Ze-Peng Liu, Yue-Liang Wu and Yu-Feng Zhou Kavli Institute for Theoretical Physics China, Institute of Theoretical Physics, Chinese Academy of Sciences arXiv: [hep-ph] Enhancement of dark matter relic density from the late time dark matter conversions 海峡两岸粒子与宇宙学研讨会 , 新竹

Outline Introduction: evidences of DM from observations DM candidates: WIMPs recent experimental results Thermal evolution of interacting multi-DM Generic case with multiple component DM models Boost factor in two-component DM model Numerical results and a simple model Conclusions

DM revealed from gravitational effects Gravitational curves Strong lensing Weak lensing Large scale structure CMB Bullet clusters

What we know about DM Massive: from gravitational interactions. Stable: lifetime longer than the age of the Universe Electro-magnetic and color neutral: dark, but can annihilate into photons Non-baryonic MACHOs: disfavored by micro-lensing survey MOND: disfavored by bullet clusters D/H from BBN: CMB: Non-relativistic motion ( from N-body simulations ) Cold DM ： substructure, halo core Warm DM ? A big challenge to the standard model of particle physics !

Stability: symmetry + kinematics Symmetries important for keeping particle stable electron: U(1) em. symmetry, lightest charged particle proton: U(1) B-L symmetry, lightest baryon neutrino: Lorentz symmetry, lightest fermion DM protected by symmetries Known examples SUSY: R-parity, LSP UED: KK-parity, LKP Little Higgs: T-parity LR model: P and CP parity W.L. Guo, L.M.Wang, Y.L. Wu, YFZ, C. Zhuang Phys.Rev.D79:055015,2009 W.L.Guo, Y.L. Wu, YFZ, Phys.Rev.D82:095004,2010 W.L.Guo, Y.L. Wu, YFZ, Phys.Rev.D81:075014,2010 DM stability

DM relic density: The WIMPs miracle Thermal freeze out: the origin of species Weakly Interacting Massive Particles (WIMPs) Particle physics independently predicts WIMPs WIMPs have just the right relic density WIMPs are testable by the current exp.

Search for non-gravitational effects ? Satellite underground Cherenkov telescope balloon collider

Hint of DM ? Positron fraction if interpreted as DM signal Large annihilation cross section now, boost factor problem. Sommerfeld enhancement ? Resonance enhancement ? Non-thermal DM ? DM may slightly decay ? Mainly annihilation/decay into leptons, not quarks Light final states <1GeV ? Leptophilic interaction ? background PAMELA Nature 458, 607 (2009)

Hint of DM? electrons plus positrons ATIC/PPB-BETS Excess in the total flux peak at ~600 GeV rapid drop below 800GeV Fermi LAT Spectrum harder than expected background with power index around ~3. Nature, 456, 2008, Phys.Rev.Lett.102:181101,2009

Direct searches

CRESST

EDELWEISS-II EDELWEISS-II, arXiv:

The boost factor problem The std. WIMP annihilation cross section is too small to account for the PAMELA/Fermi data Positron flux Boost factor Need a large boost factor B~ Bergstrom, Edsjo, Zaharijas, PRL103,031103,09’

Boot factor for DM annihilation Local clumps Via Lactea II: in subhalo? B~ 4-15, Temperature-dependent ann. cross section Sommerfeld enhancement Resonance enhancement Possible origins of boost factor Diemand, et al, , Nature Sommerfeld, Ann. Phy 403, 257 (1931). J. Hisano, S. Matsumoto and M. M. Nojiri, Phys. Rev. D 67 (2003) Phys. Rev. Lett. 92, (2004) Feldman, Liu, Nath, 09 Ibe, Murayama, Yanagida, 09 Guo, Wu, 09 Other mechanism: DM decay, non-thermal DM ….

Constraints from relic density Other constraints Halo shape CMB, protohalo Refined analysis at freeze-out Cut-off of resonance, recoupling Force-carrier production & decay rates Kinetic decoupling Self-interaction efficiency, non-thermality J. Zavala, M. Vogelsberger and S. D. M. White, Phys. Rev. D 81, (2010) M. Kamionkowski and S. Profumo, Phys. Rev. Lett. 101, (2008) J. L. Feng, M. Kaplinghat and H. B. Yu, Phys. Rev. Lett. 104, (2010) arXiv:

Boost factor in multi-component DM models Large boost requires 1. Large annihilation cross section 2. Still the correct relic density Impossible for one- component thermal DM? Multi-component DM Models with hidden sectors naturally have multi-DM DM may have SUSY partners Neutrinos are already (tiny) part of DM boost from simply mixed thermal multi-DM ? (No) Boost factor from interacting multi-DM ?(Possible) For thermal relic large cross section Always reduces signal Z.P.Liu, Y.L.Wu and YFZ, arXiv:

Thermal evolution of interacting multi-DM The components can be converted Thermal evolution for interacting DM Use common variable

the DM conversion process Maintain thermal equilibrium between the DM components, after decoupling from the SM thermal bath Convert the heavy DM into the light

Thermal evolution of the total density The total density at equilibrium The total density evolves like an ordinary WIMP at early time effective cross section is temperature-dependent

The effective cross section A interesting limit Approximate form The two-component case

Thermal evolution for two-component DM 1. Thermal equilibrium with SM 2. Decouple from SM, but still in equilibrium with each other 3. Late time DM conversion at large z Slow conversion characterized by r(z) Crossing point 4. Complete decouple (freeze-out) after Freeze-out condition Y1(z) increased eventually

Numerical results Equilibrium Equilibrium density Y2

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1 If no conversion Decoupling of Y2

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1 If no conversion Decoupling of Y2 Decoupling of Y1

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1 If no conversion Decoupling of Y2 Decoupling of Y1 With conversion Evolution of Y2

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1 If no conversion Decoupling of Y2 Decoupling of Y1 With conversion Evolution of Y2 Evolution of Y1

Numerical results Equilibrium Equilibrium density Y2 Equilibrium density Y1 If no conversion Decoupling of Y2 Decoupling of Y1 With conversion Evolution of Y2 Evolution of Y1 Evolution of Y1+Y2

Numerical results B vs mass difference B vs relative cross sections

Conditions for a large boost factor Large internal degree of freedom of Y2: Small mass difference: Cross sections satisfy: Approximate expression for the boost factor

A simple 2dm model Add to the SM

Cross sections

Summary In multi-DM models, DM conversion can significantly modify the thermal evolution of each DM component. The relic density of the DM component may not always inversely proportional to it’s annihilation cross section. Through conversions from heavier DM components, the relic density of light DM can be enhanced, leading to large boost factors. The boost factor is independent of DM velocity. For generic models with large conversion rate the boost fact can reach ~ Thank You !

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