Investigation of the reaction. pi. /sup -/p. -->. eta'n in the momentum region from 15 to 40 GeV/c

The cross sections of the reaction ..pi../sup -/p..-->..eta'(958)n, eta'--2..gamma.. were measured at the momenta 15, 20, 25, 30, and 40 GeV/c. The experiments were performed on the 70-GeV IHEP accelerator using the NICE 648-channel hodoscopic spectrometer. A total of 6000 eta' mesons were registered. The observed dip of the differential cross sections of the reactions ..pi..'p..-->..eta'n and ..pi../sup -/p..-->..etan as functions of t are identical. The ratio R(eta'/eta)/sub t/=0=0.55 +- 0.06 of the cross sections of these reactions yields the angle ..beta..=-(18.2 +- 1.4)/sup 0/ of the singlet-octet mixing of the pseudoscalar mesons.