Soft photons
Encyclopedia
In particle physics
, soft photons are photons with energies much smaller than the energies of the particles participating in a particular scattering
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
.
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.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, soft photons are photons with energies much smaller than the energies of the particles participating in a particular scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
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
Feynman diagram
Feynman diagrams are a pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, first developed by the Nobel Prize-winning American physicist Richard Feynman, and first introduced in 1948...
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
Scattering amplitude
In quantum physics, the scattering amplitude is the amplitude of the outgoing spherical wave relative to the incoming plane wave in the stationary-state scattering process...
.
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
Infrared divergence
In physics, an infrared divergence or infrared catastrophe is a situation in which an integral, for example a Feynman diagram, diverges because of contributions of objects with very small energy approaching zero, or, equivalently, because of physical phenomena at very long distances.The infrared ...
contribution to the scattering rate related to virtual
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
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.