Pole position of the a(1) (1260) resonance in a three-body unitary framework

Masses, widths, and branching ratios of hadronic resonances are quantified by their pole positions and residues with respect to transition amplitudes on the Riemann sheets of the complex energy plane. In this study, we discuss the analytic structure in the physical energy region of three-body scattering amplitudes on such manifolds. As an application, we determine the pole position of the a(1) (1260) meson from the ALEPH experiment by allowing for pi rho coupled channels in S and D waves. We find it to be root s(0) = (123(-0-11)(+15+9) - i266(-22-27)(+0+15)) MeV.