From Science Magazine: “Is the end in sight for famous dark matter claim?”

From Science Magazine

10 Nov 2021
Adrian Cho

New data cast more doubt on controversial result from the DAMA experiment and an alternative explanation of it emerges.

Dark matter particles should generate flashes of light in the COSINE experiment’s sodium iodide crystals. It aims to test a similar experiment’s dark matter claim.Credit: CHANG HYON HA.

The drama of the world’s most controversial dark matter claim may have reached its last act, if not its final scene. For 2 decades, physicists with an experiment called DAMA have claimed that particles of dark matter—the unseen stuff whose gravity appears to bind our galaxy—are bumping into atomic nuclei in their subterranean particle detector, even as other dark matter hunts come up empty. Now, physicists with a detector called COSINE-100, designed to mimic DAMA, present the most direct refutation yet of the findings. And in 2020, theorists identified a way in which the DAMA signal could have arisen inadvertently in the team’s analysis.

Yale COSINE-100 at Yangyang underground laboratory in South Korea

The DAMA team rejects both claims. Rita Bernabei, a physicist at Tor Vergata University of Rome [Università degli Studi di Roma “Tor Vergata”](IT) and DAMA’s leader, declined to be interviewed. But she dismissed the new explanation in an email: “We have already demonstrated that the assumptions there reported are untenable and the conclusions are worthless.”

The Milky Way is thought to whirl within a vast cloud of dark matter, which could consist of hypothetical weakly interacting massive particles (WIMPs). As the Solar System orbits the galactic center at 225 kilometers per second, Earth presumably plows into a wind of WIMPs. Because our planet also orbits the Sun at 30 kilometers per second, the wind should strengthen slightly when Earth is moving in the same direction as the Sun, in June, and abate when it’s moving the opposite way, in December.

DAMA physicists have long claimed to see this yearly cycle in their detector, which now contains 25 10-kilogram crystals of thallium-doped sodium iodide. Each crystal produces a flash of light when a particle pings a nucleus. The DAMA team says that, in a low-energy range that corresponds to WIMPs, the number of collisions has gone up and down each year since observations began in 1995.

Other detectors see no such thing. But those experiments use different target materials. So groups have built sodium iodide detectors that can test the DAMA result in an apples-to-apples comparison. One is COSINE , which comprises eight crystals totaling 100 kilograms and has been taking data since 2016 in South Korea’s subterranean Yangyang Laboratory. Since 2018, COSINE has improved its sensitivity 100-fold, says Hyun Su Lee, the team’s co-spokesperson and a particle physicist at Institute for Basic Science of Korea [기초과학연구원](KR). But in 1.7 years of data, they see no sign of WIMPs, the researchers report today in Science Advances.

There’s a caveat. The new COSINE analysis does not look for the annual cycle in the event rate, but simply for an excess number of low-energy events. That tiny signal would appear on top of a much larger background of events caused by ordinary particle radiation from sources both inside and outside the crystals. So the analysis depends on researchers’ ability to model the subtle details of those backgrounds, Bernabei notes.

COSINE is not the only experiment testing the DAMA result, however. The ANAIS-112 detector contains nine sodium iodide crystals with a total mass of 112 kilograms and has been taking data in Spain’s Canfranc Underground Laboratory since 2017.

Three years of data show no annual cycle in low-energy events, ANAIS researchers reported on 27 May in Physical Review D. However, the uncertainties were slightly too high to rule out the DAMA signal, says María Luisa Sarsa, a physicist at The University of Zaragoza | Universidad de Zaragoza](ES) and ANAIS co-leader.

If the DAMA signal isn’t real, “The field of particle physics owes itself to find out what DAMA is seeing,” says Reina Maruyama, a nuclear particle physicist at Yale University (US) and co-spokesperson for COSINE. And last year, Dario Buttazzo, a theorist at the Pisa section of The National Institute for Nuclear Physics[Institutio Nzaionale di Fisica Nucleare](IT), and colleagues identified a way in which the DAMA team might have inadvertently created the annual cycle.

DAMA researchers collect data in yearlong runs. To make the variation in each run stand out, they subtract the average rate over the year from the event rate measured each day. But if the event rate steadily rises or falls year after year, the subtraction can turn a steady trend into an oscillation, Buttazzo says. For example, if the rate were increasing by 1% every year, DAMA’s method would produce a signal that begins each run at –0.5% and ends each run at +0.5%.

“If you do the analysis like DAMA was doing it, and if the background has a particular feature, then you could have such an effect,” says Buttazzo, whose group reported its analysis in the Journal of High Energy Physics on 21 April 2020. But he cautions that because DAMA hasn’t published its data, “we cannot know if this effect is actually there or not.”

Still, the COSINE and ANAIS papers suggest the issue could be important. The ANAIS detector’s total event rate is decreasing steadily as short-lived radioactive nuclei in the crystals decay away, says María Lucía Martínez Pérez, a physicist at the University of Zaragoza and co-leader of the team. COSINE sees a similar steady decline, Lee says.

Closure could come soon. In weeks, COSINE will release a 3-year annual cycle analysis, Lee says. ANAIS’s Sarsa says that by next summer, “We expect to have a high-impact publication with the average rate measured over the year with all 5 years of data.” That may be enough data, she says, to bring the curtain down on the DAMA claim.

See the full article here .


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