In particle physics, soft photons are

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...

having photon energies much smaller than the energies of the particles participating in a particular

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

process, and they are not energetic enough to be detected. Such photons can be emitted from (or absorbed by) the external (incoming and outgoing) lines of charged particles of the Feynman diagram 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 scattering amplitude., pages 177-184 and appendix A6 Taking the soft photons into account multiplies the rate of a given process by a factor which approaches zero. However, there exists also an infrared-divergent contribution to the scattering rate related to virtual 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. The Weinberg's soft phothon theorem simplify the calculation of the contribute of soft photons to the scattering of particles.

References

Gauge theories Scattering theory {{scattering-stub