Production of theN*(1238)Nucleon Isobar by Photons of Energy up to 6 BeV

Abstract

The reaction $\gamma +p\to N^{*}\mathrm{}\left(1238\right)+{\pi }^{-}$ 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 ${\rho }^0$-meson threshold, and at higher energies accounts for a significant part of the $p{\pi }^{+}{\pi }^{-}$ events after subtraction of the $p{\rho }^0$ channel. Total and differential cross sections, and $N^{*}\mathrm{}\left(1238\right)\mathrm{}$-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—$N^{**}\mathrm{}\left(1420\right)\mathrm{}$, $N^{**}\mathrm{}\left(1512\right)\mathrm{}$, $N^{**}\mathrm{}\left(1688\right)\mathrm{}$, and possibly $N^{**}\mathrm{}\left(1924\right)\mathrm{}$. 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 $N^{*}\mathrm{}\left(1238\right)\mathrm{}$ 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 $N^{**}\mathrm{}\left(1420\right)\mathrm{}$ 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 $N^{**}\mathrm{}\left(1512\right)\mathrm{}$ ($J={\frac{3}{2}}^{-}$) 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.