I will present the recent work [Phys Rev D94(2016) 096015] about the a1(1420) peak which is experimentally and theoretically interesting. We study the decay mode of the a1(1260) into a pi+ in p-wave and the f0(980) that decays into pi+ pi- in s-wave. The mechanisms proceeds via a triangular mechanism where the a1(1260) decays into Kbar K, the K decays to an external pi+ and an internal K that fuses with the Kbar to produce the f0(980) resonance. The mechanism develops a singularity at a mass of the a1(1260) around 1420 MeV, producing a peak in the cross section of the pi p reaction, used to generate the mesonic final state, which provides a natural explanation of all the features observed in the COMPASS experiment, where a peak observed at this energy is tentatively associated to a new resonance called a1(1420). On the other hand, the riangular
singularity studied here gives rise to a remarkable feature, where a peak is seen for a certain decay channel of a resonance at an energy about 200 MeV higher than its nominal mass.