Weakly Interacting Massive Particles (WIMPs) as Candidate Particles for Cold Dark Matter (CDM)

KiDS Collaboration/H. Hildebrandt & B. Giblin/ESO via Wikimedia Commons.

The following essay was originally written as part of my English 201 course at the University of Wisconsin–Milwaukee, which I took as a high school senior. In celebration of this essay's prized role in my senior year, I have decided to include it as an entry in the De Omnibus series. Thank you all, as always, for reading through my essays.

Weakly Interacting Massive Particles as Candidates for Cold Dark Matter

Considering past scientists’ reflections on wonder, we are compelled to consider the wonders of today–cold dark matter (CDM) among them–to be the wonders they describe. Albert Einstein once wrote in his The World as I See It that “the most beautiful experience we can have is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder, no longer marvel, is as good as dead and his eyes are dimmed,” (Einstein). Cold dark matter relates to this concept: while the existence of CDM has been all but confirmed, everything from its properties to its constituent particles remains largely unknown.

In this paper, one facet of this present scientific discussion–weakly interacting massive particles (WIMPs) as constituent particles of cold dark matter–will be considered. This piece will include (1) a background of both CDM and WIMPs, (2) an overview of methods of detecting WIMPs, (3) a discussion of new developments regarding WIMPs and the scientific community, and (4) an outline of alternative candidate particles for CDM, which collectively will describe the current scholarly discussion on the composition of CDM as well as argue that, although WIMPs maintain an uneasy scientific consensus as the constituent particles of CDM, recent developments have rendered WIMPs as unlikely–although not impossible–candidates.

BACKGROUND

Cold dark matter–the mysterious matter that constitutes over four times as much of the universe’s matter-energy than baryonic matter–has been a topic of serious contention for almost a century. Analyses of galaxy clusters and galaxy rotation curves, raised more questions about our understanding of matter than presented answers about the limited scope of our cosmological models. Although the existence of CDM has been nothing but confirmed for almost a century, the questions over its composition and general mechanics remain. 

Constituent Particles and WIMPs. The prevalence of CDM, made evident by values for velocity dispersion and galactic rotation curves, led to much more scientific scrutiny; this scrutiny, upon a successful determination of its prevalence and interaction in the universe, prompted the search for its constituent particles. 

Early astronomers investigating dark matter believed it to be composed of condensed nebulous or starlike bodies that pervaded in astrophysical systems–like galaxies or star systems. But observations concluded that the matter was more diffuse, contradicting the idea that dark matter was weakly interacting baryonic matter in the form of faint, individual objects. Scientists, therefore, sought more complex–and often hypothetical–explanations.

Many particles and concepts have since been levied as candidates for dark matter, including neutrinos, primordial black holes, modified gravity, supersymmetric particles, axions, and WIMPs. Neutrinos are compelling candidates, but their likely low rest mass, coupled with the fact that cosmologically-produced neutrinos would be quite hot, renders them unlikely candidates. Supersymmetry, a hypothetical extension to the Standard Model arguing that there are “superpartner” particles to all bosons and fermions–where each Standard Model boson has a superpartner fermion, and vice versa–would be more likely to provide a viable candidate for cold dark matter than neutrinos, but there has yet to be any evidence that such superpartner particles exist. Axions–particles that solve the strong charge-parity (CP) problem in quantum chromodynamics, a contradiction between the postulate that CP symmetry can be broken in quantum chromodynamics (a model that explains quarks, gluons, and the interactions between them) and the observed nonexistence of such a violation–are other hypothetical viable candidates, and are, in part because of their consistency with Peccei-Quinn symmetry and the anthropic principle, mathematically congruous.

The perhaps most well studied candidate is the weakly interacting massive particle (WIMP), which emerged in the 1970s due to commonalities among many hypothesized candidate particles. WIMPs interact only through forces that are as weak or weaker than the weak force (e.g. the weak force and the gravitational force), so they cannot interact via the two other Standard Model forces–the strong force and the electromagnetic force. The search for WIMPs is ongoing–for as of now, no dark matter particle, including a WIMP, has been observed–and has been the motivation for several thousand papers and dozens of experimental programs.

Developing WIMPs as Candidates for CDM. Although the presently most probable candidates for CDM are neutrinos, axions, primordial black holes and WIMPs, such was not always the case; indeed, in the 1980s, the realization that dark matter is non-baryonic led physicists to propose a myriad of different candidates, from monopoles and cosmic strings to quark matter and even “pyrgons.” As dozens of new hypothetical yet mathematically-feasible candidates were proposed in the mid-1980s, two important commonalities between all of the candidates emerged.

First, in order for the CDM particle to “freeze-out” of thermal equilibrium in the early universe, it must be heavier than 1,100 keV. For the thermal relic abundance in the universe of the particle to match the observed density of CDM in the present universe, the particles must be able to self-annihilate (the collision of a particle with its respective antiparticle, resulting in the two eliminating one another and, to satisfy conservation of energy and momentum, producing two photons) with a cross section of σv ∼ 10−26 cm3/s, where v is the relative velocity between the particles, and σ is the Stefan-Boltzmann constant. As (1) the same cross section of the weak force has near-equal magnitude to the CDM particles’ cross sections and (2) many supersymmetric extensions to the Standard Model readily predict particles with these properties, the “WIMP miracle” emerged; hence, the formulation of a new hypothetical umbrella particle, the weakly interacting massive particle became, almost instantly, the most compelling candidate for CDM.

The development of WIMPs was, indeed, a miracle, for it unified many hypothetical particles of the time into one cohesive candidate: “such conclusions are not limited to neutrinos, but apply to a broad range of electroweak-scale dark matter candidates – including any number of stable particles with MeV-TeV masses and interactions that are mediated by the exchange of electroweak-scale particles” (Bertone 43). The commonalities between these candidates both created WIMPs and led them to become, at the time, the most compelling candidate particles of CDM.

METHODS OF DETECTION

Detecting WIMPs is much like detecting other CDM candidates: because they do not interact via the electromagnetic force, direct detection via optical or radio telescopes is not an option. Methods of detection for WIMPs are, therefore, limited to weak and gravitational interactions, as a weakly-interacting massive particle can only interact through forces as weak or weaker than the weak force. 

As with other candidate particles for CDM, there are two umbrella methods of detection that could observe WIMP-composed matter: direct detection and indirect detection.

Direct Detection. Direct detection involves detecting the dark matter particles directly through collisions in Earth-borne laboratories. There are three primary methods of direct detection that have since been subject to experimentation: cryogenic detectors, scintillators and bubble chambers.

As the cross section of a WIMP annihilation is as small as that of the weak interaction, the corresponding energy will require extremely high resolution to detect, especially on a particle basis. Cryogenic detectors could have such resolution: operating at extremely low temperatures–normally only a few degrees above absolute zero–these detectors are able to resolve smaller energies at greater capacities. Although sufficient cryogenic detectors should hypothetically be able to detect processes operating at as low an energy as that of a WIMP’s self-annihilation, they face one challenge: “there is not much commercial interest in cryogenic detectors” (Kraus 9). A novel and still undeveloped technology, the lack of commercial interest in the detectors severely hampers funding towards further development. Nevertheless, multiple ongoing research projects with cryogenic detectors are in progress, including the Cryogenic Dark Matter Search (CDMS), that seek to, for example, “directly detect [particles of dark matter] and thus understand the nature of the dark matter” (SuperCDMS 1). 

Scintillators are substances that, when exposed to radiation such as X-rays or 𝛄-rays, emit fluorescence. Researchers expose atoms–often noble gasses–or scintillating crystals to electromagnetic radiation, causing them to glow under the detector; while they move throughout the space, dark matter particles could be detected by observing the collisions of illuminated scintillating fluids against “invisible” particles in the space. Many experiments, including the XENON1T and ZEPLINIII projects, search for dark matter using scintillating materials. 

Bubble chambers suspend small droplets of superheated liquid into a large chamber in order to detect charged particle interactions in space. This method of detection, too, has several projects dedicated to it, including the new scintillating bubble chamber (SBC) collaboration that has commissioned a ten kilogram argon bubble chamber at Fermilab. Among the most compelling and practicable methods of detection, it has become an extremely popular method of direct detection attempts; indeed, according to the original commission that constructed the chamber, “The scalability and background discrimination power of the liquid-noble bubble chamber make this technique a compelling candidate for future dark matter searches” (Alfonso-Pita et al. 1). 

To the extent of present, publicly available information, all methods of direct detection–as well as additional methods not covered in this section–have failed to provide direct evidence for WIMPs. 

Indirect Detection. Indirect methods of detection focus on predicting, observing and measuring phenomena where CDM is known to be most prevalent–at the centers of galaxy clusters, galaxies, and in the Milky Way’s satellite galaxies, for example. Additional methods could result from WIMP self-annihilations in the solar halo: the self-annihilations, as a result of the conservation of energy and momentum, produce neutrinos and photons, which could theoretically be observed from Earth; however, according to data from the IceCube interferometer in Antarctica, “The three-year data set, with a live time of 988 days, contains a total of 37 neutrino candidate events with deposited energies ranging from 30 to 2000 TeV” (Aartsen et al. 1). As only 37 candidates have since been discovered, the distinction between WIMP-produced neutrinos and natural astrophysical neutrinos will likely be impossible to detect.

Although direct detection has so far failed to detect CDM particles (not only WIMPs, but all hypothesized candidate particles), indirect detection has produced several compelling–although still uncorroborated–detections. One apparent detection, made by NASA’s Energetic Gamma Ray Experiment Telescope (EGRET), “shows all the features expected from Dark Matter WIMP Annihilation: a) it is present and has the same spectrum in all sky directions, not just in the galactic plane. b) The intensity of the excess shows the 1/r2 profile expected for a flat rotation curve outside the galactic disc with additionally an interesting substructure in the disc in the form of a doughnut shaped ring at 14 kpc from the centre of the galaxy” (De Boer 1). The indirect evidence appeared in diffuse gamma ray data from the telescope, collected in 2004; however, no further detections have been made, rendering it unlikely that such was the detection of a WIMP annihilation. 

Although no substantial evidence for WIMPs has surfaced through indirect detection, the method remains easier, as it incorporates more accessible technologies (such as optical telescopes or interferometers); however, because of the natural caveats that come with indirect detection, such a method is less able to yield substantial evidence. 

PRESENT AND NEAR-FUTURE DEVELOPMENTS

The 2010s and early 2020s have seen significant movement towards ruling out WIMPs as candidate particles for CDM, as scientists still have yet to be successful in detecting them. Recent developments regarding the WIMP miracle itself, as well as null results from direct detection experiments and the Large Hadron Collider’s (LHC) failure so far to detect supersymmetric particles, have cast significant doubt on the WIMP hypothesis.

Null Results from Direct Detection Experiments. Several experiments in the last decade seeking to directly detect dark matter particles have been implemented, yet all of them have failed to produce evidence of a specific CDM particle. After the first indirect evidence of large amounts of WIMPs in the spirals of the Milky Way was first observed by the DAMA/LIBRA experiment in Italy’s Gran Sasso National Laboratory in 1999, “no such signature has been confirmed by any other experiments since” (Agência FAPESP 1). Although such experiments incorporated different processes and methods of analysis than the DAMA/LIBRA experiment, the apparent observation appears to have been ruled out. Furthermore, seeking both to rule on and to determine the discrepancy between the DAMA/LIBRA experiment results and those of other experiments, the COSINE-100 was designed 700 meters underground in South Korea; this direct detection experiment, spearheaded by Yale University in the Yangyang underground laboratory, sought detection through crystal scintillation. Like the prior experiments, however, COSINE-100 yielded null results: “No signal consistent with the dark matter interaction is identified and rules out model-dependent dark matter interpretations of the DAMA signals in the specific context of standard halo model” (Adhikari et al. 1).

Large Hadron Collider and Supersymmetry. Supersymmetric extensions to the Standard Model predict particles–such as the neutralino–with properties that would satisfy those of WIMPs; however, like the results from direct WIMP detection experiments, experiments probing supersymmetric particles have so far yielded null results. One such experiment–the ATLAS Detector at CERN’s Large Hadron Collider (LHC)–searched for “charginos and sleptons [in a]... particle-mass region previously unexplored due to a challenging background of Standard Model processes that mimics the signals from the sought-after particles” (CERN 1). The ATLAS Collaboration’s experiment, the results of which were made public in 2022, “revealed no significant excess above the Standard Model background” (1). As supersymmetric extensions to the Standard Model readily predict WIMP-like particles, the interest in identifying such particles is high; yet, like WIMPs, null results are leading the very existence of supersymmetric particles to be under question.

A Final Chance for WIMPs: Next Generation Detectors. The next decade will see the most extensive–and perhaps the final–search for WIMPs. Several new detectors, including XENON1T (see Direct Detection) in Italy, DARWIN, PandaX-4t in China, Lux-Zepellin in the United States, and the ongoing Large Underground Xenon experiment (LUX) in Black Hills, South Dakota; all five of these experiments incorporate xenon-based scintillators. The worldwide effort, spearheaded by the XENON collaboration, the LUX experiment, and the PandaX-4t experiment, is “pushing to build a final generation of supersensitive detectors — or one ‘ultimate’ detector — that will leave the particles no place to hide” (Gibney 1). That "ultimate detector” is DARWIN, a massive XENON-inspired effort that will coalesce all the present detectors into a single unit, and will house fifty tons of liquid xenon fuel in order to produce a high-resolution scintillating device. As the global yearly output of xenon is only 70 tons, the collaboration would expend over 70% of global production for this sole effort. This significant proportion of the world’s xenon resources, coupled with the expenses of construction and xenon production, has made the XENON team “hopeful that Chinese colleagues, who this year are starting up an experiment called PandaX-4t, or the team involved in the US-based xenon experiment called Lux-Zeppelin, might join them” (Gibney 1). The experiment–the largest potential search in the history of WIMP direct detection experiments–will be the “last hoorah” for WIMP experimentation; in the event that this experiment produces null results, the existence of WIMPs may be determined to be extremely unlikely, or even impossible.

Recent developments have cast significant doubt on the simplest theories of WIMP particles, rendering a “WIMP miracle” that is less miraculous. Nevertheless, new experiments–encompassing an all-out search for WIMPs–have the potential to close the argument over CDM’s composition: either WIMPs are the constituents of dark matter, or they are not.

ALTERNATIVE CANDIDATES

As evidence for WIMPs as candidates for dark matter grows less encouraging, we are forced to look elsewhere. Because supersymmetric particles (see Constituent Particles and WIMPs) have similar properties to WIMPs, they will not be considered separate from WIMPs. Among many postulated candidates, there are four compelling alternatives to WIMPs: axions, primordial black holes, neutrinos, and modified gravity.

Although not hypothesized to determine what constitutes dark matter, axions–assuming their mass is within a specific range–are tantalizing candidates. Axions are chargeless particles that were first introduced as consequences of the Peccei-Quinn mechanism, which seeks to remedy the strong charge-parity (CP) problem of quantum chromodynamics. They also have an extremely small rest mass: “typical bounds obtained from astrophysics require axions to be lighter than ma1-103 eV” (Peccei 11). If the mass value argued in Peccei’s paper is correct, then axions are light enough to be candidates for CDM; however, no experimental evidence has pointed to their existence.

Primordial black holes are relatively small yet abundant black holes that formed shortly after the Big Bang. They are hypothesized to have originated in the early universe, when dense packets of subatomic matter were packed tightly enough to experience progressive gravitational collapse. First studied in depth by Stephen Hawking, primordial black holes–assuming they are determined to be prevalent in the universe–are compelling candidates for CDM, for dark matter exhibits properties much like that of typical black holes (e.g. they do not interact with the electromagnetic force, they are ‘invisible,’ and they can only be observed indirectly). However, as with axions, no experiments have directly observed primordial black holes.

Although Standard Model particles have largely been ruled out as candidates for CDM, neutrinos are still compelling. They interact via only the weak force and the gravitational force (like WIMPs), and they have been experimentally confirmed for over half a century (unlike WIMPs). Their properties and experimental confirmation, coupled with the fact that they are readily present in the universe and interact much like dark matter, renders them extremely compelling candidates. One issue, however, is that their rest mass has never been directly measured; according to Cornell University’s Ask An Astronomer, “Neutrinos are one candidate for dark matter but only if they have a nonzero rest mass” (Stierwalt 1). Some hypothetical neutrinos, including the sterile neutrino–which interacts only via the gravitational force and whose properties match those observed by the effects of CDM at large scales–are also compelling alternatives. However, the so far unobserved rest mass of neutrinos is still a topic of debate, and for that neutrinos remain auxiliary candidates for dark matter.

Unlike other candidates, the idea of “modified gravity” does not refer to a particle. Modified gravity, also known as modified Newtonian dynamics (MOND), involves altering general relativity to account for the large-scale discrepancies between the universe that general relativity without dark matter would predict versus the universe itself. Instead of proposing a constituent particle, MOND theories argue that the phenomena observed at larger scales involve still unknown gravitational physics. Therefore, baryonic matter would constitute all the matter of the universe, and CDM would not exist. According to Adam Hadhazy in a publication through The Kavli Foundation, “modified gravity assumes that on the largest and/or smallest scales, gravity acts in novel ways compared to the well-tested scales in the middle” (1). The holes in general relativity–often in high- and low-mass systems–cannot yet be explained by known dynamics; new physics is required. One such theory–a relativistic theory for modified Newtonian dynamics–managed to reproduce two of the universe’s most integral phenomena with stunning accuracy: “the first RMOND [relativistic modification of Newtonian dynamics] theory which reproduces galactic and lensing phenomenology… [and] successfully reproduces the key cosmological observables: CMB [cosmic microwave background] and MPS [matter power spectra]” (Skordis 1). Modified gravity, if proven, would revolutionize modern physics, for not only could it solve the dark matter mystery, but it could also disrupt our understanding of the universe as a whole. However, observations of baryonic matter and associated dark matter casts doubt on most MOND models.

Although all the candidates mentioned above are tantalizing contenders for CDM, they, too, are purely hypothetical. Excluding neutrinos, all the other candidates have yet to be observed or, in the case of modified gravity, are derived only within the framework of modern mathematics. 

CONCLUSION

Although WIMPs are still the leading candidates for CDM, recent developments with dark matter and related experiments have made the existence of these mysterious particles much more questionable; indeed, despite maintaining the scientific consensus, the “WIMP miracle” is in jeopardy. This paper considered (1) the history and development of dark matter, as well as its candidate constituent particles; (2) the methods of detecting WIMP particles, either through direct or indirect detection; (3) the presently uneasy consensus on the existence of WIMPs, recent null results of supersymmetry, and near-future, “last push” direct detection, scintillating xenon experiments; and (4) alternative candidates, as well as each candidate’s standing in the scientific community. Further research will be needed to confirm whether or not WIMPs exist, but for the moment, the uneasy scientific consensus that WIMPs maintain is under significant pressure.

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