When you read cold fusion theories you hear a lot about collective effects, ways that atoms in a solid work together to do things individual atoms cannot. One iron atom cannot be a permanent magnet, but a trillion of them together can be.
In Widom-Larsen theory, they say that the surface of a metal hydride has very energetic (“heavy”) electrons that can undergo electron capture 
A central claim in the theory is that the electron capture process creates extremely slow neutrons. And I mean extremely slow! The neutron kinetic energy is supposed to be as low as 
As explained best in the 2008 paper, Widom-Larsen theory envisions a weird sort of surface plasma oscillation. Now, normal plasma oscillations (in solid-state physics) refer to a collective electron motion. But they’re talking about proton motion (the protons or deuterons embedded in the hydrated palladium). There is a vague suggestion that the electrons also move. But they certainly emphasize the proton motion by itself, and describe it in an explicit way here:
In Widom-Larsen theory, it is argued that there is a region at the surface of a metal-hydrogen (or -deuterium) system where electrons have an insanely high mass, as much as 10.5MeV/c², because of the electromagnetic environment they are in.
In the the previous post, I argued that you should understand that “insanely high mass” implies (or maybe is equivalent to) “insanely high energy”. In this post I will explain what exactly this energy is, according to Widom-Larsen theory. It’s simpler than you might think!
As described in the last post, Widom-Larsen theory states that the electron-capture process
Now, if you compare the energy of the two sides in
The Widom-Larson theory of cold fusion started with this paper:
A follow-up paper with more mathematical details is here, while a follow-up with slightly more qualitative discussion is here.
This is apparently the most popular theoretical explanation of cold fusion. For example, it was the theoretical justification supporting NASA’s cold-fusion program. Apparently, lots of reasonable people are convinced by it.
Is there a plausible theory of cold fusion / LENR? 