Lower Bound on the Neutralino Mass from New Data on CMB and Implications for Relic Neutralinos

TL;DR

This study relaxes the common GUT-scale gaugino mass unification and analyzes light neutralinos within an effMSSM framework. By combining the cosmological upper bound on the cold dark matter density with particle-physics constraints, it derives a robust lower bound m_chi ≳ 6 GeV and clarifies the competing Higgs-exchange and sfermion-exchange annihilation channels that shape this limit. It further connects relic abundance to direct-detection cross-sections, showing that light neutralinos could be probed by low-threshold, high-exposure experiments such as DAMA, and discusses the potential to interpret the DAMA modulation in terms of these light states for certain Galactic distribution functions. Overall, the results extend prior work on neutralinos to the sub-50 GeV mass range, highlighting both cosmological and experimental implications for detecting light WIMPs.

Abstract

In the frame of an effective MSSM model without gaugino-mass unification at a grand unification (GUT) scale, we set a lower bound on the neutralino mass based on the new WMAP data on $Ω_{CDM}$. Our lower bound, $m_χ \gsim 6$ GeV, leaves much room for relic neutralinos significantly lighter than those commonly considered ($m_χ \gsim 50$ GeV). We prove that these light neutralinos may produce measurable effects in WIMP direct detection experiments of low energy threshold and large exposure.

Figures (4)

• Figure 1: Neutralino relic abundance $\Omega_{\chi}h^2$ as a function of the mass $m_\chi$. The solid curve denotes $(\Omega_{\chi} h^2)_{min}^{Higgs}$ given by Eq. (\ref{['omegamin']}) for $T_{QCD} =$ 300 MeV. Dashed and dot-dashed curves refer to the representative values $T_{QCD} =$ 100 MeV, $T_{QCD} =$ 500 MeV, respectively. The two horizontal lines denote two representative values of $\Omega_{CDM} h^2$: $\Omega_{CDM} h^2 =$ 0.3 (short-dashed line) and $\Omega_{CDM} h^2 =$ 0.131 (long-dashed line). The scatter plot is obtained by a full scanning of the supersymmetric parameter space.
• Figure 2: Neutralino relic abundance $\Omega_{\chi}h^2$ as a function of the mass $m_\chi$. The solid curve denotes $(\Omega_{\chi} h^2)_{min}^{sfermion}$ derived from Eq. (\ref{['omega']}) when $\widetilde{\langle \sigma_{\rm ann} \; v\rangle}$ is given by the maximal value of ${\widetilde{\langle \sigma_{\rm ann} \; v\rangle}^{sfermion}}$ (see Eq. (\ref{['sf']})). $T_{QCD}$ is set equal to 300 MeV. The two horizontal lines denote two representative values of $\Omega_{CDM} h^2$: $\Omega_{CDM} h^2 =$ 0.3 (short-dashed line) and $\Omega_{CDM} h^2 =$ 0.131 (long-dashed line). The scatter plot is obtained by a full scanning of the supersymmetric parameter space with $m_A>$ 300 GeV.
• Figure 3: Dashed and solid curves give the variation of the lower bound on $m_{\chi}$ as a function of $m_A$ for $(\Omega_{CDM} h^2)_{max} =$ 0.3 and for $(\Omega_{CDM} h^2)_{max} =$ 0.131, respectively. For each value of $(\Omega_{CDM} h^2)_{max}$ the region on the left of the relevant curve is forbidden. The two lines are derived from the analytical expressions obtained in the text for the contributions to $\widetilde{\langle \sigma_{\rm ann} \; v\rangle}$ due to Higgs-exchange and sfermion-exchange. Also displayed is the scatter plot of a full numerical scanning.
• Figure 4: Scatter plot of $\xi \sigma_{\rm scalar}^{(\rm nucleon)}$ versus $m_{\chi}$. Crosses (red) and dots (blue) denote neutralino configurations with $\Omega_{\chi} h^2 > (\Omega_{CDM} h^2)_{min}$ and $\Omega_{\chi} h^2 < (\Omega_{CDM} h^2)_{min}$, respectively. The curves give the sensitivity line, $(\xi \sigma_{\rm scalar}^{(\rm nucleon)})_{min}$ versus $m_{\chi}$, for a NaI detector, whose features are discussed in the text. The intermediate curve refers to an isothermal DF with $v_0 = 220$ km $\cdot$ s$^{-1}$ and $\rho_0 = 0.3 ~ {\rm GeV \cdot cm}^{-3}$. The upper curve refers to a spherical Evans' power-law DF (denoted as A3 in Ref. bcfs) with $v_0 = 170$ km $\cdot$ s$^{-1}$ and $\rho_0 = 0.17 ~ {\rm GeV \cdot cm}^{-3}$; the lower curve refers to an axially-symmetric Evans' logarithmic DF with maximal flatness (denoted as C2 in Ref. bcfs) with $v_0 = 270$ km $\cdot$ s$^{-1}$ and $\rho_0 = 1.7 ~ {\rm GeV \cdot cm}^{-3}$.