Revisiting sterile neutrino dark matter in gauged <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>U</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>1</mml:mn><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow><mml:mi>B</mml:mi><mml:mo>−</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:msub></mml:math> model
Shintaro Eijima, Osamu Seto, Takashi SHIMOMURA
Abstract
We reexamine sterile neutrino dark matter in gauged $U(1{)}_{B\ensuremath{-}L}$ model. Improvements have been made by tracing and careful evaluation of the evolution of the number densities of sterile neutrinos $N$ and extra neutral gauge bosons ${Z}^{\ensuremath{'}}$. As a result, the cosmologically-interesting gauge coupling of $U(1{)}_{B\ensuremath{-}L}$ for freeze-in sterile neutrinos turns out to be smaller than the values reported in the literature. This avoids the overproduction of ${Z}^{\ensuremath{'}}$ so that it is consistent with the big bang nucleosynthesis and the cosmic microwave background constraints on the effective number of neutrino species. Similarly, the free-streaming length constraints exclude a large parameter space derived in previous studies. In addition to known freeze-in pair production of $N$ from the standard model fermion pairs, we find the case that $N$ is dominantly produced from a pair of ${Z}^{\ensuremath{'}}$ at the temperature characterized by the $B\ensuremath{-}L$ breaking scalar mass. Thus, the naive truncation of the $U(1{)}_{B\ensuremath{-}L}$ scalar contribution made in the literature is not valid.