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