Symmetries and their Violation
Dynamical isospin breaking
Within the standard model only electro--magnetic effects and the differences in quark masses distinguish between the different quark flavours. Thus the observation of isospin violation in low energy reactions allows to study quark mass effects in hadronic reactions.
A hadron accelerator like COSY has the unique feature that spin and isospin of a system can be prepared for NN as well as dd collisions. This allows to identify observables whose size is directly proportional to isospin violating matrix elements. In recent years an effective field theory was developed for these large momentum transfer reactions - a tool allowing to disentangle quark mass effects and electro-magnetic effects in reactions like dd → απ0. Polarization in the initial state will be essential; for a significant number of observables it is nescessary to disentangle the various contributions. These investigations will tell us if we understand the mechanisms of chiral symmetry breaking in the baryonic sector and will give access to quark mass ratios from hadron dynamics instead of hadron decays.
Isospin violating mixing of scalar mesons
Within the energy range of COSY the scalar mesons a0 and f0 can be studied whose nature is still quite unclear. In this context a measurement of the isospin violating a0-f0 mixing amplitude is very important, since it is sensitive to the overlap of the two wavefuctions and is therefore expected to elucidate information on the nature of the two states. Since isospin violation in the vincinity of the a0 and f0 is dominated by a0-f0 mixing, the cross section dd → α(π0η)s-wave is directly proportional to the square of the mixing amplitude.
Decays of η and η'
The η-η' system provides a very useful tool to study phenomenological implications of the axial U(1) anomaly of the strong interaction. A precise measurement of η and η' decays and in particular the slope parameters of the pertinent Dalitz plots will help to understand the importance of both gluonic degrees of freedom in the η' and the axial U(1) anomaly in the low-energy hadronic sector and constrain η-η' mixing parameters. The results can be compared with model-independent predictions of chiral perturbation theory with a clear theoretical connection to QCD.
The hadronic decays η, η' → 3π can be utilized for an accurate determination of the light quark mass ratio (md-mu)/(ms-(mu+md)/2), while further isospin violating effects can be studied in η' → ηππ.
With respect to η decays the energy release in η' decays is large enough to see resonance contributions. Moreover, the anomalous decays of η' into lepton pairs e+e- and μ+μ- will allow to draw conclusions on decays via two virtual photons and indicate whether double vector meson dominance holds which is, for example, of relevance for the hadronic contribution to the anomalous magnetic moment of the muon.
Possible unconventional effects from new physics beyond the Standard Model can be tested in CP or C violating rare decays. Strong CP-violating effects which arise due to a non-vanishing QCD vacuum angle θ can be searched for in the decays η, η' → ππ and η, η' → 4π0. C invariance for which only moderate experimental limits exist and which could help to understand the experimental abundance of matter over antimatter in the universe can be tested in a variety of decays, e.g., in η, η' → π0π0γ, η, η' → 3π0γ η, η' → 3γ and η' → e+e-π0 (η).