Minimal sterile neutrino dark matter

We propose a novel mechanism to generate sterile neutrinos <math display="inline"><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>s</mi></mrow></msub></mrow></math> in the early Universe, by converting ordinary neutrinos <math display="inline"><msub><mi>ν</mi><mi>α</mi></msub></math> in scattering processes <math display="inline"><msub><mi>ν</mi><mi>s</mi></msub><msub><mi>ν</mi><mi>α</mi></msub><mo stretchy="false">→</mo><msub><mi>ν</mi><mi>s</mi></msub><msub><mi>ν</mi><mi>s</mi></msub></math>. After initial production by oscillations, this leads to an exponential growth in the <math display="inline"><msub><mi>ν</mi><mi>s</mi></msub></math> abundance. We show that such a production regime naturally occurs for self-interacting <math display="inline"><msub><mi>ν</mi><mi>s</mi></msub></math>, and that this opens up significant new parameter space where <math display="inline"><msub><mi>ν</mi><mi>s</mi></msub></math> make up all of the observed dark matter. Our results provide strong motivation to further push the sensitivity of X-ray line searches, and to improve on constraints from structure formation.