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: ) KITPC cosmology month

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

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

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)

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

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

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

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

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

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

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× cm 3 s) is plausible

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 Non-thermal warm WIMP

Matter power spectrum 2 keV WDM NTDM r c =10 -7

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

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

(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

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

Spatial skymaps: CDM

Spatial skymaps: WDM

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

Detectability comparison

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

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)

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