New results from a decade-long experiment were initially heralded as further evidence for dark matter. But independent scientists have strongly disputed this claim, leaving many confused.
Is Dark Matter On Earth
As the Sun revolves around the Milky Way, the Earth revolves around the Sun. The changing speed of the Earth relative to a galaxy covered in dark matter causes the changes in the amount of dark matter detected in the DAMA experiment.
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A 20-year-old experiment in Italy called DAMA detected a pulsating signal from dark matter, the invisible cloud of particles that fills the universe and shapes everything else with its gravity.
One of the oldest and largest experiments hunting dark matter particles, DAMA claims to be the only one to see them. Its aim is to obtain unique interactions between predicted particles and simple molecules. But if these connections between the visible and invisible worlds do indeed produce DAMA data, more experiments will detect dark matter as well. They don’t have it.
Late last month, DAMA’s longtime leader, Rita Bernaby of the University of Tor Vergata in Rome, presented the results of another six years of measurement. He reported that the DAMA signal was as strong as ever. But researchers not involved in the experiment argue strongly against dark matter as the source of the signal.
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DAMA searches for popular dark matter candidates known as WIMPs, or “weakly interacting massive particles.” Scientists are monitoring a field of sodium iodide crystals deep beneath Mount Gran Sasso in the Apennines, looking for flashes of radiation caused by dark matter particles colliding with atomic nuclei in the crystals. As the solar system moves through the galaxy, “the WIMP wind looks like it’s coming at you,” explains University of Michigan physicist Kathryn Freese, who developed the idea for such an experiment in 1986. So while you’re driving, it looks like it’s raining on your windshield.
A technician works on detectors for the DAMA experiment, which uses 250 kilograms of sodium iodide to detect dark matter.
Consistent with this hypothesis, the DAMA scientists found that nuclear activity in their crystals varied throughout the year. The sign always peaks in June, when the Earth is moving rapidly through a galaxy full of dark matter, and in December when the planet is tilted to the part of its orbit that opposes the Sun’s movement through the galaxy. When it reaches a minimum we slow down relatively. Galaxy. Dark matter is wind.
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The final phase of the experiment, called DAMA/LIBRA-Phase 2, began in 2011. After acquiring data for six Earth orbits, the team reported that they still see a periodic signal consistent with dark matter. As Barnaby said
Via email: “Angular modulation signature and adopted mechanisms provide sensitivity to a large number of dark matter candidates.”
Outside experts saw something else. A paper published April 4 on the physics website arxiv.org showed that three Physics Standard dark matter WIMPs failed to produce the new DAMA signal. “The vanilla that everyone loves is gone,” says Freese, who co-authored the new paper with his student Sebastian Baum and Chris Kelso of the University of North Florida.
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Freese and colleagues focused on a new feature of DAMA data. As part of the DAMA/LIBRA Phase 2 upgrade, Gran Sasso’s team modified the instruments to make their detectors more sensitive to low-energy excitations in sodium iodide crystals. Burnaby reported an annual low-power nuclear reverse modulation that resembles a high-power return signal.
But if vanilla WIMPs are indeed the source of the annual modulation, the low-energy return should shift relative to the high-energy return, Freese and his co-authors wrote. They found that nuclear activity should vary more or less between June and December at lower energies than at higher energies, depending on whether the dark matter particles are light or heavy. If WIMPs were light, DAMA should see frequent collisions with light sodium atoms at lower energies than heavy iodine atoms. In general, the DAMA signal should be stronger for the lowest power oscillations. Alternatively, massive WIMPs interact almost exclusively with iodine atoms at low energy and with very little sodium. Generally, in this case, the signal weakens when you see the lowest power events.
Instead, the DAMA/LIBRA-2 phase data show no change, “which is difficult to explain with dark matter,” said Jonathan Davies, a theoretical physicist at King’s College London.
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In their paper, Baum, Freese and Kelso show that WIMPs can still produce the observed annual modulation if they have a twist: the natural advantage of protons over neutrons causes them to interact more often with sodium than with iodine. (This means more. Neutrons). . However, many physicists argue that this “isopine-disturbing” feature affects the results of other dark matter experiments, such as XENON1T, the 3.2-tonne liquid xenon detector beneath Gran Sasso, which has shown no such effect.
The strange silence of XENON1T and other attractively named dark matter detectors such as LUX and PICO has dashed many experts’ hopes for DAMA. These experiments, which looked for different nuclear activity in different types of materials, published several null results that excluded large classes of WIMPs that could match the DAMA signal. (Other dark matter candidates, such as axons, cannot be tested with these experiments.)
However, it is conceivable that dark matter may have an unexplained affinity for sodium iodide. But the April 4 analysis changes that. “In this paper, they show very well that you can rule out DAMA on its own, not based on other experiments,” said Laura Baudis, a physicist at the University of Zurich and leading XENON1T collaboration member.
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As difficult as it is to explain the DAMA signal in terms of dark matter, it is equally difficult to interpret it otherwise. For decades, experts have pondered more simplistic explanations. “Some were introduced and spread quickly,” says University of Chicago physicist Juan Caller, who directs the CoGeNT dark matter experiment. “Personally, I can’t find a better explanation.
In 2014, Davis noted in Physical Review Letters that the annual modulation is caused by a combination of muons, which bombard the Earth heavily in July, and solar neutrinos, which peak in January. But other physicists quickly showed that the second periodic effect was too small to produce a signal, at least in the way he proposed. Daniel McKinsey, a physicist at the University of California, Berkeley, says in a new paper that the signal could be caused by argon contamination. Certain argon isotopes decay more or less radioactively depending on the time of year. However, this explanation only works if the nitrogen used in one step of the DAMA procedure contains an unknown argon.
Many researchers say a lack of transparency from Bernabe and the DAMA team has slowed efforts to understand what’s going on. For example, one limitation of Freese and co-authors’ analysis is that DAMA does not reveal whether background effects are stronger or smaller at low power, so outside researchers assume these effects have already been corrected.
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“I’m sure we’d understand what’s driving the annual modulation,” Davis said, “if they were fully open to the community by sharing their data.” He said that the policy is being taken.
Other groups had to step in. In the next few years, three new sodium iodide crystal experiments will begin producing results: ANAIS, COSINE-100 and SABER, located in an underground laboratory in Gran Sasso and Australia. By replicating the experience in the Southern Hemisphere where summer and winter are reversed, SABER eliminates seasonal effects.
“As long as physics is an experimental science, the only way to advance knowledge is through better measurements,” Kollar said. Additional experiments don’t look at modulation, in which case “we have to turn the page and write the DAMA anomaly,” he said. Or they see something like this, in which case “we have to work hard on a dark matter model that also explains all null observations with other detector materials.”
Beyond Earthly Skies: Dark Matter
“The experiment shows that it’s not dark matter,” said theoretical physicist Neil Weiner of New York University, “but I’m sure it’s an interesting possibility.”
Ultimately, Freese said, his group’s new data release and analysis won’t change the big picture. “You still have to build the detectors out of the same stuff, but it’s done by different people and in-house.”
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