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soft photons that are emitted from one of the external lines and absorbed in another (or in the same line). The two factors cancel each other, leaving a finite correction which depends on the sensitivity with which photons can be detected in the experiment.
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for the process. Even though soft photons are not detected, the possibility of their emission must be taken into account in the calculation of the
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much smaller than the energies of the particles participating in a particular
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