We study the thermal properties of the lowest multiplet of the QCD light-flavor scalar resonances, including the f 0 (500)/σ, K * 0 (700)/κ, f 0 (980) and a 0 (980), in the framework of unitarized U (3) chiral perturbation theory. After the successful fits to the meson-meson scattering inputs, such as the phase shifts and inelasticities, we obtain the unknown parameters and further calculate the resonance poles and their residues at zero temperature. By including the finite-temperature effects in the unitarized meson-meson scattering amplitudes, the thermal behaviors of the scalar resonance poles in the complex energy plane are studied. The masses of σ and κ are found to considerably decrease when increasing the temperatures, while their widths turn out to be still large when the temperatures reach around 200 MeV. In contrast, both the masses and widths of the f 0 (980) and a 0 (980) are only slightly changed.

In this work we study the resonances near the thresholds of the open heavy-flavor hadrons using the effective-range-expansion method. The unitarity, analyticity and compositeness coefficient are also taken into account in our theoretical formalism. We consider the Z c (3900), X(4020), χ c1 (4140), ψ(4260) and ψ(4660). The scattering lengths and effective ranges from the relevant elastic S-wave scattering amplitudes are determined. Tentative discussions on the inner structures of the aforementioned resonances are given. *

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We study the mixing of the QCD/QCD-like axion and light-flavor mesons π0, η, η′ within the framework of U(3) chiral perturbation theory up to next-to-leading order in this work. The axion-meson mixing formulas are calculated order by order in the U(3) δ-expansion scheme, namely the joint expansions of the momentum, light-quark masses and 1/NC. We provide axion-meson mixing relations in terms of the π0–η–η′ mixing parameters and their masses. The recent lattice simulations on the η–η′ systems turn out to be able to offer valuable inputs to constrain the unknown low-energy constants. The relation of the mass and decay constant of the axion is then further explored based on our updated calculations. The two-photon couplings of the light-flavor mesons, together with the axion, are also investigated in the U(3) chiral theory up to next-to-leading order in the δ-counting scheme.