This is a question about definitions. When two photons interact to create an electron/positron pair, does this process 'count' as annihilation of the photons? I've struggled to find a good definition of the term. Some places say that annihilation requires the end state to be electromagnetic radiation. But, on the other hand, I have found several text books which give annihilation processes ending in hadrons.
[Physics] Can two photons annihilate
particle-physicsquantum mechanics
Related Solutions
Quantum field theory does not offer a description of "how" its processes work, just like Newtonian mechanics doesn't offer an explanation of "how" forces impart acceleration or general relativity an explanation of "how" the spacetime metric obeys the Einstein equations.
The predictions of quantum field theory, and quantum electrodynamics (QED) in particular, are well-tested. Given two photons of sufficient energy to yield at least the rest mass of an electron-positron pair, one finds that QED predicts a non-zero amplitude for the process $\gamma\gamma \to e^+ e^-$ to happen. That is all the theory tells us. No "fluctuation", no "virtual particles", nothing. Just a cold, hard, quantitative prediction of how likely such an event is.
All other things - for instance the laughable description in the Wikipedia article you quote - are stories, in this case a human-readable interpretation of the Feynman diagrams used to compute the probability of the event, but should not be taken as the actual statement the quantitative theory makes.
There is no "how", what happens between the input and the output of a quantum field theoretic process is a black box called "time evolution" that has no direct, human-readable interpretation. If we resolve it perturbatively with Feynman diagrams, people like to tell stories of virtual particles, but no one forces us to do that - one may organize the series in another way, may be even forced to do so (e.g. at strong coupling), or one may not use a series at all to compute the probability. The only non-approximative answer to "how" the scattering processes happen in quantum field theory that QFT has to offer is to sit down and derive the LSZ formula for scattering amplitudes from scratch, as it is done in most QFT books. Which, as you may already see from the Wikipedia article, is not what passes as a good story in most circles.
But neither nature nor our models of it are required to yield good stories. Our models are required to yield accurate predictions, and that is what quantum field theory does.
Before the quark model became the standard model for particle physics, the prevailing model for elementary particle scattering was using the theory of Regge poles.
At the time (1960s) electromagnetic interactions/scatterings could be described very well with Feynman diagrams, exchanging virtual photons. The study of strong interactions tried to reproduce this successful use of Feynman diagrams ; for example there was the vector meson dominance model :
In particular, the hadronic components of the physical photon consist of the lightest vector mesons, ρ , ω and ϕ . Therefore, interactions between photons and hadronic matter occur by the exchange of a hadron between the dressed photon and the hadronic target.
The Regge pole theory used the complex plane and Regge trajectories to fit scattering crossections, the poles corresponding to resonances with specific spins at the mass of the resonance but arbitrary ones off. The exchange of Regge poles ( instead of single particles) was fitted to scattering crossection data. See this plot for some of the "fits" .
At the time , when it seemed that the Regge pole model would be the model for hadronic interactions, it was necessary to include elastic scattering, i.e. when nothing happened except some energy exchanges. The Regge trajectory used for that was called the Pomeron trajectory.
the particles on this trajectory have the quantum numbers of the vacuum.
If you really want to delve into the subject here is a reference. With the successes of the standard model the Regge theory was no longer mainstream, but considered old fashioned.
This abstract for , The Pomeron and Gauge/String Duality is revisiting the pomeron .
The emergence of string theories though revived the interest in regge theory and particularly the veneziano model which describes the regge poles and considers the resonances as excitations of a string.
Best Answer
Annihilation is defined as the collision of a particle and its antiparticle resulting in the destruction of both. The conversion products do not have to be photons, but usually are because probability of products created is inversely related to mass, and photons are massless. In colliders like the LHC, this can be compensated for by smashing matter at higher and higher velocities in the hopes that matter-antimatter pairs coming out of the remnants have enough kinetic energy to then create particles more interesting than photons.
In two photon interactions, the photon coupling causes a fermion-anti fermion pair, such as electron-positron pairs as exploited in in Positron Emission Tomography(PET). The resulting annihilation is thus not a direct result of the photon coupling and is a distinct event.