Phase-Space Considerations for Four-Particle Final States

This paper gives a discussion of four-particle phase space in terms of the invariant mass variables ${M_{12}}^2$, ${M_{34}}^2$, ${M_{14}}^2$, ${M_{124}}^2$, and ${M_{134}}^2$. Formulas are given for the boundary of the physically allowed region and for the distribution of states inside. Some of the simpler distributions, which result when integrations are made over some of the variables, are derived.