Less suppressed μ→eγ and τ→μγ loop amplitudes and extra dimension theories

When muegamma (or taumugamma) 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 for muegamma 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. (C) 2002 Elsevier Science B.V. All rights reserved.