The new $a_1(1420)$ state: structure, mass and width

Abstract: The structure, spectroscopic parameters and width of the resonance with quantum numbers $J<sup>{PC}=1<sup>{++}$ discovered by the COMPASS Collaboration and classified as the $a_1(1420)$ meson are examined in the context of QCD sum rule method. In the calculations the axial-vector meson $a_1(1420)$ is treated as a four-quark state with the diquark-antidiquark structure. The mass and current coupling of $a_1(1420)$ are evaluated using QCD two-point sum rule approach. Its observed decay mode $a_1(1420) \to f_0(980)\pi$, and kinematically allowed ones, namely $a_1 \to K^{\ast \pm}K<sup>{\mp}$, $a_1 \to K^{\ast 0} \bar{K}<sup>{0}$ and $a_1 \to \bar {K}^{\ast 0} K<sup>{0}$ channels are studied employing QCD sum rules on the light-cone. Our prediction for the mass of the $a_1(1420)$ state $m_{a_{1}}=1416_{-79}^{+81}\ \mathrm{MeV}$ is in excellent agreement with the experimental result. Width of this state $\Gamma=145.52 \pm 20.79 \mathrm{MeV}$ within theoretical and experimental errors is also in accord with the COMPASS data.