Neutrino-dark sector equilibration and primordial element abundances

After neutrinos decouple from the photon bath, they can populate a thermal dark sector. If this neutrino cooling occurs at a temperature above <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mo>∼</a:mo><a:mn>100</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>keV</a:mi></a:math>, it can impact light element abundances. We calculate the changes to the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mrow><c:mi mathvariant="normal">Y</c:mi></c:mrow><c:mi mathvariant="normal">P</c:mi></c:msub></c:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mi mathvariant="normal">D</g:mi><g:mo>/</g:mo><g:mi mathvariant="normal">H</g:mi></g:mrow></g:math> abundances in this scenario with a suitably customized version of the public big bang nucleosynthesis code PRIMAT. If electron neutrinos significantly participate in the cooling, we find limits down to temperatures as low as 100 keV. And if the dark sector undergoes a “step” in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>N</k:mi><k:mi>eff</k:mi></k:msub></k:math>, there can be additional, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>ω</m:mi><m:mi>b</m:mi></m:msub></m:math>-dependent constraints.