What does QFT really predict about matter/anti-matter interactions?

[hyksos]

In the context of popular science literature, when antimatter comes into contact with regular matter, what happens is described as "annihilation" or "they annihilate". It is also rumored that 100% of the mass in the constituents is converted to energy. The rumor would also suggest that there need not be co-equal particle types. (It is easy to imagine that electron / positron "collision" would yield a gamma ray, with all the requisite conservation-of-charge that entails.) However, the popular understanding also suggests that a proton colliding with an electron anti-neutrino would "annihilate" all the mass of the proton as well as all the mass of the electron anti-neutrino. Suggesting this interaction :

p⁺ ν-_e → γ

This is obviously wrong.

Many laypersons are satisfied with a doctrine that "they annihilate and produce light." But what does formal QFT actually predict about the process of matter/anti-matter interactions? Of all particles, why would a photon be the result of such collisions?

I am going to assume the answer (likely wrongly) that this has something to do with the way charge is represented in gauge theory.

Your thoughts?

 

[Orodruin]

However, the popular understanding also suggests that a proton colliding with an electron anti-neutrino would "annihilate" all the mass of the proton as well as all the mass of the electron anti-neutrino.

I don't think this is the case at all. Also, the interaction $e^+ e^- \to \gamma$ is not allowed. You need (at least) two photons for energy-momentum conservation.

 

[hyksos]

Thank for the reply. Could you expand a little bit more on what you mean by "needing" at least two photons? Are you saying that the input products also most contain two photons?

If the photons are not present somehow (this is very unlikely in high-energy situations) we would expect that the positron and electron would draw close due to Coulomb force, then form into a stable orbit around each other. (?) Not sure. I'm just asking.

 

[Orodruin]

Are you saying that the input products also most contain two photons?

No, I am saying you cannot annihilate into a single photon. You need to produce at least two.

 

[DarMM]

You're right that in a way "annihilate" implies that QFT describes some sort of process where they approach and explode into two photons.

What QFT actually says if that if you detect an electron and positron close to each other or detect them with momenta that point in each other's direction (rough phrasing for B thread) then there is a certain probability you will later detect two photons. The closer they are they higher the probability.

 

[snorkack]

A photon is likely because it has zero mass and integer spin, and participates in electromagnetic interactions. It can therefore easily be formed.
Annihilation to neutrinos is a weak process and they need to be formed pairwise, leading to low branching fraction. Weakness of gravitational interactions leads to low branching fraction for annihilation to gravitons.
Yet nucleons preferentially annihilate to pions - not photons!

 

[mfb]

Antimatter/matter reactions are not that special actually. Some particles collide, they stop existing and some other particles are created in the process. Just like every other interaction. Just a few matter/antimatter combinations will end up with only photons. This is true even if you pair a particle with its own antiparticle. A proton meeting an antiproton will typically produce a few pions instead of photons. Same for a proton and an antineutron. A proton meeting an antineutrino at low energy won't have any reaction, at higher energies you can produce all sorts of new particles in the collision.