Invariant mass distribution of pion pairs with the different components of the fit represented by lines. The number of $\Rz$ candidates is integrated over the resonant Breit--Wigner part (green, full line), the interference term between $A$ and $B$ of Eq. (\ref{eq:Soeding}) is shown by the dash-dotted blue line and the muon template $M$ (red, dashed line) is taken from STARlight. See text for more details.