Missing mass analysis of the A−2 at 6.0 GeV/c

The missing mass spectrum in the region of the A − 2 has been measured in the reaction π − p→X − p at 6.0 GeV/ c in the interval 0.27 t | c ) 2 , with an optical spark chamber system which simultaneously observed the decay X − → ηπ − . A signal of 230 events above background per five MeV interval is observed at the A 2 peak, with a signal-to-background ratio of greater than 1:1. A single D-wave Breit-Wigner distribution with a quadratic background gives a good fit to the data, yielding the parameters M 0 =(1.324±0.003) GeV/ c 2 and Γ 0 =(0.104±0.009) GeV/ c 2 . The spectrum is incompatible with a dipole shape.