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/ c2 for both, so how can you have a range of their rest mass energies?