A Flash in the Dark (Matter)

This post is about the news, earlier this week, of a possible detection of dark matter.

What dark matter is

Dark matter is an invisible, unknown substance that makes up about 27% of the universe and 85% of all matter. Unlike the ordinary matter that builds stars, planets, and human bodies, dark matter does not absorb, reflect, or emit light, making it impossible to see directly with any kind of an existing telescope. Scientists only know dark matter exists because of its massive gravitational pull on visible cosmic structures. 

The notion of dark matter was first proposed by (the mercurial) Fritz Zwicky, whose idea was thought to be speculative, as the evidence backing it at the time was not very strong. The scientific community had to take the concept seriously after the work of astronomer Vera Rubin, who discovered conclusively that stars at the outer edges of galaxies spin just as quickly as the stars near the center. Without an extra invisible mass acting as a "gravitational glue," these fast-moving outer stars should fly off into deep space.

Dark matter is therefore essential for holding the (structures in the) universe together.

What dark matter is not

It is not dark energy: They are in fact opposite in nature. Dark matter pulls things together via gravitation attraction. Dark energy pushes the universe apart, accelerating its expansion. Unlike matter or dark matter, dark energy does not clump together; it is uniformly distributed and fills empty space with a constant negative pressure. The source of dark energy is also currently a mystery. 

What they were looking for (in the dark)

Physicists strongly suspect that dark matter consists of undiscovered, exotic subatomic particles. The most popular theoretical candidates are WIMPs (Weakly Interacting Massive Particles) and Axions. Experiments have been designed to look for these particles.

How does the experiment work?

This report is about the experiment being conducted at LUX-ZEPLIN, nearly a mile underground in the Davis Cavern at the Sanford Underground Research Facility (SURF - they have a nice website) in Lead, South Dakota.

The detector has 10 metric tons of Xenon gas. The idea is that a passing WIMP particle will hit a Xenon atom. The detector is sensitive enough to measure this event. Since WIMPs interact very weakly with matter, collisions are expected to be rare, and thus a large amount of Xenon has to be used. Similar techniques have been very successful in detecting other weakly interacting particles, such as neutrinos (at the Kamiokande detector).

What they saw

While analyzing 220 days of data, scientists found an anomalous high-energy nuclear recoil event. The event occurred in the deepest, quietest part of the detector core where the surrounding xenon filters out external radiation. Researchers spent months testing every conceivable cause. They concluded there is only a 0.5% chance (a 2.6-sigma statistical significance) that this specific event was caused by known background noise, such as stray neutrons or environmental radioactivity.

Because it is just one event, scientists are not claiming a discovery. In physics, a formal discovery requires a "5-sigma" threshold (less than a 1-in-3.5-million chance of being a fluke). However, they are saying that they understand their detector and the backgrounds so well that even a single outstanding event is important, and what they have seen is the most compelling sign of a dark matter particle to date. Apparently the results will soon be submitted to the journal Physical Review Letters.

Going forward

If the discovery is genuine, it would be a breakthrough, and also a surprise, as the dark matter particle seems to be heavier than was expected and also interacts with matter in a more complicated way than suspected.

It should be mentioned that there are also other dark matter detection experiments which use other techniques and target other kinds of particle candidates. An example is the SuperCDMS experiment inside SNOLAB in Canada that uses ultra-cold, frozen crystals to detect lighter dark matter particle candidates.

Stay tuned for more!