Pionium interacting with matter: magnetic terms and relativistic corrections

Magnetic contributions to the interaction of ordinary matter with pionium, i.e. exotic atoms formed from oppositely charged pions, are calculated explicitly in the context of the experiment DIRAC at CERN. These contributions have previously been estimated to amount to about 1% of the complete cross sections. Since this experiment requires the cross sections for electromagnetic processes at the 1% level as an input, a justification by explicit calculation of the previous estimations is necessary. We also estimate the magnitude of relativistic effects particularly for the electromagnetic coupling of the pionium system to the target system. These effects turn out to be negligible. (28 refs).