Enhancement of dark matter relic density
from the late time dark matter conversions
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:1101.4148[hep-ph]
海峡两岸粒子与宇宙学研讨会2011.04.01-06, 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
Bullet clusters
Large scale structure
Strong lensing
CMB
Weak lensing
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 !
DM stability
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 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
PAMELA
background
Nature 458, 607 (2009)
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 ?
Hint of DM? electrons plus positrons
ATIC/PPB-BETS



Excess in the total flux
peak at ~600 GeV
rapid drop below 800GeV
Nature, 456, 2008,362-365
Fermi LAT

Spectrum harder than
expected background with
power index around ~3.
Phys.Rev.Lett.102:181101,2009
Direct searches
CRESST
EDELWEISS-II
EDELWEISS-II, arXiv:1103.4070.
The boost factor problem

The std. WIMP annihilation
cross section is too small to
account for the PAMELA/Fermi data

Bergstrom, Edsjo, Zaharijas, PRL103,031103,09’
Positron flux
 Boost factor
Need a large boost factor
B~100-1000
Possible origins of boost factor
Boot factor for DM annihilation
 Local clumps
Via Lactea II: in subhalo? B~ 4-15,
Diemand, et al, 0805.1244, Nature
 Temperature-dependent ann. cross section


Sommerfeld enhancement
Sommerfeld, Ann. Phy 403, 257 (1931).
J. Hisano, S. Matsumoto and M. M. Nojiri, Phys. Rev. D 67 (2003)
Phys. Rev. Lett. 92, 031303 (2004)
Resonance enhancement
Feldman, Liu, Nath, 09
Ibe, Murayama, Yanagida, 09
Guo, Wu, 09
Other mechanism: DM decay, non-thermal DM ….
Constraints from relic density
J. L. Feng, M. Kaplinghat and H. B. Yu, Phys. Rev. Lett. 104, 151301 (2010)
arXiv:1005.4678
Refined analysis at freeze-out
• Cut-off of resonance, recoupling
• Force-carrier production &
decay rates
• Kinetic decoupling
• Self-interaction efficiency,
non-thermality
Other constraints
•Halo shape
•CMB, protohalo
J. Zavala, M. Vogelsberger and S. D. M. White, Phys. Rev. D 81, 083502 (2010)
M. Kamionkowski and S. Profumo, Phys. Rev. Lett. 101,261301 (2008)
Boost factor in multi-component DM models

Large boost requires
1. Large annihilation cross
section
2. Still the correct relic density
Impossible for onecomponent 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)
For thermal relic large cross section
Always reduces signal
 Boost factor from interacting
multi-DM ?(Possible)
Z.P.Liu, Y.L.Wu and YFZ, arXiv:1101.4148
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 two-component case
 The effective cross section
 A interesting limit
 Approximate form
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


4.
Slow conversion characterized by r(z)
Crossing point
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 ~1001000.
Thank You !
Thanks !