Unitarity of the $3\times 3$ lepton flavor mixing matrix $V$ is unavoidably violated in a seesaw mechanism if its new heavy degrees of freedom are slightly mixed with the active neutrino flavors. We propose to use the atomic transition process ${ \left| \rm e \right> \to\left|\rm g \right> + \gamma + \nu^{ }_{ i} + \overline{ \nu}^{ }_{ j}}$ (for $i, j = 1, 2, 3$), where $\left|\rm e \right>$ and $\left|\rm g \right>$ stand respectively for the excited and ground levels of an atomic system, to probe or constrain the unitarity-violating effects of $V$. We find that the photon spectrum of this transition will be distorted by the effects of $V V^\dagger \neq { \bf 1}$ and $V^\dagger V \neq { \bf 1}$ as compared with the $V V^\dagger = V^\dagger V = { \bf 1}$ case. We locate certain frequencies in the photon spectrum to minimize the degeneracy of effects of the unitarity violation and uncertainties of the flavor mixing parameters themselves. The requirements of a nominal experimental setup to test the unitarity of $V$ are briefly discussed.
Categories: hep-ph physics.atom-ph