Photoproduction of Charged Pion Pairs andN*(1238)++in Hydrogen from 0.9 to 1.3 GeV

The momentum spectrum of negative pions produced in the reaction $\gamma +p\to {\pi }^{-}+{\pi }^{+}+p$ has been measured at eight photon laboratory energies from 0.9 to 1.3 GeV at c.m. angles from 7° to 150°. The reaction was produced in a liquid-hydrogen target illuminated by a bremsstrahlung beam from the Caltech synchrotron. The ${\pi }^{-}$ were detected and momentum analyzed with a magnetic spectrometer employing a combination of scintillation counters and Cherenkov counters. The incident photon energy was fixed by using the technique of bremsstrahlung subtraction. The cross section for the pseudo-two-body reaction $\gamma +p\to {\pi }^{-}+N^{*}{\mathrm{}\left(1238\right)\mathrm{}}^{++}$ was obtained by fitting the ${\pi }^{-}$ momentum spectrum at each angle and energy with a linear combination of a resonance term and three-body phase space. The angular distribution of the ${\pi }^{-}$ in $N^{*}$ production shows the small-angle peak and decrease near 0° predicted by the one-pion-exchange (OPE) model. Gauge-invariant models are in poorer agreement with the data. Moravcsik fits to the angular distributions are presented, and are extrapolated to obtain a value for the $\pi N{N}^{*}$ coupling constant of 23.1±2.0 Ge${\mathrm{V}}^{-2\mathrm{}}$, in fair agreement with the value obtained from the width of the $N^{*}\mathrm{}\left(1238\right)\mathrm{}$. The total cross section for pion pair production decreases smoothly from 78.9±2.9 μb at 0.93 GeV to 59.1±5.2 μb at 1.29 GeV, whereas the $N^{*}$ part of the cross section decreases from 45.0±2.4 μb to 18.2±3.5 μb over the same range. There is no strong evidence for formation of the $N^{*}\mathrm{}\left(1688\right)\mathrm{}$ as an intermediate state.