Triangle mechanism in τ → f1(1285)πντ decay

We show that the τ− decay into f1(1285)π−ντ is dominated by a triangle loop mechanism with K⁎, K¯⁎ and K (or K¯) as internal lines, which manifests a strong enhancement reminiscent of a nearby singularity present in the narrow K⁎ limit and the near K¯⁎K⁎ threshold of the internal K⁎ propagators. The f1(1285) is then produced by its coupling to the K⁎K¯ and K¯⁎K which is obtained from a previous model where this resonance was dynamically generated as a molecular K⁎K¯ (or K¯⁎K) state using the techniques of the chiral unitary approach. We make predictions for the f1π mass distribution which significantly deviates from the phase-space shape, due to the distortion caused by the triangle mechanism and the K⁎K¯ threshold. We find a good agreement with the experimental value within uncertainties for the integrated partial decay width, which is a clear indication of the importance of the triangle mechanism in this decay and supports the dynamical origin of the f1(1285) as a K⁎K¯ and K¯⁎K molecular state.