We perform an analysis of the three-pion system with quantum numbers JPC=1++ produced in the weak decay of τ leptons. The interaction is known to be dominated by the axial meson a1(1260). We build a model based on approximate three-body unitary and fix the free parameters by fitting it to the ALEPH data on tau- -> pi+ pi- pi- nu_tau decay. We then perform the analytic continuation of the amplitude to the complex energy plane. The singularity structures related to the ππ subchannel resonances are carefully addressed. Finally, we extract the a1(1260) pole position m(a1(1260)) - i Gamma(a1(1260))/2 with m = (1209 +- 4 -9+12) MeV, Gamma = (576 +- 11 -20+89) MeV.
Date of publication:
Phys.Rev. D98 (2018) no.9, 096021