Less suppressed mu-e-gamma and tau-mu-gamma loop amplitudes and extra dimension theories

When mu-e-gamma (or tau-mu-gamma) loop involves a vector boson, the amplitude is suppressed by more than two powers of heavy particle masses. However we show that the scalar boson loop diagrams are much less damped. Particularly, the loop amplitude in which the intermediate fermion and scalar boson have comparable masses is as large as possible, as allowed by the decoupling theorem. Such a situation is realized in the "universal extra dimension theory", and can yield a large enough rate to be detectable in current experiments. Our investigation involves precise calculation of the scalar boson loop's dependence on the masses of the intermediate states.