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Sterile Neutrinos As Subdominant Warm Dark Matter

Sterile Neutrinos As Subdominant Warm Dark Matter. D.T. Cumberbatch Oxford University, Astrophysics Dept. A. Palazzo, D. Cumberbatch , A. Slosar and J. Silk (arXiv:0707.1495). Outline. Introduction Sterile neutrino properties Motivations for keV sterile neutrino dark matter (SN DM)

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Sterile Neutrinos As Subdominant Warm Dark Matter

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  1. Sterile Neutrinos As Subdominant Warm Dark Matter D.T. Cumberbatch Oxford University, Astrophysics Dept. A. Palazzo, D. Cumberbatch, A. Slosar and J. Silk (arXiv:0707.1495)

  2. Outline • Introduction • Sterile neutrino properties • Motivations for keV sterile neutrino dark matter (SN DM) • Current experimental constraints on dominant SN DM • X-rays • Lyman- • Experimental constraints on sub-dominant SN DM • HEAO-1 (diffuse X-ray background) • SDSS (Ly-) • Theoretical predictions vs. Experimental constraints • Qualitative/Quantitative treatments of hadronic uncertainties • Determination of upper limits on fs • Summary & Conclusions

  3. What are Sterile Neutrinos? • Electroweak singlet (Bruno Pontecorvo, (1967) ) • No weak interactions (i.e. “sterile” or right-handed) • Naturally explanation for small (but finite) “active” neutrino masses • Introduced through the seesaw Lagrangian (Minkowski) • Ma»100GeV or as small as ~eV • For Ma~ EEWand ya,MSM provides correct a masses • Masses of lightest neutrinos suppressed by <H>/ Ma • Variety of production mechanisms • Non-resonant oscillations • Resonant oscillations

  4. Non-resonant Oscillations • Produced through a-s oscillations involving off-diagonal elements of mixing matrix (hence nocoupling to SM Gauge bosons) • Don’t require finite lepton asymmetry • Simplest model: Dodelson-Widrow (DW) mechanism • Standard thermal history • No additional couplings required • Produces minimal relic density • Alternatives to DW • Entropy dilution (Abazajian & Fuller, Abazajian & Koushiappas) • Additional Couplings (e.g. to Inflaton (Shaposhnikov & Tkachev)) • Low TR models (Gelmini et al., Yaguna et al.) • Resonant Oscillations • Requires L0 (Shi & Fuller)

  5. Decay modes • Dominant decay to 3 active neutrinos (s  3a) • Loop-suppressed radiative decay (s   a) • Photon Energy E=(ms2-ma2)/2ms≈ms/2 for ms»ma • For ms~1keV  X-rays • Use X-ray observations to constrain ms and sin2(2s) (and later, fs for keV SN DM (Barger et al., Pal & Wolfenstein)

  6. Motivations for keV SN DM • Resolution to inconsistencies from CDM halo simulations • Central cores in low-mass galaxies (Dalcanton & Hogan) • Overpopulation of small-scale satellites (Kauffman et al., Klypin et al.) • Contribution to unresolved diffuse X-ray background (Abazajian et al.) • Facilitates HI production/star formation  Early Reionisation (Mapelli et al., Biermann & Kusenko) • Pulsar “kick” velocities (Kusenko & Segre, Fuller et al.) • Re-generation of Supernova shockwaves (Hidaka and Fuller) • Formation of early SMBH (109Mat z≈6.5) (Munyaneza & Biermann) • Baryon asymmetry (Asaka et al.) • Neutrino oscillations (LSND/MiniBoonE require at least 2 light (~eV) SNs) (Goswami & Rodejohann)

  7. Experimental constraints on ms and sin2(2s) • X-rays (Clusters, dSphs, MW, Galaxies, XRB) (Ruchayskiy, arXiv:0704.3215) • All these sources have similar NH, but different thermal emissions • F.O.V and Energy resolution of instruments dictate sensitivity

  8. Experimental constraints on ms and sin2(2s) • Lyman- • Sensitive to small-scale matter perturbations where SN are significant • Operates at high-z before non-linear growth erases free-streaming effects (Abazajian, astro-ph/0511630) (Seljak et al., astro-ph/0602430) •  ms >14 keV includes 10% suppression from non-thermal nature of SN • Recent calculations imply <ps>/<pa> can be as low as 0.8 ms>11.5 keV (<ps>/<pa>~0.8)

  9. DW SN DM for fs=1 (DM=0.24) If fs=1 is excluded, how large can fs be? Is Dodelson-Widrow SN DM still permitted? (Yuskel et al., arXiv:0706.4084)

  10. Spectral Analysis of HEAO-1 XRB data • Fit HEAO-1 data to a prescribed model (Gruber et al.) • Add MW and extragalactic (EG) contributions from SN DM (for given ms), correcting for finite energy resolution of HEAO-1 (E/E~0.25) • Vary CXRB, TXRB, XRB and sin2(2s) until 2worsens by 2 corresponding to a 3 C.L. • Determine sin2(2s) for all ms within range of data

  11. Spectral Analysis of HEAO-1 XRB data (Boyarsky et al., astro-ph/0512509 )

  12. EG and MW flux from SN DM • Extragalactic differential energy flux • Flat, -matter dominated Universe (dm  0.21) • Local differential energy flux from MW

  13. X-ray constraints on ms and sin2(2s) For fs<1we expect a “rigid” shift in constraints to larger sin2(2s) by 1/fs

  14. Ly- constraints on ms and sin2(2s) (fs<1) • ms>11.5 keV (fs=1) from limiting suppression of P(k) for k>~Mpc-1 • For fs<1 its most appropriate to run a grid of hydrodynamical simulations involving WDM+CDM and relate Ly- power spectrum to parameters. • Alternatively, we can obtain reliable constraints by “rescaling” fs=1 results • Using CAMB (Lewis et al.), we grow perturbations in a WDM(fs)+CDM(1-fs) scenario using a SN relic abundance of • Owing to “pencil-beam” nature of Ly- measurements we use P1D(k) • Determine (ms)min., for fs<1, by invoking P1D(kf, fs<1)=P1D(kf, fs=1) at kf ~2h Mpc-1, where SDSS Ly- data is most sensitive. • Reliable results for fs>0.1for1<k_f/Mpc-1<5 (<10% variation).

  15. Ly- constraints on ms and sin2(2s) (fs<1) P(kf,fs)~0.23PCDM(kf,fs=1)

  16. Experimental constraints & Theoretical predictions (2) A C D E F B DW SN DM excluded for fs=1 DW SN DM permitted for fs=0.2 (2.5<ms/1keV<16, 10-10<sin2(2s)<2.5x10-9)

  17. Theoretical uncertainties • SN which are non-resonantly produced have a relic abundance (Asaka et al.) • Accounting for hadronic uncertainties during QCD epoch, best-fit relation between fs, ms and sin2(2s) is • Pushing all errors in the same direction, one either obtains the minimal… …or the maximal abundances • We conservatively consider these extreme cases to represent a 2 C.L.

  18. Constraints on the DW scenario (2) • Plot corresponding points (A,B), (C,D) and (E,F) for all fs<1  0.55<(fs)max.<0.75

  19. Theoretical uncertainties (Quantitative treatment) • Vary fs around fsav. given by best-fit formula. • Adopt a normal distribution of log10(fs) with a s.d. equal to half the excursion (wrt log10(fsav.) determined by extreme formulae. • This corresponds to adding a penalty factor to the total 2 with 1 (asymmetric) errors • Marginalising 2 wrt fs, ms or sin2(2s), we obtain new constraints…

  20. Constraints on the DW scenario fs1 fs0.7  (fs)max.0.7 (2), (fs)max.=1 (~3)

  21. Summary & Conclusions • Recent results disfavour keV dominant sterile neutrino dark matter produced via the Dodelson-Widrow mechanism. • Relaxing the presumption that s=dm, we have shown how X-ray and Ly- constraints can be re-interpreted for s<dm. • Most importantly, we have shown how current data provides a conservative upper bound on the fraction fs of dark matter constituting of sterile neutrinos produced via the DW mechanism. • We obtained the limits fs<~0.7 (2), with fs=1 rejected at ~3. • We found that these limits are robust with respect to the large hadronic uncertainties affecting theoretical predictions. • More sensitive X-ray observations, a better understanding of the systematics in Ly- measurements and a reduction in the theoretical uncertainties all have a crucial role in improving our results.

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