Nature of the a1 (1420)

The resonance like signal with axial-Vector quantum numbers J(PC) = 1(++) at a mass of 1420 MeV and a width of 140 MeV, recently observed by the COMPASS and VES experiments in the f(0)(980)pi final state and tentatively called a(1)(1420), is discussed. Instead of a genuine new meson, we interpret this signal as a dynamical effect due to a singularity (branching point) in the triangle diagram formed by the processes a(1)(1260) -> K*(K) over bar, K*-> K pi, and K (K) over bar -> f(0)(980) (+c.c). The amplitude for this diagram is calculated. The result exhibits a peak in the intensity with a sharp phase motion with respect to the dominant a(1)(1260) -> rho pi S-wave decay, in good agreement with the data. The branching ratio of a(1)(1260) -> f(0)(980)pi via the triangle diagram is estimated and compared to the dominant decay a(1)(1260) -> rho pi.