How to test the quark-line rule by production of neutral mesons in proton-antiproton annihilation

Production of several neutral mesons in reactions involving $u$, $d$, $\overline{u}$, and $\overline{d}$ quarks only can be used to test the quark-line rule for a given hadronic system. Recent data on $p\overline{p}\to \left(\mathrm{two}\mathrm{neutral}\mathrm{pseudoscalars}\right)$ are analyzed yielding restrictions on the pseudoscalar mixing angle ${\theta }_{\mathrm{PS}}$ in agreement with theoretical estimates. It is emphasized that production of neutral mesons at LEAR can provide meaningful tests of the quark-line rule and theoretical predictions of ${\theta }_{\mathrm{PS}}$.