Observation of a New Narrow Axial-Vector Meson $a_1$(1420)

The 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.