Dark Matter particles

In de Scientific American van April 2021 las ik een interessant artikel over “Dark Mattter detection”. Hieronder geef ik een samenvatting (in het Engels), aangevuld met nog wat extra info over dit onderwerp.

Based on astronomical observations we know that 95% of our Universe is unaccounted for, but we have not yet identified what this 95% is made of. What we do know is that around 70% is made of a mysterious substance called Dark Energy, and we know as well that there is another 20% made out of so called Dark Matter.
If this Dark Matter would have been made of known particles, we would have already observed it a long-time ago. Since we did not, we suspect that Dark Matter is made out of a new kind of particle that has thus far escaped detection at particle colliders.

Dark Matter Detection

While you read this text, there are literally billions of these Dark Matter particles streaming through your body. And it is tempting to try and catch these particles in a physics experiment. Detecting a particle of Dark Matter would be the next big thing to happen in physics, after the discovery of the Higgs Boson in 2012, and the first detection of a gravitational wave in 2015.

Back in the 1990s, experiments began trying to detect the particles that make up Dark Matter, the ubiquitous yet untouchable invisible material that apparently fills the cosmos. Since then, physicists have found more and more evidence that dark matter is real but not a single sign of the stuff itself. Although scientists are fond of saying negative results are just as important as positive results, but after several decades of not finding something, researchers can be forgiven for feeling impatient. A new version of the long running XENON experiment that started up late last year (2020) aims to finally break that pattern.

Dark Matter is WIMPs?

One of physicists’ best guesses about the identity of Dark Matter has long been that it is made of particles called WIMPs—weakly interacting, massive particles. These elementary bits of matter could be anywhere between the mass of the proton and 1,000 times the mass of the proton, and they would interact with regular atoms only through gravity and the weak nuclear force, which governs radioactivity. The appeal of WIMPs comes from the fact that they match well with supersymmetry – a theory that suggests that each known particle has a heavier counterpart that is yet to be discovered, and that is the best contender among the physical theories that try to overcome the limitations of the Standard Model of particle physics.

But over the years, as experiment after experiment failed to find anything, some of the enthusiasm has faded.

Xenon Experiment in San Grasso

You do start to scratch your head and think maybe that was the wrong horse to bet on,” says Rafael Lang, a physicist at Purdue University, who has been working on the XENON experiment at the Gran Sasso National Laboratory in Italy for more than a decade. For now, though, Lang says he is keeping his money on WIMPs, pointing out that experiments have falsified many of the theories predicting what WIMPs might look like but certainly not all. “If you believed in WIMPs 10 years ago, only half of those WIMPs have been ruled out,” he says. “The other half are still alive.

Other Dark Matter candidates

There are many other Dark Matter candidates. Another top contender is the axion, a much lighter theorized particle that has lately morphed into a fluid category of possibilities called axionlike particles. Some scientists are excited about the idea that Dark Matter may be a composite particle—a conglomerate of “dark quarks” and “dark gluons” that stick together just like regular quarks and gluons to create “dark nuclei.”
It is also possible that Dark Matter is not a particle at all and that it is made of primordial black holes that formed soon after the big bang.

The XENON Experiment

The newest version of the XENON experiment, XENONnT, was under construction in March 2020. Even with the problems posed by COVID-19, the project was able to finish construction and move forwards into commissioning phase by mid 2020. Full detector operations commenced in late 2020
Its goal is to catch dark particles on the very, very rare occasions when they might bump into regular atoms.
The atoms uised in this case are xenon (hence the name of the experiment). The xenon is kept in liquid form in a giant vat buried under a mile of rock to shield it from cosmic rays and other forms of contamination.
Xenon, with its 54 protons and electrons and even more neutrons, is a good, dense target for Dark Matter to bump into. If an exotic particle were to hit a xenon nucleus, it might send the nucleus or an electron flying through the liquid, creating a flash of light that photomultipliers on the top and bottom of the vat can detect.
This latest iteration of the experiment contains four times more xenon than the previous version, which means it is four times more likely to see a signal.

Better detection systems

Other upgrades include improved purification of the xenon and better systems to detect cosmic rays and trace amounts of radioactive elements in the experiment that could masquerade as Dark Matter signals. “Every nut and bolt on the detector is custom handmade from carefully selected materials,” Lang says, “because if you just buy a stainless­steel bolt at the hardware store, it’s too radioactive for what we need.

To the outside world, these years of painstaking work without the reward of a discovery might seem disappointing, even ridiculous, but physicists see it differently. “If you’re judging by whether it has detected Dark Matter, they haven’t, but in the eyes of the community it is a dramatically successful experiment,” says theoretical physicist Dorota Grabowska of CERN, who is not part of the project. Its success, she says, lies in the many possibilities it has ruled out and the ever increasing sensitivities it has achieved. Finally, the experiment might even rule out WIMPs as a possibility.
The Xenon collaboration currently (2021) led by Italian professor of physics Elena Aprile from Columbia University.
In the Netherlands we have the Dark Matter research group, which is housed at Nikhef in Amsterdam, and which is connected to Utrecht University through prof. dr. A.P. Colijn. This group is member of the XENON collaboration.

New ways of detection

There is a lot of excitement and creativity around identifying new ways to detect Dark Matter candidates,” says theoretical physicist Tongyan Lin of the University of California, San Diego. One idea she works on involves using crystals to catch dark particles. In crystal form, elements such as silicon might register an interaction with Dark Matter at lower energies than traditional detectors, opening up a new avenue for discovery.

Star Trek

Although Dark Matter has proved more elusive than some had initially hoped, physicists are far from giving up.
A lot of people have a view of science that is like Star Trek,” says theoretical physicist Tim Tait of the University of California. “You see something and take out a tricorder and get an answer. But it’s actually a very messy process, and you try lots of things until you find something that works. All the things that didn’t work were an important part of the process.”

To be continued.

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