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## Main Question or Discussion Point

The interaction p + π- → n + π- + π + may proceed by the creation of an intermediate 'particle' or resonance called a rho. This can be detected as a peak in the plot of invariant rest mass energy of the emergent pions versus frequency of pions observed. My question is quite simply, invariant rest mass is

*invariant*. So, how can you have a spectrum of rest mass energies? The rest mass of the pi plus and pi minus are well known to be 139.6 Mev/ c^{2}for both, so how can you have a range of their rest mass energies?