[neutralino]

Originally Posted by Profpat

Thanks Neutralino;

I'll leave you alone on this one since you're cosmologist and not a nuclear physicist but here is one instance where a proton decays into a neutron.

Whilst this is an instance that shows a proton being converted into a neutron, it is not an instance of proton decay. For the reaction you showed to occur, one would need more than one proton to collide. However, unlike the proton, there are instances where the neutron would, on its own, decay into a proton and other products.