Coherent Photoproduction of Neutral Pseudoscalar Mesons $$\boldsymbol{\pi^{0}\pi^{0}}$$ and $$\boldsymbol{\pi^{0}\eta}$$
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EMENTARY PARTICLES AND FIELDS Experiment
Coherent Photoproduction of Neutral Pseudoscalar Mesons π 0π 0 and π 0η on Light Nuclei M. V. Egorov1)* Received April 22, 2020; revised April 22, 2020; accepted April 22, 2020
Abstract—The coherent photoproduction of π 0 π 0 and π 0 η on nuclei in the mass-number region of A 7 is studied systematically over the range between the threshold and the laboratory photon energy of Eγ = 2 GeV. The amplitude for photoproduction on nuclei is constructed on the basis of the impulse approximation with allowance for a number of processes responsible for meson–nucleus final-state interaction. The pion-rescattering processes πN → πN and πN → ηN occurring on spectator nucleons, π +/− π 0 π 0 and π +/− π 0 η production followed by π +/− absorption by spectator nucleons, and three-body π–NN and η–NN interactions are described for the first time within a unified microscopic approach. DOI: 10.1134/S1063778820050117
At the present time, searches of connections between excited nucleon states and multimeson decay modes play a dominant role, along with proton– antiproton annihilation, in solving the problem of missing resonances [1, 2]. On the other hand, the photoproduction of extremely light pairs of pseudoscalar neutral mesons in coherent processes on light nuclei is characterized by some other special features, including (i) the effect of nuclear medium on the elementary photoproduction operator and (ii) the distortion of the meson yield because of a strong meson–nucleus final-state interaction. A systematic investigation of these features within a unified model of the photoproduction of pairs of pseudoscalar mesons on light nuclei has yet to be performed. Instead, the appearance of precise experimental data on polarized total and differential cross sections [3– 6] gave impetus to independent simulations of γ → π 0 π 0 and γ → π 0 η reactions in partial-wave channels associated with one baryon resonance or another. In coherent processes of π 0 π 0 and π 0 η photoproduction on light nuclei, the influence of nuclear medium manifest itself both in the modification of the production operator itself as a function of the target charge number and in the effect of nonresonance contributions, which change the shape of the differential cross section with respect to the invariant meson mass [7] or increase its absolute value [8]. The presence of strong nonresonance contributions arising in γ → π 0 π 0 and γ → π 0 η processes because of final-state 1)
Russian Federal Nuclear Center–Zababakhin All-Russia Research Institute of Technical Physics, Snezhinsk, 456770 Russia. * E-mail: [email protected]
interaction should be reflected in the structure of the unified model of the photoproduction of π 0 π 0 and π 0 η pairs. Since only isoscalar photons are able to initiate γ → π 0 π 0 processes on nuclei of zero isospin, the respective total cross sections are small (about several tens of nanobarns) and are commensurate with the contribution of π 0 π 0 π 0 photoproduction [9] in the same energy region. The role of triple
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