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The electron in the beam with $k^{\mu}$ four-momentum scatters into a detector with $k'^{\mu}$ four-momentum, the associated virtual photon has four-momentum: $$q^{\mu} = k^{\mu} - k'^{\mu} = (E, 0, 0, E) - (E', E'\sin{\theta}, 0, E'\cos{\theta} )$$ where the electron energies $E$ and $E'$ are much greater than $m_ec^2$ and $\theta$ is the lab angle. Note ...

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One calculates interactions in elementary particles using Feynman diagrams, which have strict one to one correspondence with mathematical integrals. this is a simple diagram but the concepts hold for all. The solid lines describe real particles,i.e. on mass shell. The exchanged line is a virtual photon in this case. The integral is defined by the incoming ...

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On some level, no particles are "physical"; they're all tools that we use to describe the interactions of quantum fields. This is especially true for virtual particles, which don't exist outside of the reaction they're part of. Typically, we call the particles that enter and exit Feynman diagrams "real". They propagate "to infinity&...

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