![]() The photon carries momentum that allows the initial rightward momentum of the electron to be conserved. Here is a simple vertex representing an electron 'releasing' a photon.Įxamining this vertex allows us to consider how various physical quantities must be conserved. Solid lines represent particles and a variety of wiggly lines represent the exchange particle. I learned it as time flows up and space is horizontal. It is aggravating that there is no solid convention for which is which. The two directions in this vertex (horizontal and vertical) represent space and time. Typically two represent particles and one is an exchange particle. With some limited exceptions a vertex is a 3 legged creature. At each vertex various quantities must be conserved. As far as I am aware gravitational interactions between particles are not described with Feynman diagrams.Ī vertex in a Feynman diagram is a representation of a fundamental interaction that is possible. The primary difference is that there are multiple exchange particles for these nuclear forces.įeynman diagrams are a tool for describing the way in which we imagine particle interact through the EM, weak, and strong forces. The other two forces we know about in the universe, the weak and strong nuclear forces, must therefore have exchange particles as well. For gravity we imagined a particle like a photon that was the result of gravity waves and we called it the graviton. Light is a result of changes in the EM field and it already had name for it's particle - the photon. In this conceptual model each type of field had a particle that was formed by changes in the field that carried the information. The exchange of information had to be 'carried' by something which led to the idea of 'exchange particles'. What exactly that information is/was is still fuzzy for me but I think of it as field 'information' whatever that is. One important idea that emerged during this time was the idea that when objects interacted they did so through the exchange of information. Over time people tried a number of ways to represent what seemed to be going on with varying degrees of success. In industrial radiography where high atomic number elements are irradiated, pair production can become the major attenuation process assuming the incident radiation energy exceeds 1.022 MeV.In the 60's particle physics was really starting to get off the ground and it seemed like every day there was some new confusing data or particle that we needed to make sense of. Pair production in reality does not become the dominant process in water below about 30 MeV (due to its dependence on the 'Z' of absorber) and is therefore of less importance in the low atomic number soft tissue elements. These photons are then absorbed or scattered within the medium. Here, the two particles are converted back into two photons of electromagnetic radiation, each of 0.511 MeV energy traveling at 180 degrees to each other (a concept utilized in positron emission tomography - PET). As it comes to a rest, it combines with a neighboring electron and the two particles neutralise each other in a phenomenon known as annihilation. The electron is quickly absorbed, however the fate of the positron is not so straight forward. The electron and positron, once liberated within the medium are dissipated through successive interactions within the medium. PP is related to the atomic number (Z) of attenuator, incident photon energy (E) and physical density (p) by Z E (- 1.022) p. If the energy of the incident photon is greater than 1.022 MeV, the excess is shared (although not always equally) between the electron and positron as kinetic energy. The reason at least 1.022 MeV of photon energy is necessary is because the resting mass (using E=MC² ) of the electron and positron expressed in units of energy is 0.511 MeV (or 9.1 x 10 -31 kg) each, therefore unless there is at least 0.511 MeV *2 (i.e., 1.022 MeV) it is not possible for the electron-positron pair to be created. Rest mass is the mass observed when the particle is stationary relative to the observer. However, much higher incident photon energy is required to create them as both have far higher rest masses (1776 MeV for the tau and 105 MeV for the muon) than the electron and positron. Pair production not only produces electron-positron pairs, other types of particles can also be produced such as muon - antimuon and tau - antitau pairs. These two particles form a matter-antimatter pair thus leading to the name of "pair production".
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