“There is really no conclusion to be drawn at this point, other than mounting suspense,” says Juan Collar, a physicist at the University of Chicago who has worked on several dark-matter experiments about the fact that for more than two decades, only one experiment in the world has consistently reported detecting a signal of dark matter — the missing mass of the cosmos that physicists have long tried to identify. “But the instruments seem to have sufficient sensitivity to give conclusive results soon,” Collar adds.
Dark matter is one of astronomy’s most embarrassing conundrums: despite comprising 84.5 percent of the matter in the universe, no one can find it. Proposed dark matter candidates span nearly 90 orders of magnitude in mass, from ultralight particles like axions to MACHOs.
Despite dedicated searches, no signs of a dark matter particle have turned up. Physicists hope they will be able to find some dark force, a portal into the dark world –a “dark photon” that would be dark matter’s equivalent of a photon, the way that dark matter particles interact with one another.
“Dark photon searches are simultaneously straightforward and challenging, straightforward because the concept is general and simple enough that designing experimental searches is pretty easy, but challenging because we really have no clue where in the parameter space the dark photon could live,” says CERN Physicist James Beacham.
Several theorists have proposed scenarios in which there are multiple types of dark matter. But if dark matter consists of several unrelated components, each would require a different explanation for its origin, which makes the models very complex.
Physicists at the Gran Sasso National Laboratory detector in Italy have long claimed to see the universe’s missing mass — but duplicate experiments elsewhere have yet see the same result. Now, reports the journal Nature, two experiments designed to replicate these results using the same detector technology have presented their first findings.
Although a definitive answer remains elusive, initial data from one experiment seem to be compatible with the original results. The other detector’s findings, however, point in the opposite direction. Scientists say that thanks to these experiments and others scheduled to come online soon, a final answer on the mysterious signal’s nature is now within reach.
The invisible ‘dark’ matter would make its presence known almost exclusively through gravitational interactions with other objects, but a host of experiments have been attempting for decades to pick up signs of its other interactions with ordinary matter without success.
Since 1998, the underground DAMA detector at the Gran Sasso National Laboratory in central Italy, and its successor DAMA/LIBRA, have recorded flashes of light created as particles hit atomic nuclei in a highly purified sodium iodide crystal.
These flashes could be signs of dark matter, reports Nature, or stray background radiation — “but the experiment’s physicists say that the seasonal change arises because the Earth is moving through a halo of dark-matter particles that surrounds the Milky Way, resulting in a repeating pattern. In March 2018, the DAMA collaboration presented the first results from the detector after it was upgraded in 2010. The dark-matter signature still appeared to be there.”
Over the years, several experiments using various techniques have come up with results that apparently contradict DAMA’s.
Two projects, COSINE-100 and ANAIS are the first to have come online that aim to test DAMA’s claims using the same materials, and each has been operating for more than a year.
ANAIS in the Canfranc Underground Laboratory in the Pyrenees, Spain, reported its first results on 11 March. On the basis of 18 months of data, the findings seem to disagree with DAMA’s2. The ANAIS data show fluctuations, but they aren’t the same as DAMA’s yearly cycle, in which signals peak in early June and bottom out in early December.
According to Nature, COSINE-100, another sodium iodide experiment located under a mountain in South Korea, unveiled a similar analysis to ANAIS’s at conferences this month. The detector also sees a fluctuation in its data. However, “ours is a little bit closer to DAMA’s”, says Reina Maruyama, the co-leader of COSINE-100 and a physicist at Yale University in New Haven, Connecticut. (Both ANAIS and COSINE-100’s results notes Nature are still preliminary, and have not yet been peer reviewed.)
The ANAIS analysis has “no impact” on the results of DAMA/LIBRA and its predecessors — the data have already confirmed over 20 independent annual cycles, says Rita Bernabei, a physicist at the University of Rome Tor Vergata who has long led the DAMA collaboration.
Even if one experiment were to find strong evidence against the DAMA/LIBRA claim, it will be worthwhile for several detectors to keep taking data for several more years, Collar says: “When one experiment is seeing something like that and the other one isn’t, you are left wondering if someone screwed up.”
“With a few more years of data,” says David Spergel, a theoretical astrophysicist and Princeton University professor known for his work on the WMAP mission, who first predicted the seasonal oscillation in 1986 with two co-authors. “They should be able to make a definitive statement.”
The Daily Galaxy via Nature
Image credit: Dark Matter, Berkeley Lab