Asymmetric WIMP dark matter

Abstract

In existing dark matter models with global symmetries the relic abundance of dark matter is either equal to that of anti-dark matter (thermal WIMP), or vastly larger, with essentially no remaining anti-dark matter (asymmetric dark matter). By exploring the consequences of a primordial asymmetry on the coupled dark matter and anti-dark matter Boltzmann equations we find large regions of parameter space that interpolate between these two extremes. Interestingly, this new asymmetric WIMP framework can accommodate a wide range of dark matter masses and annihilation cross sections. The present-day dark matter population is typically asymmetric, but only weakly so, such that indirect signals of dark matter annihilation are not completely suppressed. We apply our results to existing models, noting that upcoming direct detection experiments will constrain a large region of the relevant parameter space.

Figures (6)

• Figure 1: Evolution of $Y^{\pm}(x)$ illustrating the effect of the asymmetry $\eta$. After freeze-out both $Y^-$ and $Y^+$ continue to evolve as the anti-particles find the particles and annihilate. The $Y^\pm_{\eta =0}$ curve shows the abundance for $\eta=0$, a mass $m=10$ GeV and annihilation cross-section $\sigma_0=2$ pb. In contrast, with a non-zero asymmetry $\eta=\eta_B = 0.88 \times 10^{-10}$ and same mass and cross-section, the more abundant species (here $Y^+$) is depleted less than when $\eta=0$. Also shown is the equilibrium solution $Y_{eq}(x)$.
• Figure 2: Here we plot the annihilation cross section $\sigma_0$ required to reproduce the correct DM abundance $\Omega_{DM}$ via a s-wave process $n=0$ (above plot) and p-wave $n=1$ (bottom plot) for a given dark matter mass $m$, and for various values of the primordial asymmetry $\eta=\epsilon\eta_B$. The line for $\epsilon=0$ corresponds to the usual thermal WIMP scenario. Notice that the fractional asymmetry runs from $r_\infty=0$ in the upper part of the curves to $r_\infty=1$ when the lines converge on the standard thermal WIMP curve. The effect of the QCD phase transition appears as a bump at $m\lesssim20$ GeV, as anticipated in the text. Note that the bottom plot is basically enhanced by a factor $\Phi_{n=0}/\Phi_{n=1}\sim (n+1)x_f$ compared to the former. As a reference, recall that $1$ pb $\simeq2.6\times10^{-9}$ GeV$^{-2}$.
• Figure 3: Same as in Figure \ref{['fig1']} showing contours of constant $r_{\infty}$, for s--wave process. The completely asymmetric scenario $r_{\infty} \ll 1$ corresponds to the top region of the plot having cross-sections always larger than the thermal WIMP cross-section. For reference we show the $\epsilon=0$ line which corresponds to the usual thermal WIMP scenario ($r_{\infty}=0$).
• Figure 4: Present fractional asymmetry $r_\infty=Y^-_\infty/Y^+_\infty$ (see (\ref{['rinf']})) for various values of the thermally averaged cross section $\sigma_0$ (defined as $\langle\sigma_{\textrm{\small{ann}}}v\rangle\equiv\sigma_0 (T/m)^n$ in the first section) in units of the value $\sigma_{0,WIMP}$ obtained for an exactly symmetric species of the same mass, see eq.(\ref{['WIMP']}). All the points in the curve, i.e. all solutions of (\ref{['rinf']}), account for the observed DM density. The dashed line is the suppression factor (\ref{['sup']}) for indirect detection.
• Figure 5: Contour plot for $r_\infty$ in the allowed parameter space for the model (\ref{['transf3']}). Below the lower line and above the upper line the DM density is smaller and higher than experimentally observed, respectively. In the dark blue area $0.9\leq r_\infty<1$, in the blue area $0.1\leq r_\infty\leq0.9$, and in the light blue area $0\leq r_\infty\leq0.1$. See the text for more details. Here we have taken $m_{\tilde{\nu}} = \Lambda = 1$ TeV and $\tan \beta = 20$.