Triangle Singularity as the Origin of the a_{1}(1420)

Area of Science:

• Particle Physics
• Hadron Spectroscopy
• Quantum Chromodynamics

Background:

• The COMPASS experiment observed a novel isovector axial-vector resonance, a_{1}(1420), decaying into f_{0}(980)π.
• Its properties, near the a_{1}(1260) with a smaller width, suggested it might be a light exotic meson.

Purpose of the Study:

• To investigate the origin of the a_{1}(1420) resonancelike signal.
• To determine if the signal can be explained by known resonance decays and rescattering effects.

Main Methods:

• Utilized a novel approach based on dispersion relations to calculate the decay amplitude.
• Modeled the process as a_{1}(1260) decaying into K*K̄, followed by rescattering via a triangle singularity into the f_{0}(980)π channel.
• Fitted the triangle-singularity model to COMPASS partial-wave data.

Main Results:

• A resonancelike signal matching experimental data was reproduced by the triangle-singularity model.
• The model provided a better fit to the data than a simple resonance hypothesis, despite having fewer parameters.
• This suggests the observed signal is a three-body effect rather than a distinct exotic resonance.

Conclusions:

• The a_{1}(1420) resonancelike structure can be explained by rescattering through a triangle singularity.
• This finding provides the first evidence for such a mechanism in the light-meson sector.
• The need for an additional resonance to explain the data is eliminated.