Study of Narrow Mesons near Their Thresholds

A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the η, ω, X⁰(958), and φ mesons near their respective thresholds in the reaction π^-+p→missingmass+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 φ to about 30 MeV/c at the η. A somewhat low value for the ω mass, 782.3 ±{} 0.6 MeV, is found. The width of the X⁰ is 1.9 MeV (95% confidence level). All four mesons show evidence of S-wave production, with values of σP* of 21.2 ±{} 1.8, 0.35 ±{} 0.03, and 0.29 ±{} 0.06 {μ}b/(MeV/c) for the η, X⁰, and φ, respectively. A rapid rise in the ω cross section appears to be modified by a final-state interaction. The effect of this rise can probably be seen in some S₁₁ pion-nucleon phase-shift solutions. Evidence is also presented of a sudden drop in the π⁺π^- 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.