| Abstract: | We evaluate the size of isospin breaking corrections to form factors f and g of the  decay process  which is actually measured by the extended NA48 setup at CERN. We found that, keeping apart the effect of Coulomb interaction, isospin breaking does not affect the moduli. This is due to the cancellation between corrections of electromagnetic origin and those generated by the difference between up and down quark masses. On the other hand, electromagnetism affects considerably the phases if the infrared divergence is dropped out using a minimal subtraction scheme. Consequently, the greatest care must be taken in the extraction of  phase shifts from experiment.Received: 27 December 2004, Revised: 24 January 2005, Published online: 3 March 2005 |
