A continuous-energy partial-wave analysis of 4148 data points on the processes $\gamma p\to {\pi }^{+}p$, $\gamma p\to {\pi }^{0}p$, and $\gamma n\to {\pi }^{-}p$ in the range of center-of-mass energy from 1160 to 1780 MeV has been made. The method used is parameterization of resonances and background in the imaginary parts of the amplitudes, with the real parts being calculated from fixed-$t$ dispersion relations, thus ensuring a proper treatment of the Born terms. It is found that the imaginary parts of the amplitudes are resonance dominated, though not resonance saturated. Many $N^{*}N\gamma$ couplings (of both isospin-½ and isospin-$\frac{3}{2}$ resonances, $N^{*}$ are determined for the first time. A comparison of the signs and magnitudes of the various resonance formation partial-wave amplitudes (depending on the product of the $N^{*}N\gamma$ and $N^{*}N\pi$ couplings) is made to the predictions of the essentially parameterless naive quark model. The critical comparison of the signs is found to be extremely favorable to that model, while there is an over-all qualitative agreement in magnitude.

Close

PrevNext

View figure in article