Recent results on light-meson exotics from COMPASS experiment

<!--HTML--><p><span style="color:#000000">The talk will discuss two prime candidates for exotic hadrons in the</span><br /> <span style="color:#000000">light-meson sector: the hybrid candidate $\pi_1(1600)$ with manifestly</span><br /> <span style="color:#000000">non-$q\bar{q}$ quantum numbers, $J^{PC}=1^{-+}$, and the tetraquark</span><br /> <span style="color:#000000">candidate $a_1(1420)$ located in the vicinity of the $K^*\bar{K}$ threshold.</span><br /> <br /> <span style="color:#000000">Based on high-precision data from the COMPASS experiment at CERN, we</span><br /> <span style="color:#000000">observe the $\pi_1(1600)$ hybrid candidate as a resonance in the</span><br /> <span style="color:#000000">$\rho(770)\pi$ and $\eta^\prime\pi$ systems. Moreover, our studies bring</span><br /> <span style="color:#000000">a long-awaited understanding of the seemingly contradictory claims made</span><br /> <span style="color:#000000">in previous analyses of the $\rho(770)\pi\,P$ partial wave.</span><br /> <br /> <span style="color:#000000">Studying the tetraquark candidate $a_1(1420)$, we find evidence for a</span><br /> <span style="color:#000000">so-called triangle singularity, a much sought-after effect of the strong</span><br /> <span style="color:#000000">interaction. Our new analysis suggests that the resonance-like signal</span><br /> <span style="color:#000000">seen in the $f_0(980)\pi$ system is produced by such a triangle</span><br /> <span style="color:#000000">singularity, which causes the particles produced in the collision to</span><br /> <span style="color:#000000">change their identities via mutual interaction right at the interaction</span><br /> <span style="color:#000000">point.</span><br /> <br /> <span style="color:#000000">I will demonstrate how progress in the understanding of the strong</span><br /> <span style="color:#000000">interaction benefits from a tight collaboration between theory and</span><br /> <span style="color:#000000">experiment on the example of the cooperation of the COMPASS experiment</span><br /> <span style="color:#000000">and the Joint Physics Analysis Center (JPAC).</span></p> EP seminar 27 July: https://cern.zoom.us/j/64131202472?pwd=Mk5MWUxQWjF3UkVxdzl1WUFQU1FVQT09