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In the proton-proton chain reactions which happen, for instance, in our Sun, two protons collide and form a proton and a neutron. However, this just blows my mind.

What is the mechanism by which a proton simply loses its charge, becomes slightly more massive, and turns into a neutron?

Hi, Steven - you've stumbled across what physicists know as the "weak interaction."

The weak interaction is the fundamental force which is involved in radioactive decay and nuclear reactions like the pp chain. The first step in the pp chain (which is actually two steps) can be written as:

p + p -> p + n + e+ + νe

where the protons (p) react to form a deuteron (p+n) while emitting a positron (otherwise known as an anti-electron: e+) and an electron neutrino (νe), as well as releasing about 400 keV of energy. The weak interaction dictates the change from proton to neutron + positron + neutrino. There are conservation laws that these reactions must follow: the total charge must be conserved, the total lepton number must be conserved, and the lepton family must be conserved, as well as the conservation of energy and momentum that applies everywhere else (there are more specific rules but we won't get into them here). So look again at the portion of the reaction that's changing one proton into a neutron:

p -> n + e+ + νe

Solar Flare

A solar flare erupts on the sun.
Image Credit: SDO/AIA



We can see that the proton on the left hand side of the reaction has a charge balanced by the positron on the right hand side. We can also see that the lepton number is conserved: electrons and neutrinos are "leptons", while protons and neutrons are "baryons" (these names refer to the type of matter - leptons are fundamental, while baryons are made of three quarks each).

So there is one baryon on the left and one on the right: balanced. And there are zero leptons on the left and an anti-electron (-1 lepton) and neutrino (+1 lepton) on the right: zero and zero, balanced (the trick here is to know that having an anti-particle is like subtracting). And our lepton family is also conserved: because we have an anti-electron (positron), we must have an electron neutrino.

A tau or anti-tau particle would have a tau neutrino, and a muon or anti-muon would have a muon neutrino.

As for the change in mass, there's one extra thing we're missing, and that's Einstein's famous E=mc2. When two protons combine into a deuteron (p+n) like in the pp chain, the energetics are favorable. It's like putting two buckets together and ending up with a bucket that's more than twice as deep as the originals. It means that a deuteron is actually less massive than two protons. Because of this, the reaction releases energy (400 keV worth!).

It's worth noting that, if you have just the proton "decay" by itself (p -> n + e+ + νe), without the extra proton as in the pp chain, the energetics are not favorable. A neutron does have more mass than a proton. And that's why protons don't decay in free space (currently, the lifetime of the proton is speculated to be about 1032 years... way longer than the age of the universe!).


Answered by:

Kelly Chipps (AKA nuclear.kelly)
Postdoctoral Fellow
Department of Physics
Colorado School of Mines

Asked by:

Steven from Louisiana