Dark Matter

A new twist on dark matter is challenging our understanding of the cosmic mystery

In March 2019, astronomers observed what they believed to be a clump of dark matter left behind afte...

Located deep under the Gran Sasso mountain in Italy, 25 crystals are sealed inside a copper box and surrounded by concrete made from the mountain’s rocks.

The intricate installation is a particle detector known as the DAMA experiment, which is on the hunt for the most elusive scientific mysteries of all: dark matter. And the scientists running the experiment are certain they have found it, much to the skepticism of several other groups with the same objective.

In an underground lab in South Korea, one such group went looking for the same dark matter signal and came up with nothing, ruling out DAMA’s claim of a dark matter signal detection.

The latest findings are detailed in a study published Wednesday in the journal Science Advances.

HERE’S THE BACKGROUND — Dark matter makes up around 85 percent of all matter in the universe and about 27 percent of its total mass, outweighing visible matter around six to one.

Scientists know dark matter exists because of the effect it has on galaxy rotation rates, although they have never been able to directly observe it. But that’s not for a lack of trying — several major experiments have been built for the sole purpose of detecting dark matter.

DAMA/LIBRA (Dark Matter Large Sodium Iodide Bulk for Rare Processes), the instrument at the center of this controversy, is a particle detector that uses ultrapure sodium iodide crystals, designed to give off flashes of light when they are struck by dark-matter particles.

In 2008, DAMA scientists claimed to have directly detected dark matter when they reported an increase in the number of flashes every June and a decrease each December.

They claimed that this was due to Earth plowing through dark matter particles scattered across the Milky Way as it orbits around the Sun. The signal increases in June as the planet moves faster through the galaxy, while decreasing in December as it goes against the Sun’s motion in the Milky Way.

During an international meeting of particle physicists in Venice, Italy in 2008, the DAMA group reasserted their claim with an additional six years of observations that prove that their signal has persisted.

Following the group’s announcement, rival groups headed back to their own labs in search of the same signal, but came up empty-handed. Additionally, scientists in the community have complained that the DAMA experiment does not make its data public, which makes it difficult to test their claim.

WHAT’S NEW — The COSINE-100 experiment placed 106 kilograms worth of crystals similar to those at DAMA at the Yangyang underground laboratory in South Korea.

The crystals are submerged into approximately 2000 liters of liquid scintillator to help reduce low energy background noise.Yale University

Using the same detectors, the scientists behind the experiment wanted to test DAMA’s claim that they had detected dark matter particles.

Hyun Su Lee, a researcher at the Institute for Basic Science in South Korea, and a member of the COSINE-100 experiment, says that it was natural for them to begin looking into the DAMA experiment with the aim of either reproducing or refuting their results.

“Based on a good understanding of the detector, we have searched for event excess in the averaged event rate that could be a dark matter signal,” Lee tells Inverse “Unfortunately, our data shows no signal excess that could set the limits of the dark matter interactions.”

How they did it —The COSINE-100 experiment looked for the dark matter signal within the same timeline as the one detected by DAMA based on a dark matter model known as WIMPS.

WIMPs (Weakly Interacting Massive Particles) are hypothetical exotic particles that don’t interact strongly with other particles, nor do they absorb or emit light. Scientists aren’t sure which particles could qualify as WIMPs, but certain kinds of neutrinos could be a strong candidate.

Rita Bernabei, a researcher at the National Institute of Nuclear Physics in Rome and DAMA’s longtime leader, dismisses the latest paper since the experiment uses a hypothesized background model.

“To compare the results in this case it is necessary to adopt a model,” Bernabei tells Inverse. “[But] considering that so many models are possible about related astrophysical, nuclear, and particle physics aspects and the many existing uncertainties on related parameters and assumptions, a serious comparison is quite a complicated and uncertain job.”

Bernabei also says that the crystals used for both experiments are not quite the same, differing in materials, growing procedures, and years spent underground.

WHAT’S NEXT — Most scientists agree that DAMA’s experiment is picking up something, but they are not quite sure what it is.

“There is still a possibility that a different model could explain both results consistently,” Lee says. “For the final conclusion, we need a model-independent test of the annual modulation signals.”

There are several other experiments currently on the lookout for dark matter such as the Large Hadron Collider, the world's largest and highest-energy particle collider.

The collider is made up of a 27-kilometer ring of superconducting magnets and two high-energy particle beams that travel at close to the speed of light before they are made to collide.

The XENON experiment in Italy is a sensor-lined tank filled with 3,500 kilograms of liquid xenon, a type of gas used in light manufacturing, that hunts for the hypothetical particles that could make up dark matter.

Meanwhile, the Sanford Underground Research Facility in South Dakota is running its own Xenon dark matter experiment with a chamber containing seven active tonnes of liquid xenon to search for dark matter particles.

“At this moment, we just need to search for various possibilities of dark matter,” Lee says.

Abstract: We present new constraints on dark matter interactions using 1.7 years of COSINE-100 data. The COSINE-100 experiment, consisting of 106 kg of tallium-doped sodium iodide [NaI(Tl)] target material, is aimed to test DAMA’s claim of dark matter observation using the same NaI(Tl) detectors. Improved event selection requirements, a more precise understanding of the detector background, and the use of a larger dataset considerably enhance the COSINE-100 sensitivity for dark matter detection. No signal consistent with the dark matter interaction is identified and rules out model-dependent dark matter interpretations of the DAMA signals in the specific context of standard halo model with the same NaI(Tl) target for various interaction hypotheses.
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