The double-differential coincidence cross section for the reaction {sup 1}H({ital e},{ital e}{prime}{ital p}){pi}{sup 0} has been measured at {ital Q}{sup 2}=0.1(GeV/{ital c}){sup 2} in a range in the invariant mass of 2--14 MeV above threshold. From the angular dependence of the data values of the pion {ital s}-wave multipoles {ital E}{sub 0+} and {ital L}{sub 0+} have been extracted. The results are compared to predictions from chiral perturbation theory and models based on an effective Lagrangian. None of the models is able to describe both the {ital E}{sub 0+} and the {ital L}{sub 0+} multipoles.

/pdf/neutral-pion-electroproduction-on-the-proton-near-threshold-4ykvpihuyb.pdf

Neutral-Pion Electroproduction on the Proton near Threshold

The differential yield of the reaction 16O(γ, π
 +
 p) at the maximum photon energy of 450 MeV was measured in the region of high momentum transfers to the residual nuclear system. The experimental results obtained for the 16O nucleus and the cross section measured earlier for the reaction 12C(γ, π
 +
 p) were analyzed on the basis of a model that takes into account the admixture of isobar states in the nuclear wave function. The probabilities for the delta-isobar configurations in the ground states of the carbon and oxygen nuclei per nucleon were estimated empirically at 0.012 ± 0.003 ± 0.002 and 0.019 ± 0.003 ± 0.003, respectively.

Delta-isobar configurations in 12 C and 16 O nuclei

We present an analysis of the spectator mechanism of $\Delta $-isobar production in the pion photoproduction on nuclei with the emission of two nucleons. The reaction mechanism is studied within the framework of the $\Delta N$-correlation model, which considers the isobar and nucleon of the $\Delta N$-system produced in the nucleus at the virtual $NN \to \Delta N$ transition, to be in a dynamic relationship. The two-particle transition operator for nuclei is obtained by the \textit{S}-matrix approach. We consider the properties of the spectator mechanism of isobar production using the example of the reaction $^{16}$O$ (\gamma ,\ \pi ^{ - }pn)^{14}$O. Numerical estimates of the cross section are obtained in the kinematic region, where it is possible to expect the manifestation of bound isobar-nuclear states.

Spectator isobar production in the $A\left( {\gamma ,\pi NN} \right)B$ reaction

Spectator isobar production in the A(γ,πNN)B reaction

A model of pion photoproduction on nuclei with emission of two nucleons that takes account of the spectator mechanism of knockout of an isobar produced in the nucleus at the virtual transition NN → ∆N in the ground state of the nuclei is presented. An analysis of experimental data for the reaction 12C(γ, π
 −
 p) at large momentum transfers is performed.

Spectator Mechanism of ∆-Isobar Knockout

Expressions for the one- and two-particle density matrices of nuclei are derived with allowance for the nucleon and isobar configurations. Mechanisms of photoproduction of pion-nucleon pairs corresponding to individual components of the density matrices are considered.

Nuclear density matrix of the A(γ,π N) B process

A model of the pion photoproduction on nuclei with emission of two nucleons is presented that takes account of ΔN correlations due to the virtual transitions NN → ΔN → NN in the ground state of nuclei.. The main components of the model are the two-particle density matrix and the γΔ → Nπ and γN → N′π transition operators. Direct reaction mechanisms are considered which follow from the structure of the density matrix. An analysis is made of the experimental data of the reaction 12C(γ, π+
 p).

Direct Mechanisms of Pion Production in the Reaction A(γ, πNN)B

The state of the art of the problem on threshold neutral-pion production on protons by photons and electrons is examined. New experimental data on the threshold of pion production are analyzed within the phenomenological model based on the gauge invariance, the hypothesis of partial conservation of the axial-vector current, and allowance for obvious chiral-symmetry breaking. The amplitudes of the p-wave threshold multipoles and the photon asymmetry are derived for the pion photoproduction on protons with a photon energy of 159.5 MeV. The differential cross sections are calculated as functions of the pion emission angle and amplitudes of the s- and p-wave multipoles for the threshold pion electroproduction on protons at the squared momentum transfer k2 = 0.05 GeV2/c2. The total reaction cross sections are also obtained as functions of k2. The results obtained are compared with new experimental data and available theoretical predictions.

Near-threshold photo- and electroproduction of neutral pions on protons

An analysis of the effective polarization of the ∆++ isobar formed in a nucleus as a result of the virtual transition pp→∆++
 n, and manifested in the reaction 16O(γ, π+ p)15C , is performed. The polarization of the isobar is analyzed within the framework of the ∆N correlation model of the A(γ, πN)B reaction, which takes account of the dynamic coupling between the isobar and a nucleon of the system ∆++
 n .

Isobar Configurations in the Ground State of Nuclei. Effective Polarization of an Isobar

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