Axinos as Dark Matter

TL;DR

This work analyzes axinos as dark matter within SUSY+PQ frameworks, highlighting that axino mass is highly model-dependent and can span from $ ext{eV}$ to $ ext{GeV}$. It develops and compares two production channels: TP, driven by the reheating temperature $T_R$, and NTP, via LOSP decays such as $ ilde ext{χ} o ilde a ext{γ}$, connecting relic abundance to $f_a$, $T_R$, and the MSSM spectrum. The paper finds that cold axino DM can arise for $m_{ ilde a}$ in the MeV–GeV range and $T_R aisebox{0ex}{$elow$}{ extstylerac{}{}} m{O}(5 imes10^4)$ GeV, while higher $T_R$ yields TP-dominated, warmer axinos; in this scenario the gravitino problem is alleviated. These results imply that the usual $ ext{Ω}_{ ilde ext{χ}} h^2 < 1$ constraint may be relaxed, as the lightest ordinary superpartner could be stable in detectors, and they motivate future collider and cosmological probes of axino DM and mixed axion-axino DM scenarios.

Abstract

Supersymmetric extensions of the Standard Model that incorporate the axion solution to the strong CP problem necessarily contain also the axino, the fermionic partner of the axion. In contrast to the neutralino and the gravitino, the axino mass is generically not of the order of the supersymmetry-breaking scale and can be much smaller. The axino is therefore an intriguing candidate for a stable superpartner. In a previous Letter [1] it was shown that axinos are a natural candidate for cold dark matter in the Universe when they are generated non-thermally through out-of-equilibrium neutralino decays. Here, we extend the study of non-thermal production and include a competing thermal production mechanism through scatterings and decays of particles in the plasma. We identify axino masses in the range of tens of MeV to several GeV (depending on the scenario) as corresponding to cold axino relics if the reheating temperature $\treh$ is less than about $5\times10^4\gev$. At higher $\treh$ and lower mass, axinos could constitute warm dark matter. In the scenario with axinos as relics, the gravitino problem finds a natural solution. The lightest superpartner of the Standard Model spectrum remains effectively stable in high-energy detectors but may be either neutral or charged. The usual highly restrictive constraint $\abundchi\lsim1$ on the relic abundance of the lightest neutralino becomes void.

Figures (4)

• Figure 1: $Y^{\rm TP}_{\widetilde{a}}$ as a function of $T_{\rm R}$ for representative values of $f_{a}=10^{11}$GeV and $m_{\widetilde{q}} = m_{\widetilde{g}} = 1\,\hbox{TeV}$.
• Figure 2: $Y^{\rm TP}_{\widetilde{a}}$ as a function of $T_{\rm R}$ for representative values of $f_{a}$ and $m_{\widetilde{q}}=m_{\widetilde{g}}=1\,\hbox{TeV}$.
• Figure 3: Lower bound on the axino mass from considering hadronic showers according to the condition (\ref{['condhadronic:eq']}), for $C_{aYY} Z_{11} = 1$ and $f_{a}/N = 10^{11}\,\hbox{GeV}$. The bound disappears for $m_{\chi} = 150 \,\hbox{GeV}$ when the lifetime drops below $0.1\, {\rm sec}$.
• Figure 4: The solid line gives the upper bound from thermal production on the reheating temperature as a function of the axino mass. The dark region is the region where non-thermal production can give cosmologically interesting results ($\Omega^{\rm NTP}_{\widetilde{a}} h^2\simeq1$) as explained in the text. We assume a bino-like neutralino with $m_{\chi}=100\,\hbox{GeV}$ and $f_{a}=10^{11}\,\hbox{GeV}$. The region of $T_{\rm R}\mathrel{\hbox{$\sim$} \hbox{$>$}}T_{\rm f}$ is somewhat uncertain and is shown with light-grey color. A sizeable abundance of neutralinos (and therefore axinos) is expected also for $T_{\rm R}\mathrel{\hbox{$\sim$} \hbox{$<$}}T_{\rm f}$gkr00 but has not been calculated. The vertical light-grey band indicates that a low range of $m_{\widetilde{a}}$ corresponds to allowing SM superpartner masses in the multi-TeV range, as discussed in the text. The division of hot, warm and cold dark matter as a function of the axino mass shown in the lower left part is for axinos from non-thermal production.