This report gives a detailed and explicit discussion of four-particle phase space in terms of invariant mass variables. The distribution of final states, after taking account of energy and momentum conservation, is integrated over orientations of the entire system and then expressed in terms of the squares of the invariant masses M12, M34, M14, M124, and M134. The results permit analysis for two-and for three-particle resonances. Formulas are given for the boundary of the allowed region in the five dimensions and for the distribution of final states inside. The simpler distributions which result when integrations can be made over some of the invariant masses are derived. The phase space proper¬ties of a three-particle system are reviewed also since they provide a guide to the discussion of the four-particle problem.
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