Tests of the fundamental symmetries in η meson decays

Patterns of chiral symmetry violation and tests of the conservation of the fundamental C, P and CP symmetries are key physics issues in studies of the π⁰, η and η' meson decays. These tests include searches for rare or forbidden decays and searches for asymmetries among the decay products in the not-so-rare decays. Some examples for the rare decays are η → 2π, η → 4π⁰ (CP tests), decays into an odd number of photons (e.g., η → 3_γ) and the decay η → π⁰e⁺e⁻ (C tests). In the conversion decay η → π⁺π⁻e⁺e⁻, the asymmetry in the distribution of the angle between the two-pion and two-lepton decay planes allows to search for CP violation in a flavor-conserving process beyond the CKM mechanism which is not constrained by the limits on the neutron dipole moment. In addition, the Dalitz decays and the decays into a lepton-antilepton pair are sensitive to contributions from a vector boson responsible for the annihilation of hypothetical light dark matter particles. The muon g – 2 and the branching ratio for the π⁰ e⁺e⁻ decay are currently the observables where hints of a deviation from the Standard Model predictions are reported. The experimental studies of the π⁰, η and η' meson decays are carried out at four European accelerator research facilities: KLOE/KLOE-2 at DAFNE (Frascati), Crystal Ball at MAMI (Mainz), WASA at COSY (Jülich), Crystal Barrel at ELSA (Bonn).