Natural SUSY with mixed axion/axino dark matter

Abstract

While supersymmetric models provide a solution to the big hierarchy problem, natural SUSY is also allowed by the little hierarchy problem. In supersymmetric models which include the Peccei-Quinn (PQ) solution to the strong CP problem, one expects the presence of an axion-axino-saxion supermultiplet with a micro-eV-scale axion and a saxion with mass of order the soft breaking scale. The axino mass is much more model-dependent, and may occur in the range of keV-TeV: over 9 orders of magnitude. This leads to the possibility of the axino as lightest SUSY particle (LSP) and the presence of mixed axion plus axino dark matter. The case of natural SUSY with higgsino-like WIMPs as LSP seems (nearly) excluded by multi-ton noble liquid WIMP detector limits, even in the case where the LSP has a depleted abundance compared to axions. We examine the case where the axino is LSP leading to mixed axion-axino dark matter in a natural SUSY context. We map out regions of PQ scale f_a vs. axino mass m_{\ta} parameter space where such a scenario remains viable in both the SUSY DFSZ and KSVZ axion models. For axino mass ~100 keV, we find solutions in accord with the measured dark matter abundance with mainly warm axino dark matter for f_a~ 10^{11} GeV and also solutions with mainly axion cold DM and a tiny axino contribution for higher f_a~ 3\times 10^{12} GeV.

Figures (7)

• Figure 1: Branching fraction of lightest neutralino $\tilde{\chi}_1^0$ into $Z\tilde{a}$ and $h\tilde{a}$ versus $m_{\tilde{a}}$ in the SUSY DFSZ axion model for both positive and negative $m_{\tilde{a}}$.
• Figure 2: Decay temperature $T_D(\tilde{\chi}_1^0)$ in the $f_a$ vs. $m_{\tilde{a}}$ plane. We also show the rough bound for the $\tilde{\chi}_1^0$ to be BBN-safe, i.e. that it decays before the onset of BBN. In white are shown contours of $\tau(\tilde{\chi}_1^0 )$ ranging from $10^{-12}--\ 1$ s.
• Figure 3: Mixed axion-axino relic abundance vs. $f_a$ for $m_{\tilde{a}}=100$ keV in the benchmark SUSY DFSZ model with $m_{3/2}=10$ TeV and $\theta_i=1$.
• Figure 4: Mixed axion-axino relic abundance vs. $m_{\tilde{a}}$ for various values of PQ scale $f_a$ in the SUSY DFSZ benchmark model with $m_{3/2}=10$ TeV, $T_R=10^6$ GeV and $\theta_i=1$.
• Figure 5: Color-coded mixed axion-axino relic abundance in the $m_{\tilde{a}}$ vs. $f_a$ plane in the SUSY DFSZ model with $m_{3/2}=10$ TeV, $T_R=10^6$ GeV and $\theta_i=1$.