By Don Lincoln, Fermilab
If dark matter moves differently than the solar system, there’s another feature of the solar system we can exploit, and that’s that the Earth orbits the Sun. If we combine the motion of the solar system with the motion of the Earth, the Earth moves at a different speed throughout the year. There are famous experiments that try to capitalize on this variation in speed called DAMA experiments.
The DAMA Experiment
DAMA is sort of a modified acronym for the term dark matter. The DAMA experiment is buried under a mountain in Italy to shield it from cosmic rays. It consists of an array of crystals that, more or less, look like blocks of glass. The idea is that dark matter particles will hit an atom in the blocks. The interaction will cause a blink of light and that blink of light will be recorded by light-sensitive detectors.
DAMA first just wanted to see dark matter collisions, but the real signature they were looking for is to see a difference in the rate of dark matter collisions over the course of the year, with more collisions when the velocity of the Earth relative to dark matter is highest, and fewer when it is lowest. So, DAMA took data for many years.
It began operations in 1995 and was substantially upgraded in 2002. Over the past quarter century, they report seeing a signal that is exactly what they hoped for, with more interactions in their detector in June and less in December, over and over. The data is really very persuasive.
No Other Experiment Could Confirm the Results
Except that there have been dozens of other experiments and not one of them can confirm the DAMA results. The conclusion is that either the DAMA scientists have figured out a way to see dark matter that nobody else can duplicate, in which case one day they will be elevated as heroes of science and will be awarded the Nobel Prize, or the other possibility is that the group has spent their entire career making the same mistake over and over again.
But of all of the other experiments that do not confirm DAMA, what are they? Some of them are crystals of germanium or some other semiconductor, cooled to a fraction of a degree above absolute 0, about 460 degrees below 0 F, or −273 degrees Celsius. In those detectors, the atoms are nearly stationary and if a dark matter particle hits the atom, it will shake it and the shaking will be detected.
Another technology uses liquid xenon, cooled to about −100 degrees Celsius or about −150 degrees Fahrenheit. The most impactful experiment using this technology is the LUX experiment, short for Large Underground Xenon detector.
It used 370 kilograms of liquid xenon and it was located nearly a mile underground when it operated. It, and a follow-on experiment using nearly a ton of liquid xenon, is about 100,000 times more sensitive than DAMA, and it sees no evidence of dark matter. This is why I think it’s safe to say that the DAMA experiment is somehow fooling itself.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Further Ideas about the Nature of Dark Matter
We are quite confident that dark matter doesn’t interact via any of the known subatomic forces. And that brings us full circle. We haven’t found dark matter yet, but we’ve learned a great deal about what it isn’t. If it exists, it could be a massive subatomic particle. But if that’s what it is, it doesn’t interact via any known force except gravity.
Of course, that doesn’t mean that there’s not another force that hasn’t been discovered, one weaker than the weak force, but stronger than gravity. And that is an attractive one for some theoretical physicists, as they have postulated the existence of what is called a sterile neutrino, which is a form of neutrino that is both massive and doesn’t experience any of the known subatomic forces.
If they exist, these hypothesized sterile neutrinos do participate in the neutrino oscillation phenomenon. You shouldn’t believe in sterile neutrinos. They’re merely a proposed solution to some lingering mysteries in neutrino oscillation data. The fact that they would also solve the dark matter mystery is just an attractive side benefit.
Then there is another idea about dark matter, called axions. Axions were proposed to solve a mystery in the behavior of the strong nuclear force. Axions haven’t been found but, if they did, they’d be incredibly light, lighter than neutrinos, so there would have to be a bunch of them. Again, don’t believe in axions. They’re a thing that is possible, but there is no experimental evidence that they exist.
Our Knowledge Is Very Limited
What we actually know about dark matter is slim. We can say that the preponderance of the evidence is leaning towards the existence of some sort of actual matter that doesn’t interact via any of the known subatomic forces. The range of possible masses of dark matter is staggering.
Researchers are looking for axions using super-strong magnetic fields. Finally, some astronomers are looking for dark matter in space in those indirect measurements. One exciting recent prospect has turned their attention away from the center of the Milky Way to what are called dwarf galaxies. Dwarf galaxies have a handful of stars, but they have, relatively speaking, a lot of dark matter.
So, that’s kind of what we know about dark matter. We have spent years looking for it, to no avail. Hopefully one day soon, one of these experiments will finally point us to the right leaf.
Common Questions about Some Experiments and Ideas for Studying Dark Matter
This experiment consists of an arrangement of glass block-like crystals buried under a mountain in Italy to protect and guard it against cosmic rays. Originally intended to see dark matter collisions, DAMA’s idea is that dark matter particles will hit an atom in the blocks, and this interaction will cause a blink of light, which will be recorded by light-sensitive detectors.
The DAMA experiment took persuasive data regarding the collision of dark matter for many years. However, the results have not been confirmed by any other experiments so far.
One of these experiments is LUX, in which they used 370 kilograms of liquid xenon and located them a mile under the ground. The LUX experiment was 100,000 times more sensitive than the DAMA experiment but it couldn’t confirm the existence of dark matter.