Speaker
Description
Astronomical observations indicate that dark matter exists, but what it is and how it is produced remains unsolved. We consider the possibility that dark matter is made up of a sea of cold degenerate fermions. Such particles may be produced out of thermal equilibrium in the very early universe, just after inflation. Production happens through the non-perturbative mechanism of fermionic preheating, during coherent oscillations of the inflaton field. The production mechanism is described by numerical solutions of an oscillator-like differential equation that has a natural frequency represented by $\Omega_{\kappa}$ (having momentum $\kappa$) and depends on a parameter $q$ that quantifies how strongly the inflaton field couples with the fermions. We find that the actual momentum spectrum of the fermions produced by this mechanism is not degenerate. For small $q$ ($\lesssim 0.01$), instead of a momentum sphere, the major contributions to the total number density of fermions come from momentum shells which correspond to resonance peaks in the momentum distribution. For larger $q$, we observe a major contribution from an approximately half-filled sphere and sub-dominant contributions from the resonance peaks. We find a simple semi-analytic relation using the average of $\Omega_{\kappa}$ over time that predicts the momentum values where resonance peaks are present without the need to numerically solve a differential equation. We then obtain analytic power-law approximations for the total number density of fermions, accounting for contributions from the full momentum distribution. We find good approximations in two regimes: the total number density is proportional to $q^{1/2}$ for $q\lesssim 0.01$ and proportional to $q^{3/4}$ for $q\gtrsim 10$.
| Keyword-1 | dark matter production |
|---|---|
| Keyword-2 | post-inflationary preheating |
| Keyword-3 | degenerate fermion dark matter |