Thermalisation of light sterile neutrinos in the early universe

Details Thermalisation of light sterile neutrinos in the early universe Thermalisation of light sterile neutrinos in the early universe

2012-04-26

arxiv.org/abs/1204.5861v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

19 pages, several figures. Prepared for submission to JCAP

Scientific paper

Recent cosmological data favour additional relativistic degrees of freedom beyond the three active neutrinos and photons, often referred to as 'dark' radiation. Light sterile neutrinos is one of the prime candidates for such additional radiation. However, constraints on sterile neutrinos based on the current cosmological data have been derived using simplified assumptions about thermalisation of the sterile neutrino at the Big Bang Nucleosynthesis (BBN) epoch. These assumptions are not necessarily justified and here we solve the full quantum kinetic equations in the (1 active + 1 sterile) scenario and derive the number of thermalised species just before BBN begins (T~1MeV) for null (L=0) and large (L=0.01) initial lepton asymmetry and for a range of possible mass-mixing parameters. We find that the full thermalisation assumption during the BBN epoch is justified for initial small lepton asymmetry only. Partial or null thermalisation occurs when the initial lepton asymmetry is large.

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