Enhancement of DM relic density from the late time

DM conversions

Ze-Peng Liu, Yue-Liang Wu, and Yu-Feng Zhou(arXiv: 1101.4148)

2-component case The effective cross section

A interesting limit

Approximate form

• Thermal equilibrium with SM

1. Decouple from SM, but still in equilibrium with each other

1. Late time DM conversion at large z2. Slow conversion characterized by r(z)

3. Crossing point

3. Complete decouple (freeze-out) after

Themal evolution

Freeze-out condition

Y1(z) increased eventually

Numerical Result

Equilibrium• Equilibrium density Y2

Numerical Result

Equilibrium• Equilibrium density Y2• Equilibrium density Y1

Numerical Result

Equilibrium• Equilibrium density Y2• Equilibrium density Y1If no conversion• Decoupling of Y2• Decoupling of Y1

Numerical Result

Equilibrium• Equilibrium density Y2• Equilibrium density Y1If no conversion• Decoupling of Y2• Decoupling of Y1With conversion• Evolution of Y2• Evolution of Y1

Numerical Result

Equilibrium• Equilibrium density Y2• Equilibrium density Y1If no conversion• Decoupling of Y2• Decoupling of Y1With conversion• Evolution of Y2• Evolution of Y1• Evolution of Y1+Y2

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

boost factor (B)

B vs mass difference B vs relative cross sections

A simple 2CDM Model

Add to the SM

A simple 2CDM Model

Add to the SM

mediators

A simple 2CDM ModelCross sections

Other processes are suppressed by p-wave.

Enhanced by the resonance

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 relative velocity. For generic models with large conversion rate the boost fact can reach the value required by PAMELA etc.