THEORY OF PHOTOPRODUCTION OF PIONS FROM NUCLEONS

Journal Article · Tue Oct 15 04:00:00 EDT 1957 · Physical Review (U.S.) Superseded in part by Phys. Rev. A, Phys. Rev. B: Solid State, Phys. Rev. C, and Phys. Rev. D

DOI:https://doi.org/10.1103/PhysRev.108.433· OSTI ID:4323857

Enoch, J; Sachs, R G; Wali, K C

A resonance theory of the pion-nucleon system based on a very general static model is extended to give an expression for the pion photoproduction amplitude. lt is found that for photon energies less than 500 Mev there are just two important terms in the amplitude, one of them describing direct photoejection of the pion and the other, photoexcitation of the J = 3/2, I = 3/2 resonance state of the nucleon. The first term is estimated by means of a weak-coupling, finite-source theory which is made gauge-invariant by introducing line currents in the source. Comparison with the threshold pi /sup +/ production leads to a coupling constant (without recoil correction) f/sup 2/ = 0.049. The behavior of the cross section at high energies does not appear to be consistent with this result. Much better agreement with the data above threshold is obtained for the uncorrected coupling constant f/sup 2/ = 0.07. However, serious discrepancies within the data make it impossible to resolve this difficulty at present. The behavior of the photoejection part of the pi /sup +/ cross section above the resonance is found to be sensitive to the form of the source function. With f/ sup 2/ = 0.049 and a Gaussian source function, inflnite cutoff comes closest to fitting the data. However a considerably better fit is obtained with f/sup 2/ = 0.07 and a cutoff of about 4 pion masses. The resonance term is sensitive to the form of the pionnucleon phase shift alpha /sub 33/ above the resonance. We find that the simple one-level form for the energy dependence of alpha /sub 33/ (Chew- Low curve) is not adequate to account for the data for high-energy photoproduction of pions; it is necessary to take account of the residue of higher resonances. Good agreement with the resonance term is obtained in terms of just two constant parameters,(script M), the strength of the resonance matrix element, and Q, the contribution of higher resonances to the phase shift. No supporting evidence for a previously suggested S wave, I = 1/2 resonance is found. (auth)

• Research Organization: Univ. of Wisconsin, Madison
• NSA Number: NSA-12-002592
• OSTI ID: 4323857
• Journal Information: Physical Review (U.S.) Superseded in part by Phys. Rev. A, Phys. Rev. B: Solid State, Phys. Rev. C, and Phys. Rev. D, Journal Name: Physical Review (U.S.) Superseded in part by Phys. Rev. A, Phys. Rev. B: Solid State, Phys. Rev. C, and Phys. Rev. D Vol. Vol: 108; ISSN PHRVA
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

COUPLING
CROSS SECTIONS
ENERGY
ENERGY RANGE
EXCITATION
INTERACTIONS
MASS
MATRICES
NUCLEONS
PHOTOPRODUCTION
PHYSICS
PIONS
RESONANCE