Observation of a new narrow axial-vector meson $a_1(1420)$

Understand

The COMPASS collaboration at CERN has measured diffractive dissociation of 190 GeV/$c$ pions into the $\pi^-\pi^-\pi^+$ final state using a stationary hydrogen target.

• A partial-wave analysis (PWA) was performed in bins of $3\pi$ mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves.
• A narrow $J^{PC} = 1^{++}$ signal is observed in the $f_0(980)\,\pi$ channel.
• We present a resonance-model study of a subset of the spin-density matrix selecting $3\pi$ states with $J^{PC} = 2^{++}$ and $4^{++}$ decaying into $\rho(770)\,\pi$ and with $J^{PC} = 1^{++}$ decaying into $f_0(980)\,\pi$.