Invariant-mass distribution of pion pairs, with superimposed S\"{o}ding (solid line) and Ross-Stodolsky (dotted line) fits, for the range $0<\phi<\pi/7$ in the 0n0n (left) and XnXn (right) neutron classes. The different components of the pion-pair production amplitude in the S\"{o}ding model are shown: the Breit--Wigner shape that describes the \rhoz (finer dotted line), the continuum process (dash-dotted line), and the interference between the \rhoz and the continuum (dash-dot-dot-dot line). The Breit--Wigner extracted from the Ross-Stodolsky model (finest dotted line) is also shown. In this example, the background contribution from misidentified muons is fixed to zero in the fit.