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Diagram Beta decay: beta particle is emitted from an atomic nucleus Compton scattering: scattering of a photon by a charged particle Neutrino-less double beta decay: If neutrinos are Majorana fermions (that is, their own antiparticle), Neutrino-less double beta decay is possible. Several experiments are searching for this. Pair production and ...
A diagram that is not a forest diagram is called a loop diagram, and an example is one where two lines of an X are joined to external lines, while the remaining two lines are joined to each other. The two lines joined to each other can have any momentum at all, since they both enter and leave the same vertex.
Virtual particles may be mesons or vector bosons, as in the example above; they may also be fermions. However, in order to preserve quantum numbers, most simple diagrams involving fermion exchange are prohibited. The image to the right shows an allowed diagram, a one-loop diagram. The solid lines correspond to a fermion propagator, the wavy ...
The linked-cluster theorem asserts that the effective action = is the sum of all closed, connected, bare diagrams. [] = [] in turn is the generating functional for the connected Green's function. As an example, the two particle connected Green's function reads:
Interactions in the Standard Model. All Feynman diagrams in the model are built from combinations of these vertices. q is any quark, g is a gluon, X is any charged particle, γ is a photon, f is any fermion, m is any particle with mass (with the possible exception of the neutrinos), m B is any boson with mass. In diagrams with multiple particle ...
Interactions in the Standard Model. All Feynman diagrams in the model are built from combinations of these vertices. q is any quark, g is a gluon, X is any charged particle, γ is a photon, f is any fermion, m is any particle with mass (with the possible exception of the neutrinos), m B is any boson with mass. In diagrams with multiple particle ...
However, since they can be off shell, wherever the diagram contains a closed loop, the energies and momenta of the virtual particles participating in the loop will be partly unconstrained, since a change in a quantity for one particle in the loop can be balanced by an equal and opposite change in another. Therefore, every loop in a Feynman ...
Otherwise, the process is understood as the initial creation of a boson that is virtual, which immediately converts into a real particle + antiparticle pair. This is called an s-channel process. An example is the annihilation of an electron with a positron to produce a virtual photon, which converts into a muon and anti-muon. If the energy is ...