  The structure, spectroscopic parameters and width of the resonance with
quantum numbers $J^{PC}=1^{++}$ 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 $a_1 \to K^{\ast \pm}K^{\mp}$, $a_1 \to K^{\ast 0} \bar{K}^{0}$
and $a_1 \to \bar {K}^{\ast 0} K^{0}$ 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.
