Evidence for two-cluster formation in one-, two-, three- and four-pion production 28 GeV/c p-p and p-d collisions

We study the reactions pd→(p s ) pp π − , pp π + π − , pd→(p s ) pp π − π − π + and pp→pp π + π − π + π − at approximately 28 GeV/ c , as examples of one, two, three- and four-pion production, respectively. One-pion production occurs mainly as the fragmentation or excitation of the target neutron into a low-mass (p π − ) cluster. Likewise, the two-pion production is seen to be mainly the production of a low-mass (p π + π − ) cluster recoiling opposite the remaining proton. For three- and four-pion production, clustering of particles is much less obvious; nevertheless, the data may be interpreted as showing that single excitation is still the dominant process as in the Nova model of Jacob and Slansky. The technique for examining cluster production involves examining the distribution of rapidities of final-state particles. However, it must be realized that the reactions studied represent only the lowest multiplicities at 28 GeV/ c and that particle production in general at this momentum cannot be assumed to be represented by the low-multiplicity channels studied here. The production of secondaries as a result of cluster formation is an attractive model but our analysis shows that, except for the lowest multiplicities, it is certainly not a unique description of the data.