Supersymmetric Dark Matter - How Light Can the LSP Be?

TL;DR

The paper addresses how light the MSSM neutralino LSP can be while explaining the observed dark matter density, under minimal, model-independent assumptions. It uses a broad non-unified MSSM parameter scan with R-parity, LEP2 bounds, and relic-density constraints to compute the relic density from the full annihilation cross section. The main finding is a robust lower bound of about 18 GeV for the neutralino LSP mass, driven by slepton-mediated annihilation and LEP2 limits; stronger collider constraints can push this bound higher, and some special MSSM corners could alter the result but face additional constraints from Z width and flavor observables. The work provides guidance for future collider searches and highlights how nearly degenerate spectra or alternative annihilation channels could modify the minimal mass bound in limited regions of parameter space.

Abstract

Using a very minimal set of theoretical assumptions we derive a lower limit on the LSP mass in the MSSM. We only require that the LSP be the lightest neutralino, that it be responsible for the observed relic density and that the MSSM spectrum respect the LEP2 limits. We explicitly do not require any further knowledge about the MSSM spectrum or the mechanism of supersymmetry breaking. Under these assumptions we determine a firm lower limit on the neutralino LSP mass of $18\gev$. We estimate the effect of improved limits on the cold dark matter relic density as well as the effects of improved LEP2-type limits from a first stage of TESLA on the allowed range of neutralino LSP masses.

Figures (3)

• Figure 1: The MSSM data points with the neutralino LSP mass on one axis. The other axis' in the four panels show the lightest slepton mass, the bino mass parameter $M_1$, the Higgsino mass parameter $\mu$ and the contribution of the decay $Z \to \tilde{\chi}^0_1 \tilde{\chi}^0_1$ to the invisible $Z$ decay width. The color coding corresponds to the light chargino mass with only the black points respecting $m_{\tilde{\chi}^+_{ 1}}>103\;{\rm GeV}$. The dashed lines are the assumed experimental limits. Note that in contrast to the black points, not all of the green (grey) points with too small chargino masses are included in the frames. Moreover the black points might hide green (grey) points below them.
• Figure 2: The MSSM data points with the neutralino LSP mass on one axis. The other axis' in the two panels show the LSP relic density and the lighter chargino mass. The color coding corresponds to the lighter stau mass with only the black points respecting $m_{\tilde{\tau}_{ 1}}>103\;{\rm GeV}$. The dashed lines are the experimental limits on the chargino mass and a possible improved limit on the relic density. Note that in contrast to the black parameter points, not all of the green (grey) points with too small chargino masses are included in the frames. Moreover the black points might hide green (grey) points below them.
• Figure 3: The MSSM data points with the neutralino LSP mass on one axis. All black and green (grey) parameter points respect the $103\;{\rm GeV}$ LEP limits as well as all other limits we impose. Upper row: versus the lightest slepton mass and versus the $Z$ invisible decay width, like in Fig. \ref{['fig:one']}. Here the color coding corresponds to the lightest chargino mass with the black points indicating $m_{\tilde{\chi}^+_{ 1}}>175\;{\rm GeV}$. Lower row: versus the LSP relic density and versus the light chargino mass, like in Fig. \ref{['fig:two']}. The color now coding corresponds to the lightest slepton mass with the black points indicating $m_{\tilde{\tau}_{ 1}}>175\;{\rm GeV}$.