Single-pion production inp¯pinteractions near threshold

Abstract

Results are reported on the reaction $\overline{p}p\ensuremath{\rightarrow}\overline{N}N\ensuremath{\pi}$ at center-of-mass energies of 2.063, 2.094, and 2.114 GeV. The cross sections for ${\ensuremath{\pi}}^{0}$ production at these energies are 22 \ifmmode\pm\else\textpm\fi{} 7, 35 \ifmmode\pm\else\textpm\fi{} 9, and 126 \ifmmode\pm\else\textpm\fi{} 26 \ensuremath{\mu}b; those for ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{+}$ production are 20 \ifmmode\pm\else\textpm\fi{} 7, 41 \ifmmode\pm\else\textpm\fi{} 10, and 79 \ifmmode\pm\else\textpm\fi{} 20 \ensuremath{\mu}b and 16 \ifmmode\pm\else\textpm\fi{} 6, 43 \ifmmode\pm\else\textpm\fi{} 10, and 47 \ifmmode\pm\else\textpm\fi{} 16 \ensuremath{\mu}b, respectively. The cross section obeys the energy dependence of phase space and joins smoothly onto data at higher energies. The cross section as a function of energy is relatively well fitted by a calculation based on current-algebra predictions of soft-pion emission. Resonance production is small, but there is a possible $I=1$ enhancement which may be the $S(1930)$. Our highest energy $\overline{p}p\ensuremath{\rightarrow}\overline{p}p{\ensuremath{\pi}}^{0}$ channel may proceed approximately 50% through $\ensuremath{\Delta}(1236)\overline{p}+\overline{\ensuremath{\Delta}}(1236)p$. Predicted nonstrange exotic mesons coupled preferentially to $\overline{N}N$ and $\overline{N}N\ensuremath{\pi}$ are not seen.