Sterile Neutrino Light Relics: The Production Window Set by Mixing, Set Against X-Ray and Bounce Scales

The required mixing falls steeply with mass, from 2.70e-10 at 1 keV through 5.51e-12 at 7 keV to 1.08e-13 at 50 keV, a span of 2.50e+03. At low masses the required mixing is comparatively large and sits in territory that X-ray non-detections exclude; at high masses structure-formation limits on warm dark matter take over from below. The production epoch, at temperatures of order 100 MeV, is far below the thermal scale at the bound density of 9.72e+54 g per cubic centimetre, so the capped history contains the whole production era unchanged. Non-resonant production requires mixings from 2.70e-10 down to 1.08e-13 across 1 to 50 keV, and the bounce, whose scales lie far above the production epoch, leaves the familiar squeeze intact. This study is computed from stated assumptions and published comparison values; it is not a measurement.