1. Sterile neutrinos with a pinch of pepper (terrible, terrible title) Cosmo 07, Brighton Anže Slosar, Oxford University

2. Introduction  Sterile neutrinos come in 2 kinds: * very light, usually thermalised species (eV range) * dark matter candidate (keV range)  I am going to talk about the latter  Theoretically attractive: * we know that neutrinos have mass and hence that the theory is incomplete * observationally constrainable from all sides * “hints” from various independent observations

3. Sterile neutrino as DM  Assume 3 thermalised, light, usual neutrinos  Add a 4 th sterile state  Populate the sterile state with sub-thermal aboundance via Dodelson-Widrow mechanism  One free parameter: mass – typically O(keV)  Lower limits on mass from Lyman-alpha observations  Upper limits on mass from X-ray observations

4. HINTS  Supernova explosion process affected  Pulsars are known to have anomalously large peculiar velocities – could be explaind by pulsar kicks  Decaying sterile neutrinos would significantly affect the reionisation by boosting formation of molecular hydrogen - potentially observable through 21-cm and other probes of reionisation

5. Lyman-alpha limits  Radiative decay channel:  Virgo cluster results: m < 8 keV (Boyarski et al)  Diffuse X-ray background: m < 5 keV (Abazajian) X-ray limits  While relativistic, neutrino free-streams and erases fluctuations on small scales – O (1 Mpc)  Ideally probed by Lyman-alpha forest at z=3  m > 10 keV at 99.9% (Seljak et al)

6. From astro-ph/0511630, Kevork Abazajian

7. How to rescule the model?  Dodelson-Widrow mechanism well-understood: difficult to escape limits  Can mix them with dark matter: Palazzo et al, arXiv:0707.1495  Can make them cooler: Kushenko, hep-ph/0609081

8. Mixing with dark matter  Constraints from X-ray easy to adapt: one simply has to assume a smaller energy density in steriles  Constraints from LYA more difficult  Adopts a rescaling procedure by matching 1D power spectra at 2 h/Mpc.

9. Mixing with dark matter

11. Making them cooler  Alexander Kusenko considers single Higgs boson coupled to neutrinos  Additional production mechanism of Higgs' S decaying to neutrinos at around 100 GeV  These neutrinos have approximately thermal momentum distribution at  A very attractive way out.

12. Making them cooler

13. Conclusions  Sterile neutrinos as DM candidate still alive and kicking.  If mixed with DM, constraints weaken considerably, without large admixture of dark matter  The model with early production very attractive theoretically, but could do with some more accurate modelling of observations