Neutral Pion Photoproduction Research Articles

Neutral-pion photoproduction on protons near threshold.

A Comment on the Letter by E. Mazzucato et al., Phys. Rev. Lett. 57, 3144 (1986).Received 13 August 1987DOI:https://doi.org/10.1103/PhysRevLett.60.748©1988 American Physical Society

DWIA in the momentum space for (?, ?0) reaction near threshold

The differential and total cross-sections of neutral pion photoproduction off4He,12C and40Ca nuclei are calculated in the framework of the DWIA in the momentum space. It is shown that the inclusion of pion wave distortion gives a less than 25% increase of the total cross sections. A good agreement with experimental data is achieved by using the full elementary amplitude that includes the pion-momentum nonlinear terms and the terms depending on nucleon momenta.

Neutral pion photoproduction on carbon near threshold

An absolute measurement of π 0 photoproduction on carbon has been carried out in the threshold region (from 135.8 to 190 MeV) using tagged annihilation photons and a large solid angle π 0 detector. The cross section which includes inelastic production in addition to the dominant elastic coherent process is found to be significantly larger than the one measured in a previous experiment. It is compared to existing theoretical predictions.

Precise measurement of neutral-pion photoproduction on the proton near threshold.

An absolute measurement of ${\ensuremath{\pi}}^{0}$ photoproduction on the proton has been carried out in the threshold region (from 144.7 to 173 MeV) by use of tagged annihilation photons. The measured cross sections, differential in the recoiling-proton energy, are used to perform a multipole analysis which gives a value , in disagreement with low-energy-theorem predictions. Total cross sections and coefficients of the ${\ensuremath{\pi}}^{0}$ angular distribution are presented.

Photoproduction of neutral pions on 12C and 6Li nuclei at threshold

The total cross sections of photoproduction of neutral pions on 6Li and 12C nuclei have been measured in the photon energy region 10 MeV above threshold with steps of 2 MeV. The results are compared to the theoretical elastic cross section calculated in the plane wave impulse approximation. Calculated within the different models, the photopion cross sections disagree with each other by a factor of about 2 and are sensitive enough to discern the approximately equivalent formulations of the elementary amplitude.

Theoretical estimates of photoproduction cross sections for neutral subthreshold pions in carbon-carbon collisions.

Using the Weizsacher-Williams method of virtual quanta, total cross section estimates for the photoproduction of neutral subthreshold pions in carbon-carbon collisions at incident energies below 300 MeV/nucleon are made. Comparisons with recent experimental data indicate that the photoproduction mechanism makes an insignificant contribution to these measured cross sections.

Photoproduction of neutral pions to discrete nuclear states

We study the photoproduction of neutral pions on nuclei to discrete final states at intermediate energies. Both photoproduction and distortion of the π 0 are dominated by Δ-excitation, which requires a careful treatment of the Δ-nucleus dynamics. We use the Δ-hole approach, which has been used to describe a variety of pion- and photon-induced nuclear reactions. It is shown that π 0 photoproduction is very sensitive to medium effects of Δ-propagation and can therefore be used to investigate the Δ-nucleus interaction. The validity of the distorted-wave impulse approximation for this reaction is examined. Results for low-lying excitations in 12C are compared with the available data.

Low-energy pion-nucleus interaction in relativistic mean-field nuclear theory

The relativistic many-body approach to the pion-nucleus interaction is applied to shifts and widths of pi -atomic states, low-energy pion-nucleus elastic scattering and coherent photoproduction of neutral pions. The approach contains only two adjustable parameters describing the real and imaginary parts of the pion-nucleus optical potential. The agreement with experiment is at least as good as that in conventional theory which contains six parameters.

Measurement of coherent π0 photoproduction on3He and3H in the resonance region

Neutral pion photoproduction has been measured on3He and3H nuclei in theΔ(1232) resonance region. Resonance averaged cross-sections are presented as a function of momentum transfer and compared to theoretical calculations.

Coherent pion photoproduction

The photoproduction of neutral pions is studied in the distorted wave impulse approximation. The cross sections are calculated for pion production from $^{4}\mathrm{He}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{40}\mathrm{Ca}$ and are found to be in good agreement with the available data. The present results compare well with the theoretical calculations carried out in the framework of the isobar-doorway model.

Amplitude analysis for pion photoproduction in the optimal formalism

Data for neutral-pion photoproduction between 1.3 and 2.1 GeV and at four different angles are analyzed in terms of the four complex amplitudes, by using the optimal formalism. Although the sets of data are incomplete for a complete determination of these amplitudes, with the help of the optimal formalism one can find frames in which the incomplete sets of data yield an accurate determination of most of the amplitude parameters. The results show many interesting features which can be considered as clues toward finding the proper dynamics of such a reaction. Among these features we see evidence that it is possible to find an amplitude formalism in whichall four complex amplitudes determined from the data have the same phase (and hence can all be considered real).

Regge cuts and neutral pion electroproduction

A complete FESR-plus-Regge analysis is made of neutral pion photoproduction and electroproduction to understand the Regge cut and its dependence onk2, the square of the virtual photon mass. A SCRAM type of model is essential, but contrary to previous expectations the range of the photon-proton interaction which gives rise to the cut increases with-k2, and is not related to the pointlike, scaling behaviour of deep inelastic scattering, which arises from much simpler parton-model diagrams.

Pionic and electromagnetic excitation of A* resonances

The nuclear excitation spectrum in the Delta (3,3) energy range is studied by comparing elastic pion scattering with coherent neutral pion photoproduction. The corresponding multipole excitation strength distributions are found to be quite different. This emphasises the role of the electromagnetic excitations of nuclei for the investigation of the A* resonances which is complementary to their mesonic excitation. Results are presented for 12C( gamma , pi 0) 12C(GS); they are compared with those of the phenomenological isobar doorway model and of DWIA calculations.

Differential cross sections of neutral pion photoproduction from hydrogen in the energy region from 390 MeV to 975 MeV

The differential cross sections of the reaction γp→ π 0p have been measured at photon energies between 390 MeV and 975 MeV with an energy step of 20 or 25 MeV. The angular region covers 11 angles between 15° and 130° in the c.m.s. A combination of a magnetic spectrometer and photon detector was used for the measurement in the angular region from 50° to 130°. At forward angles of 15°, 35° and 50°, π 0 mesons were detected using a π 0 detector consisting of a pair of photon detectors. The data obtained with two different detection systems overlap at the angle of 50° and coincide with each other within the experimental error. The results are compared with those of phenomenological partial-wave analyses.

Recoil proton polarization of neutral pion photoproduction from protons in the energy range between 400 MeV and 1142 MeV

The recoil proton polarization of the reaction γp → π 0p was measured at a c.m. angle of 100° for incident photon energies between 451 and 1106 MeV, and at an angle of 130° for energies from 400 to 1142 MeV. One photon, decayed from a π 0 meson, and a recoil proton were detected in coincidence. Two kinds of polarization analyzer were employed. In the range of proton kinetic energy less than 420 MeV and higher than 346 MeV, carbon plates and liquid hydrogen were used for determining the polarization, respectively. The data given by the two polarimeter systems are in good agreement. Results are compared with recent phenomenological analyses. From the comparison between the present data and the polarized target data, the invariant amplitude A 3 can be estimated to be small.

Photoproduction of neutral pions at backward angles in the resonance region and the ?-cusp

At the Bonn 2.5 GeV electron synchrotron we have measured the differential cross section of the reaction γp→π0p at a pion CM angle of 170° and at photon energiesKγ between 0.6 and 1.8 GeV. In comparison to previous measurements the accuracy of the data was improved substantially. For the first time in neutral pion photoproduction a cusp structure at the η-threshold has been confirmed [1].

Threshold effects in neutral pion photoproduction reactions on few-body targets

We have constructed a dynamical model for the elementary γN→N π 0 reactions, which allows us to reproduce also the γD→D π 0 and γ 3He→ 3He π 0 reaction cross sections measured at threshold.

Measurements of the double polarisation parameters G and H in neutral pion photoproduction

Measurements have been made of the double polarisation parameters G and H in the photoproduction of neutral pions from protons, for incident photon energies between 1300 MeV and 2300 MeV and for pion c.m. angles between 50δ and 80δ. The results are compared with predictions from a recent comprehensive analysis of earlier photoproduction data.

Photoproduction of neutral pions in the region of the fourth nucleon resonance

We report the measurement of the differential cross section of the reaction γ +p→π0 +p at a photon energy of 1.4 GeV and pion c.m. angles between 60 and 175 degrees. The angular distribution confirms the simple quark model prediction of a pure magnetic excitation of theF37 (1950) resonance.

Neutral-pion photoproduction and proton Compton scattering at large angles

Neutral-pion photoproduction and proton Compton scattering at large angles