Triangle Singularity as the Origin of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>a</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mn>1420</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math>

The COMPASS experiment recently discovered a new isovector resonance-like signal with axial-vector quantum numbers, the $a_1(1420)$, decaying to $f_0(980)\pi$. With mass too close and width smaller than ground state $a_1(1260)$, it was immediately interpreted as light exotic meson, similar $X$, $Y$, $Z$ states in hidden-charm sector. We show that fully matching experimental data is produced by decay of $a_1(1260)$ resonance into $K^\ast(\to K\pi)\bar{K}$ subsequent rescattering through triangle singularity coupled $f_0(980)\pi$ channel. amplitude for this process calculated using approach based on dispersion relations. triangle-singularity model fitted partial-wave experiment. Despite having less parameters, fit shows slightly better quality one hypothesis thus eliminates need an additional order describe data. thereby demonstrate first time light-meson sector structure can be described singularity, providing evidence genuine three-body effect.