Gamma-ray emission from warm WIMP annihilation

1. Gamma-ray emission from warm WIMP annihilation Qiang Yuan Institute of High Energy Physics Collaborated with Xiaojun Bi, Yixian Cao, Jie Liu, Liang Gao, Pengfei Yin & Xinmin Zhang (arXiv:1203.5636) KITPC cosmology month 2012-09-05

2. Outline • Introduction of cold/warm dark matter • Gamma-ray emission of warm WIMP based on numerical simulations • Conclusion

3. Structure evolution: cold dark matter Bottom-up structure formation pattern instead of top-down pattern (fragmentation): cold dark matter Springel et al. (2006) Nature CDM simulation vs. galaxy survey

4. How cold is dark matter? The coldness of dark matter depends on the free-streaming scale during the formation of structures • Hot dark matter (eV neutrinos) that washes out fluctuations on cluster scale (10 Mpc/h) • Warm dark matter (sterile neutrinos) that washes out fluctuations on galaxy scale (1 Mpc/h) • Cold dark matter that has effectively zero thermal velocity From Jing’s Nanjing talk (2012)

5. CDM WDM How cold is dark matter: matter power spectrum Tegmark et al. (2004)

6. How cold is dark matter: number of satellites Jing (2001)

7. How cold is dark matter: circular velocity of Milky Way satellites Lovell et al. (2012)

8. How cold is dark matter: velocity width function of galaxies (ALFALFA survey) Papastergis et al. (2011)

9. How cold is dark matter: central density of dwarf galaxies Burkert (1995) S. Shao’s talk on Friday

10. Observational summary • Large scale structures are very close to CDM • At (sub-)galactic scales, many discrepancies between observations and CDM expected (abundance, density profile, velocity profile) • WDM can better explain the observations

11. Detection of WDM particles? • Traditionally, WDM is light (e.g., sterile neutrinos) • Most of DM experiments are dedicated on WIMPs; it is fatal if DM is warm and light • Nevertheless, if non-thermally produced, WIMPs could also be warm (Jeannerot et al., 1999; Lin et al., 2001; Bi et al. 2009) • Another feature of non-thermal WIMPs is that larger annihilation cross section (compared with 3×10-26 cm3 s) is plausible

12. Non-thermal warm WIMP • DM particles are produced through decay of very heavy particles (e.g., from cosmic string) and carry very large initial momentum • Large initial momentum will correspond to a large free-streaming length

13. 2 keV WDM NTDM rc=10-7 Matter power spectrum

14. Outline • Introduction of cold/warm dark matter • Gamma-ray emission of warm WIMP based on numerical simulations • Conclusion

15. Simulations CDM: Aquarius Springel et al. (2008) 2keV WDM Lovell et al. (2012)

16. (Sub-)halo density profile • Core in the center • Core size is anti-correlated with halo mass • For Milky-Way halo, CDM and WDM profiles are identical within resolution

17. Subhalo statistics M vs. L≡∫2dV M vs. F≡L/d2

18. Spatial skymaps: CDM

19. Spatial skymaps: WDM

20. Two supersymmetric benchmark models Total skymaps with diffuse background (E>10 GeV)

21. Detectability comparison

22. Impact on direct detection • Velocity distribution? • Nucleon-DM scattering cross section?

23. Conclusion • Subhalos are less abundant for WDM, resulting a very flat subhalo luminosity function • It is currently difficult to detect either the cold or warm WIMPs, but the detectability of warm WIMP can be in principle better than cold WIMP due to a potentially larger cross section • For DM indirect search strategy, the Galactic center may be prior to dwarf galaxies for warm WIMP scenario (different from that for cold WIMPs)

24. Thank you