A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the g, w , 2!(958), and @mesons near their respective thresholds in the reaction a-+ p -missing 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 @ t o about 30 MeV/c at the 7. A somewhat low value for the w mass, 782.3 1-0.6 MeV, is found. The width of the 2! is < 1.9 MeV (95% confidence level). All four mesons show evidence of S-wave production, with values of u/P* of 21.2 1-1.8, 0.35 i0.03, and 0.2910.06 pb/(MeV/c) for the q, A ! , and @, respectively. A rapid rise in the w cross section appears to be modified by a final-state interaction. The effect of this rise can probably be seen in some Sli pion-nucleon phase-shift solutions. Evidence is also presented of a sudden drop in the s i n -mass spectrum just above the threshold for the production of a KiKpair. The paper includes a comprehensive discussion of the method and of the details of the spectrometer.-

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No enhancement in 6-+ nfn-ndecay mode appeared a t mass 962 MeV or a t IJ9G MeV produced in n-p -+ 6-p interaction near its production threshold in a missing mass counter experiment. This finding fully supports the fact that the 6-+ n+n-x-decay mode is not, allowed by the conRervations of the angu1a.r momentum and the parity. Studie des Zerfalls 6--+ X+R-R-Inhaltsubersicht. Es wurde keine Steigernng der Zerfallsrate des von 6---t n+n-n-bei den Masson 962 MeV oder 996 MeV beobachtet, welche bei der Reektion n-p -+ 6-p in der Xihe ihrer Schwelle crzeugt wnrden. Dieses Ergebnis entspricht im vollen MaBe der Tataache, daB der Zerfall 6--+ n+n-n-wegen der Erhaltung des Drehimpulses und der Paritat verboten ist.According to the quark model [l], a meson is made up of quark and anti-quark.