Role of two-nucleon mechanisms in pion photoproduction on nuclei in the region of high momentum transfers
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EMENTARY PARTICLES AND FIELDS Experiment
Role of Two-Nucleon Mechanisms in Pion Photoproduction on Nuclei in the Region of High Momentum Transfers M. V. Egorov1)* and A. I. Fix2) Received April 25, 2012; in final form, September 12, 2012
Abstract—The role of two-nucleon mechanisms in pion photoproduction on nuclei was studied in the region of high momentum transfers to the residual nucleus. The process in which the photoproduction of negative pions on a 12 C nucleus is accompanied by proton emission was considered by way of example. The results of the calculations were compared with available experimental data. DOI: 10.1134/S1063778813050049
1. INTRODUCTION Meson photoproduction on p-shell nuclei of mass number in the range of A ≤ 16 traditionally stands out among meson–nucleus reactions. This is explained, first of all, by the existence of comparatively simple and simultaneously reliable models of such nuclei. For example, the extremely simple shell model involving a harmonic-oscillator potential makes it possible to describe electromagnetic form factors for the majority of p-shell nuclei over a substantially wide momentum-transfer range. The electromagnetic nature of the main process, which is pion photoproduction on a nucleon, is yet another advantage. Owing to the relative weakness of the electromagnetic interaction, we can restrict ourselves to the lowest order in the coupling constant, and this facilitates substantially both the calculations themselves and the interpretation of their results. The smallness of the electromagnetic coupling constant also means that photons are able to penetrate deep into the interior of nuclei, undergoing virtually no scattering and exerting no effect on their properties; therefore, they are an efficient tool for probing the nuclear structure. The majority of modern theoretical approaches to studying photomeson reactions on nuclei are based on the so-called single-interaction approximation, whose concept was developed, for example, in [1]. Within this approximation, one assumes that, because of the smallness of the incident-photon wavelength in relation to the characteristic nucleon– nucleon distance, the photon can interact at each 1)
National Research Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia. 2) National Research Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia. * E-mail: [email protected]
instant with only one target nucleon. As a consequence, the total transition operator T is accurately approximated by a coherent sum of respective onenucleon photoproduction operators tˆ; that is, T =
A
tˆb (i),
(1)
i=1
where the index b indicates that the nucleons in question are bound. In photoproduction reactions, the single-interaction approximation can be thought to be justified even at rather low energies. By way of example, we indicate that, at the laboratory photon energy of ωγ = 140 MeV, which corresponds to the threshold for π − -meson photoproduction on a proton, the photon wavelength of λγ = 1/ωγ ≈ 0.75 fm is shorter than the nucleon–nu
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