Resonant behaviour in double charge exchange reaction of \pi^+ mesons on   the nuclear photoemulsion
  The invariant mass spectra of the $pp\pi^-$ and $pp$ systems produced in the double charge exchange (DCX) of positively charged pions on photoemulsion are analysed. A pronounced peak is observed in the $pp\pi^-$ invariant mass spectrum, while the $M_{pp}$ spectrum exhibits a strong Migdal-Watson effect of the proton-proton final state interaction. These findings are in favor of the $NN$-decoupled $NN\pi$ pseudoscalar resonance with T=0 called $d'$. 
Yu.A. Batusov, T.D. Blokhintseva, F. Balestra, M.P. Bussa, G.   Piragino, G.B. Pontecorvo, M.G. Schepkin