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

Abstract

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.

Comments: 11 Pages and 3 Figures, To be published in Phys. Rev. D