D. Cline, J. English, R. Terrell, W. Wilke, B. Chaudhary, H. Courant, E. Marquit, K. Ruddick “Observation of "Peripheral" Backward Multipion Production at 2.15 BeV/c” Physical Review Letters 23 (9): 491-495 (1969) https://ui.adsabs.harvard.edu/abs/196

Abstract: Using the Princeton-Pennsylvania Accelerator rapid-cycling bubble chamber in a flash-lamp triggered mode we have observed backward (π+π-) and (π+π-π0) multipion production in the reaction π+n-->pm(π) at 2.15 BeV/c. The main characteristics of the data are a sharp forward peaking of the proton relative to the incident π+ direction for all multipion masses below 1100 MeV/c and the strong production of the ω meson. Backward ω production is observed to be more probable than backward ρ production. The sharp forward peaking of the proton in these reactions is suggestive of the dominance of baryon exchange.