Resonance Particles and Complexes

The eta particle has xenicity 0, and the pions have xenicity 0. There is, accordingly, no change in xenicity accompanying the decay of the eta particle, and the reaction is very fast. 

The mass of N is about 1237 MeV. Its half-life is about I0 - s and, in accordance with the uncertainty relation between energy and time (Section 3-13), the mass (energy) of the particle is not well defined, but has an uncertainty of about 60 MeV. (The half-life 10 s is in fact calculated from the observed distribution function for the mass of the N complex.) 

Since 1952 about 100 of these short-lived particles or complexes have been discovered. They are called resonance particles or resonance complexes. One of them, rf has been included in our listing of the mesons (Table 20-2). It is produced by reaction of a pion and a neutron (within a deuteron)  

It is the lightest of the resonance particles, with mass 550 MeV. It decomposes, with half-life lO - s, by the reaction 

The next lightest resonance particles are the p particles, p, p , and p, with mass 760 MeV. They constitute a charge triplet (inherent charge 0, 

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