Totalp−pand "p−n" Cross Sections at Cosmotron Energies

Abstract

The total proton-proton cross section (excluding Coulomb scattering) has been measured at energies from 410 Mev up to 2.6 Bev, using external beams from the Cosmotron. Fast counting equipment was used to measure the attenuation of the beams through polyethylene, carbon, and liquid ${\mathrm{H}}_{2}$ absorbers. At each energy $E$, ${\ensuremath{\sigma}}_{p\ensuremath{-}p}(E, \ensuremath{\Omega})$ was measured as a function of the solid angle $\ensuremath{\Omega}$ subtended by the rear counter at the center of the absorber. The total cross section ${\ensuremath{\sigma}}_{p\ensuremath{-}p}$ was obtained by a least squares straight line extrapolation to $\ensuremath{\Omega}=0$. The measured ${\ensuremath{\sigma}}_{p\ensuremath{-}p}$ as a function of energy rises sharply from 26.5 mb at 410 Mev to 47.8 mb at 830 Mev and then remains approximately constant out to 1.4 Bev, above which energy it decreases gradually to about 42 mb at 2.6 Bev.Using the same equipment and procedure, we have also measured the ${\mathrm{D}}_{2}$O-${\mathrm{H}}_{2}$O difference cross section, called "${\ensuremath{\sigma}}_{p\ensuremath{-}n}$," for protons over the same energy range. From a comparison of "${\ensuremath{\sigma}}_{p\ensuremath{-}n}$," and ${\ensuremath{\sigma}}_{p\ensuremath{-}p}$, with the $n\ensuremath{-}p$ and $n\ensuremath{-}d$ measurements of Coor et al. at 1.4 Bev, it is apparent that one nucleon is "shielded" by the other in the deuteron. This effect is not present at energies below 410 Mev.Comparing the measured $p\ensuremath{-}p$ and "$p\ensuremath{-}n$" (corrected) cross sections with the results of other high-energy experiments, one may infer the following conclusions: (1) The sharp rise in ${\ensuremath{\sigma}}_{p\ensuremath{-}p}$ from 400 to 800 Mev results from increasing single pion production, which may proceed through the $T=\frac{3}{2}$, $J=\frac{3}{2}$ excited nucleon state. (2) Above 1 Bev the inelastic (meson production) $p\ensuremath{-}p$ cross section appears to be approximately saturated at 27-29 mb. (3) The rise in cross section for $n\ensuremath{-}p$ interaction in the $T=0$ state, associated with the rise in double pion production, implies that double meson production also proceeds through the $T=\frac{3}{2}$ nucleon state. (4) The probable equality of ${\ensuremath{\sigma}}_{p\ensuremath{-}d}$ and ${\ensuremath{\sigma}}_{n\ensuremath{-}d}$ at 1.4 Bev implies the validity of charge symmetry at this energy.