Eur. Phys. J. Spec. Top. (2021) 230: 1121-1130
https://doi.org/10.1140/epjs/s11734-021-00093-1

Review

Present and potential future experimental evidence supporting a multicomponent dark matter scenario

Department of Physics and Astronomy, Texas A&M University, 77843, College Station, Texas, USA

^h allen@tamu.edu

Received: 22 June 2020
Accepted: 5 January 2021
Published online: 12 April 2021

Abstract

We briefly review the arguments for a multicomponent dark matter scenario which contains both the higgsino of natural supersymmetry (susy) and one or both of two other well-motivated particles—the axion and the higgson. This scenario has become increasingly compelling because of increasingly strong constraints from experiment and observation: (1) The predicted density of natural neutralinos (higgsinos with masses of $\sim$ 100–300 GeV/c${}^2$) is $\sim$ 5–25 times smaller than the observed dark matter density, because the cross section for annihilation of light higgsinos in the early universe is large. (2) The coupling to nuclei through Higgs exchange (with a natural admixture of gaugino components in a particle that is dominantly higgsino) makes it difficult to evade the very low upper limits on the cross sections for scattering off nuclei in direct detection experiments. (3) Indirect detection experiments and Planck have imposed retrictive upper limits on the annihilation cross section. (4) CMS and ATLAS have obtained lower limits on the masses of some susy particles that are relatively high, up to about 2 TeV. Since the axion and axion dark matter experiments have already been extensively reviewed, here we emphasize a candidate which is similar to the neutralino in some respects—with spin 1/2 and R-parity $=-1$—but which has a substantially smaller cross section for both annihilation and scattering, and which, therefore, has the potential to be consistent with (1) observations of the dark matter abundance, (2) spin-independent and spin-dependent scattering in direct detection experiments, (3) observations of gamma rays, antiprotons, etc. in indirect detection experiments, and (4) the LHC limits on masses of susy particles. The present theory also contains susy, with both the lightest neutralino and lightest higgson being stable dark matter particles (since neither can decay into a set of particles containing the other). This theory predicts a doubly rich variety of new particles whose existence can be tested in the foreseeable future.