A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the $\eta$, $\omega$, $X^0\mathrm{}\left(958\right)\mathrm{}$, and $\phi$ mesons near their respective thresholds in the reaction ${\pi }^{-}+p\to \mathrm{missing}\mathrm{mass}+n$. 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 $\phi$ to about 30 MeV/c at the $\eta$. A somewhat low value for the $\omega$ mass, 782.3 ± 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 $\frac{\sigma }{P^{*}}$ of 21.2 ± 1.8, 0.35 ± 0.03, and 0.29 ± 0.06 μb/(MeV/c) for the $\eta$, $X^0$, and $\phi$, respectively. A rapid rise in the $\omega$ 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 ${\pi }^{+}{\pi }^{-}$ mass spectrum just above the threshold for the production of a $K^{+}{K}^{-}$ pair. The paper includes a comprehensive discussion of the method and of the details of the spectrometer.

• Received 5 April 1973

DOI:https://doi.org/10.1103/PhysRevD.8.2789