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Considering the recent observation of two-neutrino double electron capture ($2\nu$ECEC) by the XENON Experiment:

https://arxiv.org/abs/1904.11002

I was wondering why the half-life of this process is so much longer than the quite similar process of two-neutrino double beta-decay.

Considering crossing symmetry the two corresponding Feynman diagrams are basically the same:

enter image description here

The half-life of $^{124}$Xe was measured to be $1.8\cdot 10^{-22}$y

The half life of $^{136}$Xe is $2.1\cdot 10^{-21}$y

The difference therefore is about one order of magnitude.

The only explanation I could come up with is that the phase space factor for $2\nu$ECEC should be smaller than for $2\nu\beta\beta$ since the neutrino energy is fixed. However a factor of 10 feels rather large for me (it might still be right though). Are there any factors that I did not consider?

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    $\begingroup$ This is a good question, but I would just put it down to phase space factors. In practice, a factor of $10$ difference is tiny! $\endgroup$ – knzhou Apr 30 at 9:35

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