Meson spectroscopy at VES and COMPASS

Diffractive production of $\pi^- \pi^- \pi^+$ and $\pi^- \pi^0 \pi^0$ final states is the subject of comprehensive studies performed recently by the VES and the COM-PASS experiments. COMPASS pioneered the application of novel methods of partial-wave analysis: mass-independent PWA inmultiple $(m_{3 {\pi}}, t’)$-cells, mass-dependent analysis of spin-density matrices performed simultaneously in all measured t’ bins, the analysis with freed shapes of $\pi^+ \pi^-$ isobars. In addition, COMPASS observed a new narrow state: $a_1(1420)$. VES has world-leading data samples on $\pi^-\pi^-\pi^+$ and $\pi^-\pi^0 \pi^0$, that yield compatible results and show the potential for a detailed comparison of isospin relations between different decay channels, using the PWA methods with fixed and freed shapes of $\pi \pi$ isobars.