COMPASS collaboration (205 authors)Bordalo, P.Franco, C.Nunes, A.S.Quaresma, M.Quintans, C.Ramos, S.Silva, L.Stolarski, M.2019-02-042019-02-042015-08-19http://dx.doi.org/10.1103/PhysRevLett.115.082001http://hdl.handle.net/10400.26/26762The COMPASS Collaboration at CERN has measured diffractive dissociation of 190  GeV/c pions into the π-π-π+ final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of 3π mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow peak is observed in the f0(980)π channel with spin, parity and C-parity quantum numbers JPC=1++. We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with JPC=2++ and 4++ decaying into ρ(770)π and with JPC=1++ decaying into f0(980)π. We identify a new a1 meson with mass (1414-13+15)  MeV/c2 and width (153-23+8)  MeV/c2. Within the final states investigated in our analysis, we observe the new a1(1420) decaying only into f0(980)π, suggesting its exotic nature.engjournal article2019-02-04