It does not represent a "particle" that can be measured, but a function necessary for the computation of cross-sections and lifetimes according to the limits of integration entering the problem under study. The function of the virtual line is to keep the quantum number conservation rules and help as a mnemonic. These "photons" instead of having a mass of zero, as they do when measured/observed have a varying mass imposed by the integral under which they have "existence". The internal lines in the diagrams carry only the quantum numbers of the exchanged named particle, in this example a virtual photon. Only the external lines are measurable and observable in this model, and the incoming and outgoing particles are on the mass shell. The standard model Lagrangian allows the calculation of cross-sections and lifetimes for elementary particles and their interactions, using Feynman diagrams which are an iconic representation of complicated integrals: This means that the mass of each particle, called rest mass (because it is the invariant mass the particle has in its rest frame) in the energy-momentum frame is given by: The standard model is a mathematical model based on a Lagrangian which contains the interactions of all these particles, and it is framed in the four dimensions of special relativity. These particles are completely and uniquely characterized by their mass and quantum numbers, like spin, flavour, charge. This is the table of particles on which the standard model of elementary particle physics is founded:
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