Observation of a New Narrow Axial-Vector Meson a1(1420)

Abstract

The COMPASS Collaboration at CERN has measured diffractive dissociation of 190GeV/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 a1meson with mass (1414-13+15)MeV/c2and 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.