China team captures first direct images of the Migdal effect, opening a new path in dark-matter searches

In a lab outside Beijing, an ultra-sensitive detector recently captured what physicists have been trying to see since before World War II: a tiny atomic nucleus jolting one way and a lone electron streaking off in another, both frozen in the same microscopic frame.

Those six ghostly images, extracted from about a million recorded particle collisions, mark the first direct observation of the Migdal effect, a rare quantum phenomenon predicted 87 years ago. The result, reported Jan. 14 in Nature, could sharpen one of physics’ most ambitious quests — the hunt for dark matter.

The experiment was led by researchers at the University of Chinese Academy of Sciences, working with several Chinese universities. By firing a beam of neutrons through a chamber filled with gas and reading out the faint trails left behind, the team says it has confirmed with high statistical confidence that electrons can be shaken loose when an atomic nucleus suffers a sudden jolt.

Scientists say that confirmation strengthens a key assumption behind a new class of dark matter searches that rely on this subtle effect to turn nearly invisible signals into detectable ones.

A tiny quantum misfire

The Migdal effect is named for Soviet physicist Arkady Migdal, who in 1939 proposed that when a neutral particle collides with an atomic nucleus, the nucleus can recoil so quickly that the electron cloud surrounding it briefly fails to keep up. In that instant of misalignment, one or more electrons can be ionized and ejected.

The probability of that happening in any given collision is tiny — on the order of one in 100,000 under the conditions tested in the new experiment. But when it occurs, the expelled electron can carry a few thousand electron volts of energy, far more visible to modern detectors than the feeble nuclear shove that caused it.

For decades, the effect was folded into theoretical calculations and treated as a small correction. It gained new prominence in the mid-2000s, when physicists began to explore how it might help reveal “light” dark matter, with masses far below those of the heavy particles long favored in earlier searches.

“Dark matter holds the key to understanding the origin and evolution of the universe,” said Yangheng Zheng, a physicist at the University of Chinese Academy of Sciences and one of the corresponding authors of the Nature paper, in an interview with Chinese state media. “Our work brings humanity one step closer in this ‘cosmic treasure hunt.’”

Photographing a quantum jolt

To test Migdal’s old idea, the Chinese-led team built what they liken to an atomic camera.

At its heart is a gas-filled chamber that doubles as a particle target and a tracking medium. The researchers used a mixture of 40% helium and 60% dimethyl ether, chosen because it allows electrons to leave long, easily resolved tracks as they drift through the gas.

Above the gas volume sits a microstructured gas amplification stage and a custom-designed pixelated readout chip known as Topmetal. The chip’s array of roughly 83-micrometer pixels can resolve fine-grained patterns of charge with very low electronic noise, turning each particle interaction into a crisp, two-dimensional image. With timing information, the detector reconstructs the full three-dimensional path.

A compact deuterium-deuterium neutron generator, operating at about 2.5 million electron volts, provided a steady stream of neutrons. When a neutron scatters off a gas nucleus, it can kick that nucleus into motion, leaving behind a short, dense track — a classic nuclear recoil.

What the team was searching for was rarer: a nuclear recoil track emerging from a point where a second, thinner and longer track — an electron — appears to peel off. That twin-track, common-vertex pattern is the hallmark of a Migdal event.

After roughly 150 hours of data-taking, the detector had recorded around 1 million candidate interactions. The researchers then applied a series of selection cuts, requiring the right combination of energies, track lengths, shapes and angles to distinguish true Migdal events from look-alike backgrounds such as random coincidences, cosmic-ray-induced electrons and secondary “delta” rays.

In the end, just six events passed all the criteria. Based on detailed simulations and measurements, the team estimated that ordinary background processes should have produced about 0.23 such events, with a small uncertainty. Using standard statistical methods, they reported a significance above five standard deviations — the threshold particle physicists typically require to claim a discovery.

“The direct observation of the Migdal effect has been a long-standing and widely recognized challenge,” said Yu Haibo, a professor of physics and astronomy at the University of California, Riverside, who was not involved in the work. He noted that several international groups had attempted similar measurements without success. The new result, he said, is “a genuine breakthrough and truly exciting.”

Just as important as seeing the effect was measuring how often it occurs. In the specific energy window probed — nuclear recoils above 35 kiloelectron volts (measured in electron-equivalent energy) and Migdal electrons between 5 and 10 kiloelectron volts — the team found that about 5 in 100,000 nuclear scatters produced a Migdal electron. That rate matches theoretical predictions within the experimental uncertainties.

A new handle on dark matter

The confirmation is more than a test of quantum theory. It feeds directly into the evolving strategy of dark matter detection.

Most of the universe’s matter is thought to be dark — invisible to telescopes but exerting gravitational pull on stars and galaxies. For years, experiments dug deep underground and filled massive tanks with xenon, argon or other materials, hoping that heavy “weakly interacting massive particles,” or WIMPs, would crash into atomic nuclei and produce detectable nuclear recoils.

So far, those efforts have produced no clear sign of such particles, pushing many researchers to consider lighter dark matter candidates, with masses in the million- to billion-electron-volt range. The problem is that lighter particles transfer less energy to nuclei, often below the thresholds of traditional detectors.

The Migdal effect offers a workaround. A light dark matter particle might barely nudge a nucleus, but on rare occasions that nudge can trigger a Migdal electron, whose keV-scale ionization signal is much easier to spot.

“With the Migdal effect, once an electron is ejected, our detector can, in theory, capture 100% of its energy,” Zheng said. The process effectively converts “an otherwise imperceptible low-energy jolt into a measurable electronic signal,” he added.

Several leading dark matter experiments, including the XENON and LUX collaborations in Europe and the United States and the PandaX and CDEX projects in China, have already interpreted some of their low-energy electron data under the assumption that Migdal calculations are reliable. The new measurement gives those interpretations a firmer footing.

“This work fills a long-standing experimental gap, solidifies the theoretical foundation of the Migdal effect, and represents a crucial first step toward applying it in the search for light dark matter,” said Liu Jianglai, a physicist at Shanghai Jiao Tong University and a leading scientist on the PandaX experiment.

China’s growing role and what comes next

The experiment highlights China’s growing presence in frontier physics. The project was funded by the National Natural Science Foundation of China and involved early-career researchers trained through the University of Chinese Academy of Sciences’ integrated graduate programs. The detector technology — from the gas amplification system to the Topmetal chip — was developed largely in China.

For all its significance, the new result is still based on a handful of events in a specific gas mixture and energy range. Researchers say independent confirmation with other detector technologies and target materials will be important, especially for applications to solid-state and liquid noble gas detectors widely used in dark matter searches.

The Chinese team plans to refine its setup, explore different gases and improve the detector’s performance. Dark matter collaborations are expected to incorporate the new measurement into their models and may revisit past data or adjust the design of future experiments to better capture Migdal electrons.

The observation also serves as a rare experimental benchmark for complex atomic calculations, which must be extended to elements like xenon, germanium and silicon to fully exploit the effect.

In the meantime, those six faint, forked tracks stand as a reminder of how much information can be wrung from vanishingly rare events. An idea sketched in wartime Europe has finally been seen in action in a Chinese lab, and the quantum misstep it describes may help bring one of modern science’s darkest mysteries into a slightly clearer light.