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Abstract

A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the $\ensuremath{\eta}$, $\ensuremath{\omega}$, ${X}^{0}(958)$, and $\ensuremath{\varphi}$ mesons near their respective thresholds in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}\mathrm{missing}\mathrm{mass}+n$. The incident momentum is varied in small steps through the threshold while neutrons of a given momentum are detected near zero degrees. The lower limit of the c.m. momentum ${P}^{*}$ at which measurements have been made ranges from about 50 MeV/c at the $\ensuremath{\varphi}$ to about 30 MeV/c at the $\ensuremath{\eta}$. A somewhat low value for the $\ensuremath{\omega}$ mass, 782.3 \ifmmode\pm\else\textpm\fi{} 0.6 MeV, is found. The width of the ${X}^{0}$ is 1.9 MeV (95% confidence level). All four mesons show evidence of $S$-wave production, with values of $\frac{\ensuremath{\sigma}}{{P}^{*}}$ of 21.2 \ifmmode\pm\else\textpm\fi{} 1.8, 0.35 \ifmmode\pm\else\textpm\fi{} 0.03, and 0.29 \ifmmode\pm\else\textpm\fi{} 0.06 \ensuremath{\mu}b/(MeV/c) for the $\ensuremath{\eta}$, ${X}^{0}$, and $\ensuremath{\varphi}$, respectively. A rapid rise in the $\ensuremath{\omega}$ cross section appears to be modified by a final-state interaction. The effect of this rise can probably be seen in some ${S}_{11}$ pion-nucleon phase-shift solutions. Evidence is also presented of a sudden drop in the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ mass spectrum just above the threshold for the production of a ${K}^{+}{K}^{\ensuremath{-}}$ pair. The paper includes a comprehensive discussion of the method and of the details of the spectrometer.

Study of Narrow Mesons near Their Thresholds

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Abstract

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