Muonium decay - art. no. 073001

Modifications of the mu(+) lifetime in matter due to muonium (M = mu(+) e(- )) formation and other medium effects are examined. Muonium and free mu(+) decay spectra are found to differ at O(alpha m(e) lm(mu)) from Doppler broa dening and O(alpha(2)m(e)lm(mu)) from the Coulomb bound state potential. Ho wever, both types of corrections are shown to cancel in the total decay rat e due to Lorentz and gauge invariance respectively, leaving a very small ti me dilation lifetime difference, (tau(M) - tau(mu+))/tau(mu+) = alpha(2)m(e )(2)/2m(mu)(2) similar or equal to 6 x 10(-10), as the dominant bound state effect. It is argued that other medium effects on the stopped mu(+) lifeti me are similarly suppressed.