Production of the N*(1238) nucleon isobar by photons of energy up to 6 BeV

R. Hargraves, R. Hargraves, B. Kendall, R. E. Lanou, A. M. Shapiro, M. Widgoff, G. E. Fischer, C. Bordner, A. E. Brenner, M. E. Law, U. Maor, T. A. O'Halloran, F. D. Rudnick, K. Strauch, J. C. Street, J. J. Szymanski, P. Bastien, B. T. Feld, V. K. Fischer, I. A. PlessA. Rogers, L. Rosenson, T. L. Watts, R. K. Yamamoto, G. Calvelli, F. Gasparini, L. Guerriero, J. Massimo, G. A. Salandin, L. Ventura, G. Voci, F. Waldner, A. Brandstetter, Y. Eisenberg, A. Levy, E. E. Ronat

Research output: Contribution to journalArticlepeer-review

Abstract

The reaction γ+p→N*(1238)+π- has been studied in a hydrogen bubble chamber exposed to a 4.8-6.0-BeV bremsstrahlung beam. This channel dominates the two-pion photoproduction reaction below the 1.05-BeV ρ0-meson threshold, and at higher energies accounts for a significant part of the pπ+π- events after subtraction of the pρ0 channel. Total and differential cross sections, and N*(1238)-decay angular correlations are presented over the energy range 0.5-1.8 BeV. These data are analyzed using a resonance model in which the reaction proceeds through the photoexcitation of the higher nucleon isobars-N**(1420), N**(1512), N**(1688), and possibly N**(1924). The only free parameters are the resonance strengths, which are fixed by a best fit to the total-cross-section curve. These same strengths account for the N*(1238) production angular distribution, but there remain some discrepancies between the predicted and observed N*-decay angular correlations in the lowest-energy regions covered by our fit. Thus, although the total cross sections and production angular distributions require an appreciable contribution from N**(1420) excitation, the decay angular correlations tend to minimize the contributions of this resonance. However, satisfactory agreement with all the data may be obtained with a downward shift of the N**(1512) (J=32-) resonance mass by ∼50 MeV. Comparison is also made between our data and the one-pion-exchange model with final-state interaction. Unless this model is drastically modified (as for example in the calculations of Stichel and Scholz), agreement with the observations is poor.

Original languageEnglish
Pages (from-to)1510-1522
Number of pages13
JournalPhysical Review
Volume163
Issue number5
DOIs
StatePublished - 1967
Externally publishedYes

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