New a1 (1420) state: Structure, mass, and width

Abstract

The structure, spectroscopic parameters and width of the resonance with quantum numbers JPC=1++ discovered by the COMPASS Collaboration and classified as the a1(1420) meson are examined in the context of QCD sum rule method. In the calculations the axial-vector meson a1(1420) is treated as a four-quark state with the diquark-antidiquark structure. The mass and current coupling of a1(1420) are evaluated using QCD two-point sum rule approach. Its observed decay mode a1(1420)→f0(980)π, and kinematically allowed ones, namely a1→K∗±K, a1→K∗0K̄0 and a1→K̄∗0K0 channels are studied employing QCD sum rules on the light-cone. Our prediction for the mass of the a1(1420) state ma1=1416-79+81 MeV is in excellent agreement with the experimental result. Width of this state Γ=145.52±20.79 MeV within theoretical and experimental errors is also in accord with the COMPASS data.