WIMPs are by no means the only theoretical DM constituent being searched for in current experiments. Feng’s review comprehensively explores other options, including axions and sterile neutrinos. Unlike WIMPs, neither of these candidates naturally produce the correct relic DM density. Still, they address other issues in particle physics, unrelated (at least at face value) to the gauge hierarchy problem: sterile neutrinos offer an explanation to the question of neutrino mass, whereas axions have been introduced to tackle the strong CP problem [7].
Other theories hold that DM can be explained without introducing any new particles at all. MACHOs (Massive Astrophysical Compact Halo Objects) were proposed as a particularly reactionary alternative to WIMPs, and would consist of baryonic matter. However, microlensing surveys have ruled out MACHOs as being significant components of the halo DM for a wide range of MACHO masses [56]. That said, the recent LIGO detection of black hole mergers [57] has sparked interest in the possibility of primordial black holes providing some of the dark matter. Though many mass ranges are ruled out, primordial black holes in the “intermediate-mass range” (M < M < 103M ) can still
account for some the DM [58]. Carr et al. conclude that this and the “sublunar” mass range may soon be ruled out as well, but that Planck-mass scale black holes are viable candidates and will not be constrained by near-term experiments.
It is possible that some combination of all of these candidates are responsible for the observed ΩDM. However, observational constraints imposed on a single hypothetical
species are typically calculated under the assumption that it alone constitutes the DM. The details of this calculation in the case of WIMP dark matter are explored in the next chapter.
Detection of dark matter
The “discovery” of dark matter, from an astrophysical perspective, has already been ac- complished. Debate rages over the rightful attribution of this accolade: Fritz Zwicky’s daughter Barbarina Zwicky continues to wage a fierce battle for her father’s legacy [59], notwithstanding the tendency of modern authors (including this one) to favor Rubin’s efforts [60].1Irrespective of this controversy, however, the question of dark matter’s com-
position is decidedly unanswered. If “discovery” describes the ascertaining of DM’s ex- istence, then “detection” must be the unveiling of DM’s particle nature.
1It appears that B. Zwicky scored a minor victory in the case of Ref. [60]. In the publicly available
online version of the article, the title reads (as of this writing) “How Vera Rubin confirmed dark matter” instead of “How Vera Rubin discovered dark matter,” as it appeared in the original print edition. (Ad- ditionally, an online commenter self-identified as “barbarina zwicky” lambasts the author for advancing a smear campaign against her father.) However, the HTML title field still retains the “discovered,” so Google searchers will see the original title’s wording in the hyperlink.
2.1
Detection schemes
Experiments designed to detect dark matter fall into three main categories:
Production: Colliders like the LHC produce exotic particles via high-energy inelastic scattering. Detectors like CMS and ATLAS are used to reconstruct what particles were created in the collisions. If a WIMP were generated in such a collision, it would appear as missing energy in the event reconstruction.
Indirect detection: Cosmic rays are in general a useful probe of astrophysical pro- cesses. If many DM particles were concentrated in a high-density region (e.g. in the Sun, after scattering from protons and becoming gravitationally trapped), they could annihilate and produce a tell-tale resonance in the spectrum of radiation emanating from this region. Earth- and space-based laboratories are capable of observing such a signal.
Direct detection: Perhaps the most conceptually straightforward of the three tech- niques, experiments in this category seek to observe collisions between galactic DM particles and atoms in a target material. Since the Earth is constantly passing through the dark halo, a sensitive enough detector should be able to detect such a collision. To mitigate non-WIMP backgrounds from cosmic rays, direct detection experiments must be deployed underground, where the rock overburden acts as a radiation shield.
Figure 2.1: Illustrative Feynman diagrams for the three approaches to DM detection, beneath images of representative experiments. At left, the collider search technique is shown with an LHC graphic (credit: CERN) and diagram with Standard Model (SM) particles inelastically scattering to produce DM particles (χ). At right, the Feynman diagram is flipped, and DM self-annihilates to produce SM particles, conceivably visible to indirect detection observatories like the Fermi LAT (image credit: NASA). Finally, shown in the center, an image of LUX represents the field of direct detection, schematically illustrated by a Feynman diagram obtained by rotating the others 90◦, wherein a DM
particle scatters against a SM particle (image credit: Matt Kapust, South Dakota Science and Technology Authority).
tection scenario would involve experiments in multiple categories seeing signals: for ex- ample, a new particle discovery at the LHC could be corroborated by an underground detector reporting detection of a WIMP with consistent mass and coupling properties. This would imply that the particle created in anthropogenic high-energy collisions exists cosmogenically in the galaxy.
Experiments designed for different detection schemes are often said to be “complemen- tary,” capable not only of validating one another’s results, but also of testing hypotheses to which the other is insensitive. If, for example, DM particles are too massive to be produced at the LHC, direct and indirect detection experiments might still be able to see a signal. In fact, DM searches within just the direct detection category also exhibit complementarity, due to differences in their target materials. Section2.2 will explore the specific mechanisms giving rise to these differences in sensitivity.