SEMINAR    Thursday, 14th December 2006 - h. 15:00 Auditorium B. Touschek    E. Gabrielli (Helsinki Inst. of Physics ) Photon splitting in magnetic field as a probe of ultralight axion/scalar fields Abstract Recently, the experimental observation of a light polarization rotation in magnetic field by the PVLAS collaboration pointed out the existence of a very light pseudoscalar boson, the axion. Due to the coupling with two photons, the axion can in principle induce the photon splitting in external fields, but with a different mechanism with respect to QED. After a general review about the photon splitting in QED, the same phenomena will be analyzed in the framework of effective scalar/pseudoscalar interactions with two photons, in the presence of uniform and constant magnetic/electric fields.