A notorious dark matter signal could be due to an analysis error

To capture dark matter, modules containing sodium iodide crystals are inside the COSINE-100 detector in South Korea. The experiment started in 2016.Credit: COSINE-100 Collaboration

Physicists have shown that an underground experiment in South Korea can “see” dark matter flow through the Earth – or not, depending on how its data is sliced. The results cast further doubt on a decades-old claim that another experiment detected the mystery substance.

The latest study, published on the arXiv preprint server on August 101, reports that a certain type of data analysis by the COSINE-100 detector can produce seasonal fluctuations in the results. Physicists had proposed a similar fluctuation as the signature of dark matter, the invisible but ubiquitous substance thought to be five times more abundant in the Universe than ordinary matter.

“If you apply the wrong method, you can see a fluctuation in their data where there was none,” says Nicola Rossi, an experimental particle physicist at the Gran Sasso National Laboratory near L’Aquila, Italy, who first suggested this explanation. in 20202.

Dark matter signal

For more than two decades, the experiment now known as DAMA/LIBRA, which is also housed at the Gran Sasso National Laboratory, has been reporting annual fluctuations in the flashes recorded by the sodium iodide crystals in its detector.3. The number of such events peaks in June and peaks in December, as physicists would expect if particles of dark matter were permeating the Milky Way. This is because, as the Earth orbits the Sun, its velocity in the Galaxy peaks in June, so more particles from the dark matter halo would hit the DAMA/LIBRA detector at that time – just like a highest number of flying insects hitting the windshield of a car. when the car is going faster.

But none of the many other dark matter experiments – based on various other technologies – have so far seen DAMA/LIBRA-compatible signals. So, to put the claim to the ultimate test, in the mid-2010s physicists began building detectors made of the same type of sodium iodide crystal. Preliminary results from two such experiments, COSINE-100 Yangyang Underground Laboratory in South Korea and ANAIS-112 at the Canfranc Underground Laboratory in the Spanish Pyrenees, failed to reproduce the windshield effect.

Even though most of the physics community had dismissed the DAMA/LIBRA results as probably false, the question of why the Italian experiment was seeing periodic ups and downs in its recorded events remained. Over the years, the collaboration has convincingly pushed back against a number of proposed explanations.

Changing the background

In 2020, when reading papers from the DAMA/LIBRA collaboration, Rossi and colleagues noticed that the reported fluctuations were only those that occurred against a supposed “background” of events that the team discarded as experimental artifacts, such as sources of radioactivity in the laboratory or in the detector itself.

But unless the bottom is absolutely constant throughout the year, it could be “a dangerous approach,” wrote Rossi and his co-authors. The windshield effect should add a sinusoidal term to the bottom. But the precise way a background is modeled and subtracted could affect how the data is interpreted and potentially create a false dark matter signal.

To subtract background flashes, the DAMA/LIBRA team averaged them over each year, which could have made the number of remaining events look like a jagged wave. And because real data points are noisy, it could be easy, Rossi and his colleagues wrote in their 2020 report, to mistake the fluctuations for the type of sine wave expected from dark matter. The researchers also used simulated data to show that they could create a false dark matter signal if the number of background flashes increased slowly over time.

Result recreated

The COSINE-100 collaboration has now performed a similar analysis of real-world data collected by its crystals. “We can generate a DAMA-like signal with our analysis, but our timing is in the opposite direction,” says Hyun Su Lee, a physicist at the Institute of Basic Sciences in Daejeon, South Korea, who is co-director of COSINE- 100. In other words, while the South Korea-based experiment’s background noise naturally decreases, its “dark matter” count is highest around early December and lowest around early June.

This leaves a dilemma. Dark matter detectors and other physics experiments often have to deal with impurities in their radioactive materials, but as these age and the radioactive isotopes decay, the background tends to become quieter, no stronger. “Increasing the background is unnatural,” says Lee. Still, Rossi and his team suggest that some effects, such as radioactive impurities slowly seeping into the detector from outside, could, in principle, cause an increasing background effect.

Either way, the fact that the data analysis created a false fluctuation “seems to me like a potentially good – and maybe even likely – explanation for the DAMA/LIBRA claims, says theory specialist Dan Hooper. dark matter at the University of Chicago in Illinois.

Because the DAMA/LIBRA detector was built decades ago and has been kept underground ever since, the radioactive backgrounds are more stable than in recently built experiments, says Rita Bernabei, a physicist at Tor Vergata University in Rome and DAMA/LIBRA spokesperson. “If there was a hypothetical small contribution from a decreasing rate, it might decrease the observed modulation amplitude, but it would never produce a positive signal as we observe,” she adds. The collaboration also chastised Rossi’s group’s arguments in an article.4 who calls their assumptions “untenable” and their conclusions “worthless”.

The physics community has repeatedly asked the DAMA/LIBRA team to reveal more of its data and, in particular, to provide the total number of flashes. Bernabei says it ‘doesn’t add any useful information’. But the added transparency could be to the benefit of the collaboration, says Rossi: If the total number of background noises turns out to be constant over time, it would add credence to the team’s claim that a fluctuation sinusoidal is observed. “If the DAMA collaboration had been more open, this could have been understood a long time ago,” says Hooper.

Meanwhile, attempts to replicate dark matter detections continue. Earlier this month ANAIS-112 completed five years of data collection and is working on an analysis, said spokeswoman Maria Luisa Sarsa of the University of Zaragoza in Spain. The data is sensitive enough to potentially rule out the DAMA/LIBRA outcome with high statistical confidence. Within a few years, this confidence could reach five standard deviations, which is usually the threshold for a physics result to be considered robust.

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