Heart of darkness: the significance of the zeptobarn scale for neutralino direct detection

Abstract

The direct detection of dark matter through itselastic scattering off nucleons is among the most promising methodsfor establishing the particle identity of dark matter. The currentbound on the spin-independent scattering cross section is σSI < 10 zb for dark matter masses mΧ ∼ 100 GeV, with improvedsensitivities expected soon. We examine the implications of thisprogress for neutralino dark matter. We work in a supersymmetricframework well-suited to dark matter studies that is simple andtransparent, with models defined in terms of four weak-scaleparameters. We first show that robust constraints on electric dipolemoments motivate large sfermion masses m̃ ⩾ 1 TeV,effectively decoupling squarks and sleptons from neutralino darkmatter phenomenology. In this case, we find characteristic crosssections in the narrow range 1 zb ⩽ σSI ⩽ 40 zb formΧ ⩾ 70 GeV. As sfermion masses are lowered to near theirexperimental limit m̃ ∼ 400 GeV, the upper and lower limitsof this range are extended, but only by factors of around two, and thelower limit is not significantly altered by relaxing many particlephysics assumptions, varying the strange quark content of the nucleon,including the effects of galactic small-scale structure, or assumingother components of dark matter. Experiments are therefore rapidlyentering the heart of dark matter-favored supersymmetry parameterspace. If no signal is seen, supersymmetric models must contain somelevel of fine-tuning, and we identify and analyze severalpossibilities. Barring large cancellations, however, in a large andgeneric class of models, if thermal relic neutralinos are asignificant component of dark matter, experiments will discover themas they probe down to the zeptobarn scale.