Heart of Darkness: The Significance of the Zeptobarn Scale for Neutralino Direct Detection

TL;DR

This paper explores the implications of upcoming dark matter detection experiments on neutralino models within supersymmetry, showing that experiments are approaching the parameter space where neutralinos could be detected or constrained.

Contribution

It provides a transparent supersymmetric framework analysis, linking experimental sensitivities to neutralino cross sections and discussing implications for model fine-tuning.

Findings

Characteristic cross sections range from 1 zb to 40 zb for neutralino masses above 70 GeV.

Experiments are nearing the sensitivity to probe most of the neutralino parameter space in supersymmetric models.

If no detection occurs, models will require fine-tuning, but detection is likely if neutralinos are dark matter.

Abstract

The direct detection of dark matter through its elastic scattering off nucleons is among the most promising methods for establishing the particle identity of dark matter. The current bound on the spin-independent scattering cross section is sigma^SI < 10 zb for dark matter masses m_chi ~ 100 GeV, with improved sensitivities expected soon. We examine the implications of this progress for neutralino dark matter. We work in a supersymmetric framework well-suited to dark matter studies that is simple and transparent, with models defined in terms of four weak-scale parameters. We first show that robust constraints on electric dipole moments motivate large sfermion masses mtilde > 1 TeV, effectively decoupling squarks and sleptons from neutralino dark matter phenomenology. In this case, we find characteristic cross sections in the narrow range 1 zb < sigma^SI < 40 zb for m_chi > 70 GeV. As sfermion masses are lowered to near their experimental limit mtilde ~ 400 GeV, the upper and lower limits of this range are extended, but only by factors of around two, and the lower limit is not significantly altered by relaxing many particle physics assumptions, varying the strange quark content of the nucleon, including the effects of galactic small-scale structure, or assuming other components of dark matter. Experiments are therefore rapidly entering the heart of dark matter-favored supersymmetry parameter space. If no signal is seen, supersymmetric models must contain some level of fine-tuning, and we identify and analyze several possibilities. Barring large cancellations, however, in a large and generic class of models, if thermal relic neutralinos are a significant component of dark matter, experiments will discover them as they probe down to the zeptobarn scale.