Elastic Scattering of Photons by Protons. I

Abstract

The results of some preliminary measurements on the elastic scattering of photons by protons are reported. In the region of the (3,3) resonance and immediately above, our measurements join smoothly with those previously reported at lower energies. The scattering cross section shows a peak in the vicinity of 300 Mev. There is indication of a cos 2 0 term in the center-of-mass angular distribution. W E report the results of some observations on the proton Compton effect, i.e., the elastic scattering of photons by protons:y+p->p'+y'.(1) This reaction appears to be adequately described by a modified Klein-Nishina formula 1 below the threshold for meson production; above this threshold, however, mesonic processes dominate the scattering. By making measurements in this region, it can be hoped that important information about the meson-nucleon system can be obtained. 2,3 Observation of reaction (1) above the meson threshold is made more difficult by the presence of neutral photopions, y+P -> p'+Tfi -> p'+y'+y",whose production kinematics resemble those of reaction (1), and whose decay photons can have energies as high as that of the elastically scattered quantum. Reaction (2) has a cross section about two orders higher than that of (1). EXPERIMENTAL METHODThe photons to be scattered were obtained in the form of a beam of bremsstrahlung from the Cornell 1.2-Bev electron synchrotron. The machine energy was adjusted so as to produce a photon spectrum with a selected upper limit Wo for each run. Discrimination in favor of reaction (1) relied on two factors: First, measurements were carried out at such angles that the recoil protons from (1) were of longer range than any that could be produced by (2), given the maximum incident energy Wo; second, the angular correlation between the scattered photon and the recoil proton was studied. This correlation is exact (determined by experimental geometric factors) for reaction (1), but is smeared out by the decay kinematics of the neutral pions from reaction (2).Our apparatus is shown in Fig. 1. A beam of brems-* Supported by joint contract of Office of Naval Research and the U. S. Atomic Energy Commission.

Proton Compton effect in the ?(1232) energy region

“…Nevertheless, the information provided by these experiments is still rather incomplete due to the special difficulties which will be discussed below. In particular in the resonance region there exist only few measurements [1][2][3][4][5] compared with, e.g., the data of pion photoproduction which are nowadays available. Systematical measurements of angular distributions are needed for a theoretical analysis of the process (1).…”

Model-independent connections between pion photoproduction and Compton scattering in theΔ(1232)region

“…In this energy region, there exist four data sets on differential cross sections: The three oldest data sets are from the Cornell group [32], the Tokyo group [33], and the Illinois group [34]. Of these, the Tokyo group had the best photon energy resolution to data ( + 5 MeV), but their work suffers from very poor statistics.…”

“…3. (a) Experimental differential cross section for the Compton scattering on protons at e,,, =90" in the A (1232) excitation region, as measured in various laboratories. Circles, Genzel et al [lo]; triangles, De Wire et al [32]; diamonds, Nagashima [33]; and squares, Gray and Hansen [34]. Notice the long horizontal error bar for the energy resolution.…”

“…The dark bands are due to the uncertainties in the photo-pion multipoles. The differential cross-section data in (a)-(c) are indicated by circles [lo], triangles [32], diamonds [33], and squares [34]. tion, the smallness of real parts of Compton amplitudes around E,=320 MeV encourages us to discuss photon asymmetry and related observables, assuming the absence of the real parts of Compton amplitudes.…”

Polarized Compton Scattering from the Proton

“…These can be grouped into two general categories, those based on the baryon resonance spectrum or its underlying quark structure [5][6][7], and those relying upon unitarity and dispersion relations to phenomenologically describe elastic scattering in terms of photopion production [3,4,8-10]. These calculations give significantly different predictions, particularly in the region of the D resonance.A number of measurements of proton Compton scattering have been reported [1,[11][12][13][14][15][16], and several authors of dispersion calculations have pointed out a significant inconsistency between many of these experiments and p photoproduction data near the peak of the D [3,8-10]. Compton scattering can be described by six independent amplitudes.…”