Since all the physical quantities can be written in terms of the
masses, when A decays at rest, the energies of B and C are
fixed. This was expected to happen in beta decays of nuclei, and when the energy of the beta particle was not found to be unique,
neutrinos were postulated.
Just like the energies of all particles in a two-body decay can be
determined in terms of their masses, conversely the mass of any
of the particles involved can be measured. This has been useful
in putting an upper bound on the mass of through the process (This measurement is obsolete now.)
Even for the decay to multiple particles, if we know the energies
and momenta of all the decay particles, we can determine the
mass of the decaying particle through
(8)
Indeed, this is the way the masses of unknown particles are
often determined in High Energy Physics.
through the process
