This is a follow up question to this question.

The process of annihilation is very well explained. However while the annihilation is a step-by-step process the pair production does not seem to follow this but happens in one step as mentioned here.

How is this asymmetry explained?

  • $\begingroup$ Why do you say it's a step-by-step process? $\endgroup$ – Javier Feb 27 '18 at 23:46
  • $\begingroup$ This is unclear: are you comparing $p\bar{p} \rightarrow X $ with $\gamma N\rightarrow e^+e^-N$? $\endgroup$ – JEB Feb 28 '18 at 0:18
  • $\begingroup$ Following @JEB's reasoning it might be instructive to look at the Drell-Yan process ($q + \bar{q} \to l^+ + l^-$ at the vertex level, and sometimes used as an expreimental probe used with muons for the leptons). $\endgroup$ – dmckee --- ex-moderator kitten Feb 28 '18 at 2:08

I wonder if the Feynman diagram for annihilation has misled you. The tree level diagram for annihilation is (from Wikipedia):


But this diagram is not an illustration of the physical processes happening during annihilation. Feynman diagrams are a graphical illustration of a mathematical equation, and they are not to be taken literally as an illustration of what is physically happening. Specifically this diagram does not imply that annihilation is a two step process.

There is not a simple description of what happens during annihilation in terms of point particles. When interactions are relatively strong, as in annihilation, we do not know what the particle states of the quantum field are. Particles are only well defined objects in the limit of infinitely weak interactions, when they are described by Fock states. The particles present, and even the number of particles present, are not defined during the interaction itself.

  • 1
    $\begingroup$ “There is no simple description ... point particles” this is so important. The name “particle physics” is misleads people into thinking in terms of (point) particles being the fundamental object. The subject should be called Quantum Field Theory, as the fields are always there, while the particles are only useful approximations under certain conditions. $\endgroup$ – Andrea Jul 10 '18 at 8:03

Do you mean this first sentence in the link:

Pair production is the creation of an elementary particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton.

It is a confusing statement since it does not stress in that first sentence,that these neutral bosons need an energy input, which is extra steps. It does say it further on. One should not stick to first sentences in a physics article.

These neutral bosons that carry energy and zero charge and quantum numbers, are the gamma $γ$, gluon $g$ , the zed $Z$, gauge bosons of the electromagnetic, strong, and weak interactions. The $g$ and the $Z$ can only exist within a diagram, as off mass shell virtual particles, there is a step of input interaction input interactions, as


gluon exchange

The photons also exist as real particles due to the long range nature of electromagnetism, but even so, a photon cannot generate a particle antiparticle pair in a vacuum, without extra interactions, because energy and momentum would be violated: the created pair has at least the mass of two electrons, whereas the photon has zero mass . The real photon has to interact with the fields of a nucleus in order to generate an e+e- pair, as the article you quote states:


No unstable state in particle physics is really a one step process. Pions and kaons and muons live long enough to be able to ignore their creation state and their decay may be considered a one step process, but not gluons and zeds into particle pairs. Also the need of conservation of energy and momentum adds further steps when necessary.


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