1 Introduction
In 1933 Fritz Zwicky published his paper on the redshifts of
extragalactic nebulae, in which he noted that the velocity dispersion
he observed in the Coma Cluster corresponded to an average density
400 times larger than the observable luminous matter
(Zwicky
1933). Zwicky’s application of the virial theorem (Zwicky 1933) to
the cluster, together with his insight that some unseen matter must be
holding its galaxies gravitationally bound, began the search for the
universe’s missing matter. Since Zwicky, evidence for dark matter has
accumulated from many directions, from rotation curve analysis of the
Andromeda galaxy(Rubin and Ford Jr 1970), to the missing mass required for a flat
\(\Omega=1\) universe (Gaskins 2016a). Additional
evidence has come from strong (Tyson et al. 1998) and weak (Refregier 2003)
lensing of background galaxy cluster light caused by dark matter, where
the luminous foreground matter is not present in sufficient quantity or
density to account for the lensing observed.
Aside from a handful of properties that can be inferred, dark matter's
actual composition has remained frustratingly difficult to pin down.
Its gravitational effects are easily observable, yet it must barely
interact with baryonic matter (Peter 2012). Thus, the hunt for dark
matter has embraced a multi-messenger approach to its detection, using
direct, indirect and collider detection strategies (Bertone et al.
2005). All three are discussed below, with the greatest focus given to
indirect detection techniques.
Taken together, this breadth of detection strategies and the wide range of viable candidates make the hunt for dark matter an example of truly modern, \(21^{\text{st}}\) century multi-messenger astronomy.
2 Theoretical Framework
Traditional astronomy such as the work of Zwicky(Zwicky 1933), Rubin and Ford (Rubin and Ford Jr 1970), and later experiments has placed a number of cosmological constraints on dark matter candidates. However, these constraints are insufficient on their own to practically narrow the search(Feng 2010). Particle physics is therefore brought in to place further constraints on the possibilities for these nearly invisible particles.
The most written about (Vecchi 2021) dark matter candidate, WIMPs, are motivated by their ability to address an open issue in the standard model known as the gauge hierarchy problem(Feng 2010). In the standard model, the mass of the Higgs boson is predicted to lie on the order of the Planck mass, or to be zero if enforced by a symmetry(Feng 2010). However, electroweak symmetry in the standard model is broken, and the Higgs boson mass is observed to be nonzero but far smaller than the Planck mass(Feng 2010). This discrepancy can be eliminated, however, if WIMPs are introduced at the weak scale mass range of\(10\) GeV - TeV (Feng 2010). This is a strong particle physics motivation for searching for dark matter WIMPs, and the remainder of this paper focuses on them, both because they are an excellent candidate for the indirect detection techniques that are this report's primary focus, and because they are the most well-studied. Other particle physics motivations exist for other dark matter candidates, including the new physics flavour problem, the neutrino mass problem and the strong CP problem (Feng 2010), but covering all of these and their resulting dark matter candidates is beyond the scope of this report.
2.1 Characteristics of Dark Matter WIMPs
The first and most important requirement on any dark matter candidate is that it satisfies the simple, predictive description of a thermal relic of the Big Bang(Zeldovich 1965). The future density and distribution of dark matter can therefore be predicted: as the universe cools, WIMPs become Boltzmann-suppressed and their interactions with standard model particles fall below the rate required to maintain equilibrium, causing them to thermally freeze out(Roszkowski et al. 2018) once their mean free path grows too large to sustain that equilibrium(Kahlhoefer 2017). The relic density of cold dark matter (CDM) inferred from gravitational effects matches the density expected from this freeze-out picture, making WIMPs a strong candidate(Roszkowski et al. 2018). The relic density argument, combined with the role WIMPs play in resolving the gauge hierarchy problem, also implies that WIMPs can interact with and decay into standard model particles (Feng 2010). These interactions mean that WIMPs are in principle detectable via all three detection strategies. However, their small interaction cross sections, together with the difficulty of confirming a WIMP signal, make direct and collider searches challenging, leaving indirect searches as the most promising avenue of discovery(Roszkowski et al. 2018).
3 Detection Strategies
As argued above, indirect detection is both the most promising avenue for finding evidence of dark matter and the most astronomical of the three approaches, and will be discussed at greatest length. A brief treatment of collider and direct searches is included for context.
3.1 Collider Searches
In the WIMP paradigm, where the existence of WIMPs is motivated by the gauge hierarchy problem, particle collider operators expect to produce stable particles at the electroweak scale(Kahlhoefer 2017). Using the annihilation cross section derived from the electroweak scale, and under the strong assumption that this cross section also governs WIMP pair-production(Kahlhoefer 2017), colliders expect to find evidence for cold dark matter through the characteristic missing-energy signatures it would leave(Kahlhoefer 2017). However, it remains uncertain whether even the largest particle colliders can produce WIMPs via hadron collisions; larger colliders reaching higher energies may be required(Feng 2010). This is a significant disadvantage of collider searches: they take operating time away from other research, and they may not even be feasible on existing machines.
3.2 Direct Searches
Direct detection rests on the assumption that WIMPs are prevalent in space, so the Earth is continuously traversed by a flux of dark matter particles(Feng 2010). These WIMPs can be detected directly via elastic scattering off ordinary matter, made possible by their finite weak coupling (Liu et al. 2017). Theoretical models predict this scattering cross-section to be of the order of \(10^{-42}\) cm\(^{-2}\) (Liu et al. 2017), so the expected signal is extremely small and great effort must go into suppressing background noise. Direct search experiments are therefore heavily shielded from the environment and built from high-purity materials such as xenon(Liu et al. 2017). In general, these detectors house a pure collision medium and use photomultiplier tubes and scintillation detectors to register a signal. A direct detection would be the most conclusive evidence available, but it requires substantial investment in highly sensitive single-purpose machinery that diverts funding from other research.
3.3 Indirect Searches
Indirect detection looks for the photons and cosmic rays produced when
dark matter particles annihilate(Bertone et al. 2005). The
resulting flux is proportional to the annihilation rate, which is in
turn proportional to the dark matter density. One particularly
promising channel is the flux of charged antimatter particles in
cosmic rays, where the background noise is low(Gaskins 2016b),
since standard astrophysical processes do not produce them in large
quantities. Experiments such as AMS-02 have been measuring the spectra
of cosmic-ray antiprotons and positrons(Lu and Zong 2016); at energies above
\(10\) GeV the observed positron diffusion falls short of the level expected
by standard cosmic-ray spectrum models(Liu et al.
2017). WIMP annihilations are the expected cause. Since WIMPs themselves
cannot be charged(Feng 2010), identifying the likely charged products of their annihilation
constrains what dark-sector particle(Lu and Zong
2016) the WIMP can be.
Charged cosmic rays are not the only indirect channel; neutral gamma ray photons are equally informative. Space-based telescopes such as Fermi-LAT search for the most telling signature of WIMP annihilation or decay, a monochromatic photon line(Ackermann et al. 2012). Photons travel in straight lines from their source, providing a useful directional diagnostic(Feng 2010). Fermi-LAT has tentatively noted a\(130\) GeV (Gaskins 2016b) feature that could be evidence of WIMPs, but more data is needed to confirm it, work that will most likely fall to the next generation of gamma-ray telescopes.
Indirect methods carry an additional advantage: data can be gathered by existing astronomy infrastructure. Reusing telescopes and detectors already in operation, sometimes from data captured for unrelated projects, makes indirect detection a highly economical strategy.
4 Conclusions
Dark matter detection, and the search for WIMPs in particular, highlights how broad a discipline multi-messenger astronomy has become. The ability to combine direct, indirect, and collider searches for relic dark matter is a powerful tool. Time did not allow a treatment of other dark matter candidates, and future work would examine the differing requirements they each impose on detection. For WIMPs, however, it is the opinion of the author that indirect detection offers the most versatile and economical strategy.