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

TL;DR

This paper investigates the structure, mass, and decay width of the $a_1(1420)$ meson as a four-quark state using QCD sum rules, finding results consistent with experimental data.

Contribution

It provides a QCD sum rule analysis of the $a_1(1420)$ meson as a four-quark state, estimating its mass and decay width in agreement with experiments.

Findings

Predicted mass $m_{a_{1}}=1416_{-79}^{+81}$ MeV matches experimental data.

Calculated decay width $ ext{Gamma}=145.52 ext{MeV}$ agrees with COMPASS measurements.

Supported the four-quark diquark-antidiquark structure hypothesis for $a_1(1420)$.

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.