text: The above-described process with the capture of two electrons and emission of two neutrinos (two-neutrino double electron capture) is allowed by the Standard Model of particle physics: No conservation laws (including lepton number conservation) are violated. However, if the lepton number is not conserved, or equivalently the neutrino is its own antiparticle, another kind of process can occur: the so-called neutrinoless double electron capture. In this case, two electrons are captured by nucleus, but neutrinos are not emitted. The energy released in this process is carried away by an internal bremsstrahlung gamma quantum.
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  This mode of decay has never been observed experimentally, and would contradict the Standard Model if it were observed.