The structure of the $a_0(980)$ and $f_0(980)$ resonances is investigated with the $a_0(980)\text{−}{f}_0(980)$ mixing intensity from the viewpoint of compositeness, which corresponds to the amount of two-body states composing resonances as well as bound states. For this purpose, we first formulate the $a_0(980)\text{−}{f}_0(980)$ mixing intensity as the ratio of two partial decay widths of a parent particle, in the same manner as the recent analysis in BES experiments. Calculating the $a_0(980)\text{−}{f}_0(980)$ mixing intensity with the existing Flatte parameters from experiments, we find that many combinations of the $a_0(980)$ and $f_0(980)$ Flatte parameters can reproduce the experimental value of the $a_0(980)\text{−}{f}_0(980)$ mixing intensity by BES. Next, from the same Flatte parameters, we also calculate the $K\overline{K}$ compositeness for $a_0(980)$ and $f_0(980)$. Although the compositeness with the correct normalization becomes complex in general for resonance states, we find that the Flatte parameters for $f_0(980)$ imply a large absolute value of the $K\overline{K}$ compositeness, and the parameters for $a_0(980)$ lead to a small but non-negligible absolute value of the $K\overline{K}$ compositeness. Then, connecting the mixing intensity and the $K\overline{K}$ compositeness via the $a_0(980)$- and $f_0(980)\text{−}K\overline{K}$ coupling constants, we establish a relation between them. As a result, a small mixing intensity indicates a small value of the product of the $K\overline{K}$ compositeness for the $a_0(980)$ and $f_0(980)$ resonances. Moreover, the experimental value of the $a_0(980)\text{−}{f}_0(980)$ mixing intensity implies that the $a_0(980)$ and $f_0(980)$ resonances cannot be simultaneously $K\overline{K}$ molecular states.

• Received 12 November 2014

DOI:https://doi.org/10.1103/PhysRevD.92.034010