Indirect detection of dark matter
Indirect detection of dark matter is a method of searching for dark matter that focuses on looking for the products of dark matter interactions rather than the dark matter itself. Contrastingly, direct detection of dark matter looks for interactions of dark matter directly with atoms. There are experiments aiming to produce dark matter particles using colliders. Indirect searches use various methods to detect the expected annihilation cross sections for weakly interacting massive particles. It is generally assumed that dark matter is stable, that dark matter interacts with Standard Model particles, that there is no production of dark matter post-freeze-out, and that the universe is currently matter-dominated, while the early universe was radiation-dominated. Searches for the products of dark matter interactions are profitable because there is an extensive amount of dark matter present in the universe, and presumably, a lot of dark matter interactions and products of those interactions ; and many currently operational telescopes can be used to search for these products. Indirect searches help to constrain the annihilation cross section the lifetime of dark matter, as well as the annihilation rate.
Dark matter interactions
Indirect detection relies on the products of dark matter interactions. Thus, there are several different models of dark matter interactions to consider. Dark matter is often considered stable, as a lifetime greater than the age of the universe is required for large amounts of DM to be present today. In fact, it seems that the abundance of DM has not changed significantly while the universe has been matter-dominated. Using measurements of the CMB and other large scale structures, the lifetime of DM can be roughly constrained by s. Thus, annihilating DM is the focus of most indirect searches.Annihilating dark matter
An annihilation cross section on the order of is consistent with the measured cosmological density of DM. Thus, the objects of indirect searches are the secondary products that are expected from the annihilation of two dark matter particles. When observations of those secondary products reveal cross sections on the order of the expected the source of those products may become a dark matter candidate, or an indication of dark matter. In general, the DM is expected to be for the cross section given above.Note that the "J-factor" of a given potential source of dark matter interaction products is the energy spectrum integrated along the line of sight, taking only the term dependent on the distribution of the DM mass density. For annihilation, that J-factor is commonly given as,
where is the mass density of DM. The J-factor is essentially a predictive measurement of a potential annihilation signal. The J-factor depends on the density, so if the density of a given region is not well-known or well-defined, then it can be difficult to determine the size of the expected signal. For example, since it is difficult to distinguish and remove backgrounds near the galactic center the calculated J-factor for that region varies by several orders of magnitude, depending on the density profile used.
Decaying dark matter
However, if DM is unstable, it would decay and produce decay products that could be observed. Since decay only involves one DM particle, the flux of DM decay products is proportional to the DM density,, rather than in the case of annihilation. There have been efforts to search for DM decay products in gamma rays, X-rays, cosmic rays, and neutrinos. For unstable dark matter of mass in the GeV–TeV range, the decay products are high-energy photons. These photons contribute to the extragalatic gamma ray background. Studies of the EGRB using the Fermi satellite have revealed constraints on the lifetime of dark matter as s, for masses between about 100 GeV and 1 TeV. The constraints derived from the EGRB are relatively unaffected by additional astrophysical uncertainties. NuSTAR observations have been used to search for X-ray lines to further constrain decaying DM for masses in the 10 to 50 keV range. For sterile neutrinos, there are several existing constraints based on X-ray limits. For DM masses keV and keV, there are well-defined constraints on the mixing angle,. Neutrinos have been used to derive constraints for DM masses in the range GeV. Combined data from Fermi gamma-ray observations and IceCube neutrino observations give constraints depending on energy and defined by the criterion,, with defined as the given signal, as the muon neutrino background, and as the Gaussian significance. For low energies, the constraints improve with time as. For high energies, the constraints are not well-defined, as neutrino flux is no longer dominant. Thus, there are constraints on the properties of decaying DM for masses ranging from keV to TeV. Additionally, in the case of decay, the signal strength is dependent only on density, rather than density squared:. For sufficiently distant sources, the signal strength can then be approximated as, where is the source mass.Methods of indirect detection
There are currently many different avenues through which indirect searches for dark matter may be carried out. In general, indirect detection searches focus on either gamma-rays, cosmic-rays, or neutrinos. There are many instruments that have been used in efforts to detect dark matter annihilation products, including H.E.S.S., VERITAS, and MAGIC, Fermi Large Area Telescope, High Altitude Water Cherenkov Experiment, and Antares, IceCube, and SuperKamiokande. Each of these telescopes participates in the search for a signal from WIMPs, focusing, respectively, on sources ranging from the Galactic center or galactic halo, to galaxy clusters, to dwarf galaxies, depending on allowable energy range for each instrument. A DM annihilation signal has not yet been confirmed, and instead, constraints are placed on DM particles through limits on the annihilation cross section of WIMPs, on the lifetime of dark matter, as well as on the annihilation rate and flux.WIMP annihilation limits
Gamma-ray searches
In order to detect or constrain the properties of dark matter, observations of dwarf galaxies have been carried out. Limits may be placed on the annihilation cross section of WIMPs based on analysis of either gamma-rays or cosmic rays. The VERITAS, MAGIC, Fermi, and H.E.S.S. telescopes are among those that have been involved in the observation of gamma-rays. The air Cherenkov telescopes are most effective at constraining the annihilation cross section for high energies.For energies below 100 GeV, Fermi is more effective, as this telescope is not constrained to a view of only a small portion of the sky. From six years of Fermi data, which observed dwarf galaxies in the Milky Way, the DM mass is constrained to GeV. Then, combining data from Fermi and MAGIC, the upper limit of the cross section is found to be (that is, with no uncertainties in. This collaboration produced constraints for DM masses in the range. Note that Fermi data dominates for the low mass end of the range, while MAGIC dominates for the high masses.
VERITAS has been used to observe high energy gamma-rays in the range 85 GeV to 30 TeV, for the mass range.
Cosmic-ray searches
Cosmic ray analyses primarily observe positrons and antiprotons. The AMS experiment is one such project, providing data on cosmic ray electrons and positrons in the 0.5 GeV to 350 GeV range. AMS data allows for constraints on DM masses GeV. Results from AMS constrain the annihilation cross section to for DM masses GeV. The upper limit for the annihilation cross section can also be used to find a limit for the decay width of a DM particle. These analyses are also subject to substantial uncertainty, particularly pertaining to the Sun's magnetic field, as well as the production cross section for antiprotons.Galactic center
The galactic center is hypothesized to be a source of large amounts of dark matter annihilation products. However, the background at the galactic center is both bright and not yet well understood. The Galactic center is a unique source of high mass dark matter, which cannot be replicated in colliders. Thus, telescopes like Fermi and H.E.S.S. have observed the excess of gamma-rays coming from the galactic center, as backgrounds are lower for gamma-rays. The annihilation cross section is consistent with the expected, and thus, In the case that those excess gamma-rays are products of dark matter annihilation, they must originate from dark matter with a mass.H.E.S.S., an imaging atmospheric Cherenkov telescope, has been used to observe this excess of very high energy gamma-rays emanating from the galactic center. Probing energies in the range GeV to TeV, H.E.S.S. data allowed for limits on internal bremsstrahlung processes to be determined, which then allowed for upper limits on DM annihilation flux to be defined.
Overall, the galactic center is a focus for indirect searches due to its excess of gamma-rays. That excess has which is on the order of the thermally averaged annihilation cross section, making the gamma-ray excess a potential dark matter candidate.