Sterile Neutrinos and WDM

1. Sterile Neutrinos and WDM R.Svoboda

2. Sterile Neutrinos as DM? • As WIMP searches getting well below the weak interaction scale for large range of masses and collider limits now limit smaller masses. What if no WIMPs are found to well below weak scale? Spin-dependent Spin-independent

3. "Satellite" problem • CDM seems to imply many more small galaxies (e.g. dwarf galaxies around the Milky Way) than is actually seen. Like 100's predicted and 10's see, • Are small galaxies broken up at some point? • Are their selection effects that cause them to not to be found yet?

4. Leo –I (800k LY)

5. Willman and Strader 2012

6. Constraints on Sterile Neutrino DM • How would it be produced? • Tremaine-Gunn bound • X-ray constraints • Lyman-a Forest and large scale structure • Direct Experimental Limits?

7. Thermal vrs Non-Thermal Production In general, direct production from thermal equilibrium with freeze out occurring when relativistic causes problems with the relic density being too high. Need non-thermal. Way too much! Rapid expansion density usual stuff m/T determines density At freeze out e.g. production by Oscillation!

8. Tremaine-Gunn Bound • A phase space argument based on velocity and mass of WDM particle relativistic freeze out compared to observed size of galactic halos today • MDM > ~300-500 eV

9. X-Ray constraints • heavy sterile neutrinos that mix with light neutrinos have a radiative decay mode proportional to M5 sin22q • Can look in DM rich regions for x-rays from such decays • Non definitively found thus far

10. J.W. den Herder, et al arXiv:0906.1788

11. Lyman-a • Observation of QSO spectra show red-shifted La lines due to neutral hydrogen clouds between us and QSO • The "forest" of lines is a measure of the matter density (assuming neutral hydrogen clouds trace DM distribution) as a function of z • This can be turned into a "scale size" of the universe versus z • This sets limits on contributions from WDM

12. So in general WDM with sterile neutrinos prefers heavy mass >2 keV and very small mixing This is in tension with observed lack of small satellite galaxies and the theoretical motivation for leptogenesis

13. Reactor Neutrino Anomaly

14. Allowed Region from Reactors andGNO calibrations

15. Oscillation length for light/heavy mixing 1.27 Dm2 L/E = p/2 gives L ~ (1.2)E/Dm2 For MH = 1 keV and E = 30 MeV, then L << 1 mm! Essentially, a two detector experiment not possible at these energies Other experiments?

16. MINOS arXiv:hep-ex:1104.3922 Look at NC identified events In FD. Note: these look like ne to them. High background and correlation to q13 reduce sensitivity

17. 90% cl limit of 40% of oscillated nm going to sterile SNS could certainly do better

18. Where would SNS fit in? • It is possible to do a NC disappearance experiment – essentially comparing predicted and measured NC component • Unlike reactors, the flux seems easier to predict, and there is a "golden" NC (not CC) tag if you use carbon target (see OscSNS proposal)

19. NC Disappearance at SNS OscSNS Proposal, 3 years. Proposal calculates NC interaction rate in miniBooNE like detector (50% efficiency) of 1300/year. Issues: (a) cross section precision (b) absolute neutrino flux prediction (need 1% to compete with reactor measurements)

20. Conclusions • There is interest in heavy sterile neutrinos in astrophysics as well as physics community • relevant experiments will be ONE DETECTOR due to short oscillation length. Absolute flux measurement is needed. • Astrophysics would prefer very small mixing angle – but very model dependent • New Territory!