Portal de Periódicos da CAPES

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

1967; American Institute of Physics; Volume: 160; Issue: 5 Linguagem: Inglês

10.1103/physrev.160.1215

1536-6065

M. G. Hauser,

Particle physics theoretical and experimental studies

The momentum spectrum of negative pions produced in the reaction $\ensuremath{\gamma}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}+{\ensuremath{\pi}}^{+}+p$ has been measured at eight photon laboratory energies from 0.9 to 1.3 GeV at c.m. angles from 7\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{}. The reaction was produced in a liquid-hydrogen target illuminated by a bremsstrahlung beam from the Caltech synchrotron. The ${\ensuremath{\pi}}^{\ensuremath{-}}$ 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 $\ensuremath{\gamma}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}+{N}^{*}{(1238)}^{++}$ was obtained by fitting the ${\ensuremath{\pi}}^{\ensuremath{-}}$ 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 ${\ensuremath{\pi}}^{\ensuremath{-}}$ in ${N}^{*}$ production shows the small-angle peak and decrease near 0\ifmmode^\circ\else\textdegree\fi{} 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 $\ensuremath{\pi}N{N}^{*}$ coupling constant of 23.1\ifmmode\pm\else\textpm\fi{}2.0 Ge${\mathrm{V}}^{\ensuremath{-}2}$, in fair agreement with the value obtained from the width of the ${N}^{*}(1238)$. The total cross section for pion pair production decreases smoothly from 78.9\ifmmode\pm\else\textpm\fi{}2.9 \ensuremath{\mu}b at 0.93 GeV to 59.1\ifmmode\pm\else\textpm\fi{}5.2 \ensuremath{\mu}b at 1.29 GeV, whereas the ${N}^{*}$ part of the cross section decreases from 45.0\ifmmode\pm\else\textpm\fi{}2.4 \ensuremath{\mu}b to 18.2\ifmmode\pm\else\textpm\fi{}3.5 \ensuremath{\mu}b over the same range. There is no strong evidence for formation of the ${N}^{*}(1688)$ as an intermediate state.