The COMPASS Collaboration at CERN has measured diffractive dissociation of $190\mathrm{GeV}/c$ 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 peak is observed in the $f_0(980)\pi$ channel with spin, parity and $C$-parity quantum numbers $J^{PC}=1^{++}$. 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 $f_0(980)\pi$. We identify a new $a_1$ meson with mass $({1414}_{-13}^{+15})\mathrm{MeV}/c^2$ and width $({153}_{-23}^{+8})\mathrm{MeV}/c^2$. Within the final states investigated in our analysis, we observe the new $a_1(1420)$ decaying only into $f_0(980)\pi$, suggesting its exotic nature.

• Received 25 January 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.082001

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Published by the American Physical Society