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

Abstract

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 $f_0(980)\pi$. We identify a new $a_1$ meson with mass $(1414_{-13}^{+15})$ MeV/$c^2$ and width $(153_{-23}^{+8})$ 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.

8 pages, 1 figure

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