-String Model (QGSM) [35].
EXPERIMENTAL PROCEDURES AND ANALYSIS
The experimental data were obtained on the basis of processing stereophotographs from the 2-m propane (C3H8) bubble chamber of the Laboratory of High Energy of the Joint Institute for Nuclear Research (JINR, Dubna, Russia) placed in a magnetic field of strength 1.5 T and irradiated with a beam of deuterons and protons accelerated to a momentum of 4.2 GeV/c per nucleon at the JINR synchrophasotron. To select events of inelastic d12C (p12C) interactions in the total set of proton interactions with propane, we used criteria based on the determination of the total charge of secondary particles, the presence of protons emitted into backward hemisphere, the number of n- mesons produced, etc., as described in detail in [23, 24, 36, 37]. These criteria separate about 70% of the total number of inelastic d12C (p12C) events that was estimated by using the known cross
Table 2. Comparison of the mean values of momentum, kinetic energy, transverse momentum, emission angle, and rapidity of A0(1232) resonances produced in d12C and p12 C collisions at 4.2 A GeV/c in the laboratory frame
Collision type d12 C p12 C
(p) [MeV/c] 1483 ± 29+g 1103 ± 27+15
(T) [MeV] 797 ± 22+45 523 ± 20+94
(pt) [MeV/c] 455 ± 8+4 422 ± 9+31
(19) [deg] 29 ±1+2 36 ± 1+1
(y) 0.78 1 Q 0i+0.02 m u.ui_o 03 0.59 ± 0.02+0 01
sections for d(p) + p and d(p) + 12 C interactions and the proton—carbon ratio in the propane molecule. The remaining 30% interactions were extracted statistically from d(p) +p and d(p) + 12C collision events in propane molecules by introducing the relevant weights. The weights were determined in such a way that the numbers of events occurring on carbon and hydrogen corresponded to the numbers expected on the basis of the known cross sections for inelastic interactions [23, 25, 37—39]. It should be mentioned that d + p (p + p) interaction events on quasi-free protons of carbon nuclei constitute approximately 69% of all the d + p (p + p) interaction events on quasi-free protons of propane molecules. This is due to the fact that 18 out of total 26 protons in propane molecule belong to carbon nuclei. The corrections to account for the loss of particles emitted under large angle to object plane of the camera were introduced. The separation of protons and mesons was done visually, based on their ionization up to momentum p & 0.75 GeV/c. However, under conditions of the present experiment, protons and positively charged pions are identified unambiguously up to momentum of 0.5 GeV/c. Therefore all the positively charged particles with a momentum higher than 0.5 GeV/c were assigned weights determining the probability that a given particle is a proton or a meson. All the negatively charged particles were identified as n-mesons, since n- mesons make up the main fraction (>95%) among the negatively charged particles, and admixture of fast electrons and negative strange particles among them does not exceed 5%. Most of the pions and protons with momentum lower than 70 and 150 MeV/c, respectively, were not registered because of their short range in the propane bubble chamber. The average error in measuring angles of the secondary particles was 0.8°, while the mean relative error in determining momenta of the particles from the curvature of a track in the magnetic field was 11%. More details about the experimental procedures can be found in [23, 24, 36, 37].
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Fig. 1. Reconstructed experimental invariant-mass distribution (•) and the corresponding background distribution (o) forpn " pairs in p12C (a) and d12C (c) collisions at 4.2 A GeV/c. The corresponding mass distributions (•) for A0(1232)-resonances produced in p12C (b) and d12 C (d) collisions at 4.2 A GeV/c along with the Breit—Wigner fits (solid curves).
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Fig. 2. Transverse momentum distribution of all n (•) and n mesons comingfrom A0(1232) decay (o) in d12C (a) andp12C (b) collisions at 4.2 A GeV/c.
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In [23] protons with plab > 300 MeV/c (and excluding projectile spectator protons with momentum p > 3 GeV/c and emission angle relative to the beam direction d < 3° in the laboratory frame) in d12C collisions at 4.2 A GeV/c were classified as participant protons. Protons with plab > 300 MeV/c in p12 C collisions at 4.2 GeV/c were classified as participant protons in [23, 25, 34, 37, 38]. The experimental mean multiplicities of participant protons and n- mesons
in p12C and d12C collisions at 4.2 A GeV/c along with those calc
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