Utente:Vilnius/Sandbox: differenze tra le versioni
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Riga 40:
in which the soft photon attaches to an internal line. The soft photon can attach to
any one of the n + m external lines, so we must include a '''sum over all such terms'''.
The full amplitude has a Laurent expansion in q with an infinite number of terms
whose detailed form depends on what theory we are talking about. '''For the pole we'''
'''need not specify what theory we are studying except that it has a photon'''. That is
one of the beauties of this formula.
The '''LSZ rule''' for computing scattering amplitudes starts out by computing
'''the time-ordered Green’s functions using the Feynman iε prescription and then'''
'''amputating the external legs.''' The Feynman diagrams have factors for vertices and
propagators. What happens when we attach the extra photon to an external leg is,
'''since external legs are amputated, we need only add a vertex and propagator for'''
'''the particle to whose external leg the photon is added'''. The difference between the
diagram with and without the attached external soft photon is just the vertex and
propagator.
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