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Small fluctuations around a constant electric or constant magnetic field Fare analyzed in a theory with pseudo scalar [phi] with a coupling g[phi]F~F. Itis found that a magnetic external field leads to mass generation for the smallperturbations, while an electric field suffers from a tachyonic mass generationin the case in which the field strength is higher than a critical value(related to the pseudo scalar mass). The vacuum energy can be exactly evaluatedand it is found that an imaginary part is present when the external electricfield exceeds its critical value.Comment: Extended content, some mistakes amended 21 pages, JHEP 3 clas

Real and imaginary mass generation in the presence of external fields and axions