Light Relic Neutralinos

TL;DR

This paper shows that relaxing gaugino unification in an effMSSM framework allows light neutralinos ($m_{\chi} < 45$ GeV) to have cosmologically relevant relic densities and sizable elastic scattering cross sections off nucleons, potentially yielding observable direct-detection signals. The authors derive analytic scalings for the relic abundance and scattering cross sections under Higgs-exchange dominance and validate them with numerical scans across a broad parameter space, including a nonuniversal gaugino mass ratio $R = M_1/M_2$. They find that light, bino-dominated neutralinos can achieve $\Omega_{\chi} h^2$ within the allowed range and $\sigma_{\rm scalar}^{(\rm nucleon)}$ up to $\sim 10^{-7}$ nbarn when $m_h$ and $m_A$ are near their LEP bounds and $\tan\beta$ is large; these scenarios are not excluded by current experiments due to astrophysical and detector uncertainties, but remain accessible to upcoming low-threshold detectors. Overall, the work highlights a viable and testable region of neutralino dark matter parameter space beyond the conventional gaugino-unified framework, with clear implications for interpreting direct-detection results.

Abstract

The relic abundance and the scalar cross-section off nucleon for light neutralinos (of mass below about 45 GeV) are evaluated in an effective MSSM model without GUT-inspired relations among gaugino masses. It is shown that these neutralinos may provide a sizeable contribution to the matter density in the Universe and produce measurable effects in WIMP direct detection experiments. These properties are elucidated in terms of simple analytical arguments.

Figures (5)

• Figure 2: Scatter plots of the neutralino--nucleon cross section $\sigma_{\rm scalar}^{(\rm nucleon)}$ vs. the neutralino relic abundace $\Omega_\chi h^2$, for $R=0.01,0.02,0.03,0.04,0.05$ and for the standard value $R=5/3 \tan^2\theta_W\simeq 0.5$.
• Figure 3: Scatter plots of the fractional amount of the neutralino pair--annihilation cross section due to sfermion exchange vs. Higgs exchange, for $R=0.01,0.02,0.03,0.04$. Crosses denote configuration for which the neutralino--nucleon scattering cross section $\sigma_{\rm scalar}^{(\rm nucleon)}$ is larger than $10^{-8}$ nbarn.
• Figure 4: Scatter plots of the neutralino composition in terms of $\tilde{B}$ ($a_1$) and of $\tilde{H}_1^{\circ}$ ($a_3$) for $R=0.01,0.05,0.1$ and for the standard value $R=5/3 \tan^2\theta_W\simeq 0.5$. The dashed lines denote the line where $a_1^2+a_3^2=1$.
• Figure 1a: Scatter plots of the neutralino--nucleon cross section $\sigma_{\rm scalar}^{(\rm nucleon)}$ times the rescaling factor $\xi$ vs. the neutralino mass, for nonuniversal gaugino models with different values of the gaugino mass ratio $R=M_1/M_2$: $R=0.01,0.02,0.03,0.04$. Crosses denote configurations with dominant relic neutralinos ($0.05 \leq \Omega_\chi h^2 \leq 0.3$), while dots refer to subdominant neutralinos ($\Omega_\chi h^2 < 0.05$).
• Figure 1b: The same as in Fig. \ref{['fig:1a']}, for $R=0.05,0.1$, for the standard value $R=5/3 \tan^2\theta_W\simeq 0.5$ and for a generic variation of $R$ in the interval 0.01--0.5.