One of science’s most enigmatical concepts is dark matter. Despite decades of astronomical evidence for its existence, no evidence of it has yet been discovered closer to home. There have been dozens of attempts, and one of the most well-known has recently reached a major milestone: the release and analysis of eight years’ worth of data. The results from those eight years will be released soon by the IceCube Neutrino Observatory, but for now, let’s look at what they’re looking for.
There are many theories about what dark matter is, and many of them revolve around the concept of Dark Matter as a particle. Weakly Interacting Massive Particles is the most well-known of these (WIMP). One of the main driving forces behind the IceCube Experiment is the physics of WIMP.
Although it may appear to be an unusual method of searching for WIMPs, the physics behind it is well understood. WIMPS could lose energy and become gravitationally bound to the body they are traveling through when passing through large clumps of “standard model” matter (i.e., what we think of as “normal” particles). This is true of planets and the Sun, for example. Thus, a large, unseen mass of weakly interacting particles could exist at the Earth’s core.
Any such clustering of WIMPS would be impossible to detect directly. Scientists, on the other hand, could detect warning signs by measuring neutrinos, a proxy particle. Some theories in which WIMPs self-destruct by interacting with a standard particle produce neutrinos, which are notoriously difficult to detect. The neutrinos that would result from this process in any mass of WIMPs in the Earth’s core would almost certainly be able to make it through the Earth’s mass and out into space because they are so difficult to pin down.
However, they may be detected by a neutrino detector like IceCube along the way. IceCube, which is based at the geographic South Pole, is made up of 86 strings of digital optical modules with 5160 individual optical sensors that will detect Cherenkov radiation when a neutrino interacts with another particle. Scientists can backtrack the speed and direction of the neutrino by triangulating the brightness and longevity of the light pulse.
Because of the equipment’s finicky nature and the particle of interest, IceCube’s noise reduction is a critical component. Part of that strategy is isolation—the detection array is not only located in one of the world’s most remote locations, but it is also buried beneath 1450 meters of ice and spans nearly a kilometer in vertical depth.
Another component of that strategy is simulations, which are used to estimate and eliminate background noise. To eliminate false detections, the IceCube research team, which includes scientists from all over the world, employs simulations of background noise. They can also rule out some neutrinos that aren’t linked to WIMPs, such as when the system detects a neutrino traveling towards the Earth’s core rather than away from it. These neutrinos are most likely caused by “atmospheric neutrinos,” which form when cosmic rays collide with the Earth’s atmosphere.
All of this effort is focused on a single, relatively straightforward task: determining what WIMPs are. This means attempting to constrain their “mass” in particle physics terms. It’s measured slightly differently than putting something on a scale, as is the case with many things in particle physics. The researchers studied potential masses ranging from 10GeV (giga electron volts) to 10 TeV (tera electron volts) in “electron volts” (tera electron volts). These ranges include masses that are orders of magnitude “heavier” than well-known subatomic particles like the Higgs Boson (125 GeV) or the electron (.511 MeV).
The “annihilation rate,” or how often WIMPs actually destroy themselves and create a neutrino that IceCube can detect, is another characteristic of WIMPs that the research was attempting to narrow down. The researchers developed a statistical probability for various ranges of annihilation probability using advanced statistical analysis.
Despite all of the work that has been done thus far, the final results have yet to be fully analyzed. So it’s still unclear what all of these findings mean in terms of the search for WIMPs. The results will be “released soon,” according to the IceCube team. Furthermore, the data they examined only spanned the years 2011 to 2018, leaving three years of data unaccounted for.
It’s possible that all of the effort put into figuring out what dark matter is worth it. After all, it’s one of particle physics’ most puzzling phenomena. And the only way scientists will be able to fully comprehend it is to collect data for years and years at instruments like IceCube.