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

TL;DR

The paper reports the discovery of a new narrow axial-vector meson, $a_1(1420)$, observed in diffractive pion dissociation data, with unique decay properties indicating a potentially exotic nature.

Contribution

First observation of the $a_1(1420)$ meson in diffractive dissociation experiments, expanding the meson spectrum and suggesting exotic meson characteristics.

Findings

Discovery of a new $a_1(1420)$ meson with specific mass and width.

Observation that $a_1(1420)$ decays only into $f_0(980)\pi$, hinting at exotic properties.

Largest partial-wave analysis model with 88 waves used in the study.

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 $(1414^{+15}_{-13})$ MeV/$c^2$ and width $(153^{+8}_{-23})$ 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.