We study the reactions $gammagammarightarrow pi^0pi^0$, $pi^+pi^-$, $K^0bar{K}^0$, $K^+K^-$, $eta eta$ and $pi^0eta$ based on a chiral Lagrangian with dynamical light vector mesons as formulated within the hadrogenesis conjecture. At present our chir
al Lagrangian contains 5 unknown parameters that are relevant for the photon fusion reactions. They parameterize the strength of interaction terms involving two vector meson fields. These parameters are fitted to photon fusion data $gammagammarightarrow pi^0pi^0$, $pi^+pi^-, pi^0eta$ and to the decay $etarightarrowpi^0gammagamma$. In order to derive gauge invariant reaction amplitudes in the resonance region constraints from micro-causality and exact coupled-channel unitarity are used. Our results are in good agreement with the existing experimental data from threshold up to about 0.9 GeV for the two-pion final states. The $a_0$ meson in the $pi^0eta$ channel is dynamically generated and an accurate reproduction of the $gammagammarightarrow pi^0eta$ data is achieved up to 1.2 GeV. Based on our parameter sets we predict the $gammagammarightarrow $ $K^0bar{K}^0$, $K^+K^-$, $eta eta$ cross sections.
We consider the chiral Lagrangian with a nonet of Goldstone bosons and a nonet of light vector mesons. The mixing between the pseudoscalar mesons eta and eta-prime is taken into account. A novel counting scheme is suggested that is based on hadrogene
sis, which conjectures a mass gap in the meson spectrum of QCD in the limit of a large number of colors. Such a mass gap would justify to consider the vector mesons and the eta-prime meson as light degrees of freedom. The complete leading order Lagrangian is constructed and discussed. As a first application it is tested against electromagnetic transitions of light vector mesons to pseudoscalar mesons. Our parameters are determined by the experimental data on photon decays of the omega, phi and eta-prime meson. In terms of such parameters we predict the corresponding decays into virtual photons with either dielectrons or dimuons in the final state.
It is argued that the chiral partners of the lowest-lying hadrons are hadronic molecules and not three-quark or quark-antiquark states, respectively. As an example the case of a_1 as the chiral partner of the rho is discussed. Deconfinement -- or as
a precursor large in-medium widths for hadronic states -- is proposed as a natural way to accommodate for the fact that at chiral restoration the respective in-medium spectra of chiral partners must become degenerate. Ingredients for a systematic and self-consistent in-medium calculation are presented with special emphasis on vector-meson dominance which emerges from a recently proposed systematic counting scheme for the mesonic sector including pseudoscalar and vector mesons as active degrees of freedom.
The decays of light vector mesons into three pseudoscalar mesons are calculated to leading order in the recently proposed counting scheme that is based on the hadrogenesis conjecture. Fully differential as well as integrated decay widths are presente
d. Since the required parameters have been fixed by other processes, the considered three-body decays are predictions of the presented approach. The decay width of the omega meson into three pions agrees very well with experiment. The partial decay widths of the K^* into its three K-pi-pi channels are predicted.
