Nearannihilation

Nearannihilation is a qualitative term used in particle physics and cosmology to describe a regime in which a particle-antiparticle system has a very high probability of annihilating into other products, but the process does not proceed to completion in every event. The phrase is not a formal term with a single precise definition, but it is used to indicate that the annihilation cross section, times the relative velocity (sigma v), is large relative to competing processes or the expansion rate of the system.

In practice, near annihilation tends to occur in two limiting circumstances. Near a resonance, when the center-of-mass energy is close to the mass of an intermediate unstable state, the cross section can be greatly enhanced by a Breit-Wigner peak, pushing the system toward efficient annihilation. Near a production threshold, phase-space limitations can also influence the likelihood of annihilation, producing a regime where the final-state phase space is restricted but still favored.

In the early universe, near annihilation is linked to the period when the thermally averaged annihilation rate Gamma_ann = <sigma v> n becomes comparable to the Hubble expansion rate H. As the universe expands and cools, this rate falls, and relic abundances imply that not every particle-antiparticle pair annihilates.

Applications: In dark matter models, annihilation cross sections governing near-annihilation determine the present-day indirect detection signals and the thermal relic density. In collider experiments, near annihilation conditions affect the rates for final states such as photons, jets, or leptons following particle-antiparticle annihilation.

Limitations: The term is heuristic; precise predictions require calculating cross sections from a given theory, including all contributing channels and quantum numbers.