Gamma-ray emission from warm WIMP annihilation

Qiang YuanInstitute of High Energy Physics

Collaborated with Xiaojun Bi, Yixian Cao, Jie Liu, Liang Gao, Pengfei Yin & Xinmin Zhang

(arXiv:1203.5636)

KITPC cosmology month2012-09-05

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 Friday

Burkert (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 3×10-26 cm3 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

NTDMrc=10-

7

Outline

• Introduction of cold/warm dark matter

• Gamma-ray emission of warm WIMP based on numerical simulations

• Conclusion

Simulations

2keV WDMLovell et al. (2012)

CDM: AquariusSpringel 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≡∫2dV M vs. F≡L/d2

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)

Thank youThank you