Electronpositron annihilation freezeout in the early universe
Abstract
Electronpositron annihilation largely occurs in local thermal and chemical equilibrium after the neutrinos fall out of thermal equilibrium and during the big bang nucleosynthesis (BBN) epoch. The effects of this process are evident in BBN yields as well as the relativistic degrees of freedom. We selfconsistently calculate the collision integral for electronpositron creation and annihilation using the KleinNishina amplitude and appropriate statistical factors for Fermi blocking and Bose enhancement. Our calculations suggest that this annihilation freezes out when the photonelectronpositronbaryon plasma temperature is approximately 16 keV, after which its rate drops below the Hubble rate. In the temperature regime near 16 keV, we break the assumption of chemical equilibrium between the electrons, positrons, and photons to independently calculate the evolution of the chemical potentials of the electrons and positrons while computing the associated collision integrals at every time step. We find that the electron and positron chemical potentials deviate from the case with chemical equilibrium. While our results do not affect the interpretation of precision cosmological measurements in elucidating the standard cosmological model, these out of equilibrium effects may be important for testing physics beyond the standard model.
 Publication:

Physical Review D
 Pub Date:
 March 2020
 DOI:
 10.1103/PhysRevD.101.063507
 arXiv:
 arXiv:1910.14050
 Bibcode:
 2020PhRvD.101f3507T
 Keywords:

 High Energy Physics  Phenomenology;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 Phys. Rev. D 101, 063507 (2020)