Light Dark Matter

When we gaze up at the night sky, we are met with countless bright stars. The universe is filled with innumerable celestial bodies that spark our curiosity, and the study and observation of these objects—astronomy—is one of humanity’s oldest fields of science. But starting around a century ago, astronomers began to discover phenomena that could not be explained by the visible celestial bodies alone. To account for these mysterious phenomena, it became clear that much more mass was needed than what we could see. This led to the introduction of a hypothesis involving unseen, unknown matter—what we now call dark matter.

Although dark matter has not yet been directly detected, a wealth of astronomical observations supports its existence. As a result, numerous models have been proposed to explain what dark matter might be. One of the most widely supported candidates is the WIMP (Weakly Interacting Massive Particle), which has been the target of direct detection experiments for decades. Laboratories have been moving deeper underground and building increasingly large detectors to minimize background noise and capture rare signals. These efforts have made it possible to probe even the tiniest interactions with dark matter, yet the true nature of dark matter remains hidden.

Recently, growing attention has been paid to the search for lighter forms of dark matter by detecting extremely faint signals. Advances in detection and data analysis technology have opened up this new direction in research. However, because the motion of dark matter in the universe is non-relativistic, there is a natural limit to how low a mass can be effectively searched for in such experiments.

NEON? Why Search for Dark Matter at a Nuclear Power Plant?

The NEON (NEutrino Optics Neutrino) experiment is being conducted at the tendon gallery of Unit 6 of the Hanbit Nuclear Power Plant, located in Yeonggwang, Jeollanam-do (Figure 1). Aimed at studying neutrino properties and searching for dark matter, the NEON experiment began collecting data in April 2022 to explore the existence of light-dark matter. The first results were published this January in the prestigious international journal Physical Review Letters.

Dark matter searches typically take place deep underground to minimize background noise and enable the detection of extremely rare signals. Because the NEON detector is located at a nuclear power plant, and not deep underground, it might seem at first glance to be at a disadvantage. However, if the reactor itself can serve as a source of dark matter, then this location may in fact offer a highly favorable environment for such experiments.

[Figure 1] Schematic of the NEON Detector
(Left): This shows Unit 6 of the nuclear power plant and the on-site tendon gallery. The NEON detector is installed in the tendon gallery, located 23.7 meters from the reactor.
(Right): This shows the structure of the detector. The core detectors are made of sodium iodide (NaI), with six units placed at the center. To block external background radiation, the setup includes a multi-layer shielding structure composed of liquid scintillator, lead, and polyethylene.

Inside a nuclear reactor, an enormous number of fission reactions occur, producing a vast amount of gamma rays. These gamma rays can transform into dark matter via a mediator known as the dark photon. A key point here is that the gamma rays generated through fission possess energies on the order of several mega-electronvolts (MeV). As a result, dark matter with masses of several MeV could be produced — or alternatively, lighter dark matter particles may be generated, carrying energies in the MeV range.

This means that dark matter produced in a reactor can be used to search for extremely low-mass dark matter without the limitations imposed by non-relativistic motion. In contrast, experiments that search for cosmologically distributed dark matter must overcome significant challenges to detect masses below the giga-electronvolt (GeV) scale. By using a reactor, however, it becomes possible to search for dark matter with masses ranging from kilo-electronvolts (keV) to mega-electronvolts (MeV).

Achieving World-Class Sensitivity

Based on this concept, the NEON experiment achieved world-leading sensitivity using a nuclear reactor. As shown in [Figure 2], NEON not only demonstrated exceptional sensitivity but also extended its search range down to very low dark matter masses at the kiloelectronvolt (keV) scale. For comparison, the figure includes data from TEXONO, a reactor-based experiment conducted in Taiwan, and from DAMIC and SENSEI, which are dark matter search experiments being carried out in underground laboratories in France and Canada, respectively.

[Figure 2] NEON Experiment's Dark Matter Search Results
The NEON research team analyzed data collected from the nuclear reactor and presented a new exclusion limit curve for dark matter with very low masses ranging from 1 kiloelectronvolt (keV) to 1 megaelectronvolt (MeV). Compared to previous experimental results, NEON demonstrated world-leading sensitivity across most of the mass range. Notably, at a mass of 100 keV, the experiment achieved a search sensitivity approximately 1,000 times greater than earlier studies, setting a new benchmark for experimental sensitivity. Additionally, the results include the first-ever search outcomes for dark matter masses below 100 keV, which had not been explored in prior research.

The NEON experiment team developed their own proprietary sodium iodide (NaI) detector technology to construct the detectors and significantly reduced background noise by using a liquid scintillator—originally serving as shielding material—as a detector to tag external radiation. In particular, they introduced a novel algorithm to their data analysis that effectively distinguishes between scintillation signals containing potential dark matter signatures and background noise, greatly enhancing signal interpretation capabilities. As a result, the experiment achieved approximately 1,000 times higher sensitivity for detecting dark matter with a mass of 100 kiloelectronvolts (keV) compared to the earlier TEXONO reactor-based experiment. It also expanded the search into previously unexplored regions, setting a new benchmark in dark matter research.

Looking ahead, the team plans to more than double the amount of data collected and apply more refined analysis techniques. They also aim to include detection channels that account for interactions with electrons, thereby further improving the results and continuing the search for light-dark matter.