The first step, in the pp-chain, is the fusion of two protons, into a deuteron, a neutrino, and a positron:

 

What would happen, if you "flipped" the positron, on the right (product side), to an electron, on the left (reactant side). If you conducted pp collisions, in an electron-rich environment, could you catalyze the fusion physics?

Please ponder protons & electrons combining, into neutrons & neutrinos:

 

[math]p^{+} + e^{-} \rightarrow n^0 + \nu_e[/math]

If you "flipped" this reaction, then neutrinos could "stimulate" the fission, of neutrons, into protons & electrons. Indeed, in the OP, neutrinos could "stimulate" the fission, of deuterium nuclei, into hydrogen nuclei. Moreover, in a standard nuclear fission chain-reaction, neutrons essentially "stimulate" the emission, of more neutrons -- a little like a "neutron laser" effect. Could not neutrinos do something similar? There seems to be a physical parallel, between spontaneous-and-stimulated photon emission, and spontaneous-and-stimulated nuclear fission. In both cases, there seems to be a process parallel to the spontaneous one.

 

If "neutrino-stimulated nuclear fission" is possible, then one could construct a "neutrino detector", by using a short-lived radio-active element. Then, when a high flux of neutrinos -- e.g., from GRBs -- impinged upon the detector, the neutrinos would induce additional fission events, so that the sample would decay more rapidly, than its half-life would indicate. To wit, your "geiger counter" would start clicking faster, with the increase in frequency, due to neutrino-induced fission events.

What would happen, if you "flipped" the positron, on the right (product side), to an electron, on the left (reactant side). If you conducted pp collisions, in an electron-rich environment, could you catalyze the fusion physics?

 

In general, three-body interactions have a much lower probability that two-body interactions, since three particles colliding at the same time is a significantly rarer event. That says nothing about whether the electron would actually increase the cross-section.

 

AFAIK catalysts generally add an intermediate step, not increase the number of participants in an individual reaction.

It definitely wouldn't catalyse a fusion reaction; in the normal/chemical sense of the word a catalyst is not consumed in the reaction. As SonT said - it often forms an intermediate step/compound, but it reverts to original form and is preserved by the end of the reaction.

in a standard nuclear fission chain-reaction, neutrons essentially "stimulate" the emission, of more neutrons -- a little like a "neutron laser" effect. Could not neutrinos do something similar? There seems to be a physical parallel, between spontaneous-and-stimulated photon emission, and spontaneous-and-stimulated nuclear fission. In both cases, there seems to be a process parallel to the spontaneous one.

 

 

There's a huge difference, though. Neutrons interact via the strong force and neutrinos via the weak. Cross sections are very different. And thermal neutrons only induce fission in fissile materials.

The "terminal S-process cycle" involves "feeding" a Pb-206 nucleus 4x neutrons, which the nucleus "alchemizes" into 1x helium:

 

Pb-206 + 4n ----> Pb-206 + He + radiation

Thus, the lead nucleus acts like a "nuclear catalyst", a "fempto-fusion reactor" within which 4n are "glued" together, via the Color Force, into a He. Given a neutron source, a lead brick would do the fusion for you.

 

Stars use Gravity, to supply pressure, to squeeze 4x protons, into 1x He. The Color Force is (proverbially) 40 orders-of-magnitude stronger than Gravity -- which is why a single nucleus, via the CF, can fuse nucleons, like an entire star, via GF.