Searching for Exotic Scalars at Fusion Reactors

original article: https://doi.org/10.1007/JHEP10(2025)215

Introduction

Physicists are actively searching for hidden, lightweight subatomic particles known as "dark sector" particles or exotic scalars. These theoretical particles interact so weakly with normal matter that they are practically invisible, making them incredibly difficult to detect. Discovering them could help explain major mysteries of the universe, such as the nature of dark matter. Historically, scientists have looked for these particles using emissions from the sun or traditional nuclear power plants that split atoms. However, a research study by Baruch et al. proposes a completely new approach: using modern nuclear fusion reactors to create and capture them.

The main motivation behind this research is that nuclear fusion reactors—which generate clean energy by smashing together heavy forms of hydrogen atoms—produce an exceptionally intense storm of subatomic particles called neutrons. Compared to traditional nuclear plants, fusion reactors produce neutrons that carry significantly higher energy and are released in quantities roughly 100 times greater. The researchers argue that this combination of extreme energy and high volume creates the perfect environment to generate these elusive, invisible particles.

Inside a fusion reactor, these high-energy neutrons fly outward and crash into the inner walls of the facility. These walls are lined with specialized protective shielding layers containing lithium, which are designed to absorb the neutrons and help regenerate reactor fuel. The study suggests that when neutrons slam into the atoms within these wall linings, the violent collision can occasionally create and release the mysterious dark sector particles instead of ordinary radiation. Because these exotic particles can easily pass right through solid concrete and steel shielding, they would escape the reactor entirely, creating a stream of new physics just outside the facility where scientists can attempt to measure them.

Method/Procedure Simplified

Rather than building a physical experiment, Baruch et al. used mathematical modeling to estimate how these elusive particles could be generated and captured using the designs of current and future fusion reactors. Their proposed method is divided into two main phases: production and detection. 

For the production phase, the team calculated the creation of these particles through two primary interactions occurring within the reactor's protective walls: 

1. Neutron Capture: When the materials in the reactor walls (such as lithium or iron) absorb a high-energy neutron, they become highly unstable. Normally, they would release ordinary radiation to stabilize, but the researchers calculate they could occasionally emit a dark sector particle instead. 

2. Neutron Scattering: As neutrons continuously bounce and scatter off the atoms in the wall shielding, the sudden shift in their movement could cause them to radiate one of these dark sector particles. 

   
   

For the detection phase, the researchers theorised placing a massive detector just ten meters outside the fusion facility. They modeled this detector after a famous real-world setup known as the Sudbury Neutrino Observatory. It consists of a giant spherical tank filled with 1,000 tons of "heavy water," a type of water that contains a slightly heavier, loosely bound form of hydrogen. 

The researchers calculated that when an escaping dark sector particle flies into this tank and collides with a heavy hydrogen atom, it will transfer its energy and split the atom apart. This splitting process leaves behind a free neutron. While the dark particle itself remains completely invisible, the newly freed neutron can be caught and measured by standard scientific equipment, serving as proof that the invisible particle passed through the tank. 

Key Results Simplified

The mathematical modeling and simulations conducted by Baruch et al. demonstrated that the proposed fusion reactor setup is highly effective for discovering previously undetectable subatomic particles. The study calculated the potential performance of a 1,000-tonne water-based detector positioned near a commercial-scale fusion power plant over several years of continuous operation.

The primary finding is that this experimental setup can probe entirely unexplored regions of particle physics. Specifically, the high volume and extreme energy of the particles emerging from the fusion reactor allow the detector to look for hidden particles that interact with normal matter at much weaker levels than any current technology can track. The calculations show that if these exotic particles exist within a specific lightweight mass range, the detector will record a significant number of atom-splitting events, providing clear statistical proof of their existence.

Additionally, the researchers analyzed the potential interference from natural space radiation, primarily from the sun, which can mimic the same signal in the detector. The results indicate that the sheer volume of particles produced by the fusion reactor is large enough to overwhelm this natural background noise. Over a standard experimental run of a few years, the expected signal from the reactor-generated exotic particles would be distinct and easily distinguishable from any background interference, confirming that modern clean-energy facilities can double as highly sensitive physics laboratories.

What’s New?

Prior to this study, laboratory-based searches for hidden dark sector particles relied almost entirely on traditional nuclear power plants that split heavy atoms. However, those traditional setups were strictly limited by the lower energy levels of their particle emissions, meaning they could only search for hypothetical particles that interact relatively strongly with ordinary matter. While scientists have also gathered data by observing the sun and distant stars, those space-based observations are subject to complex cosmic uncertainties that are impossible to control in a laboratory setting.

The research by Baruch et al. provides entirely new insights by mapping out how modern nuclear fusion plants can access previously unreachable scientific territory. By utilizing the much higher energy and vastly greater volume of particle emissions unique to fusion, this study establishes a brand-new search parameter for physicists. The calculations prove that a fusion-based experiment can detect hidden particles that interact with normal matter at levels significantly weaker than what traditional atomic plants could ever probe.

Ultimately, this work changes our understanding of energy infrastructure, proving that future commercial fusion power plants can simultaneously function as the world's most sensitive laboratories for exploring fundamental physics. It fills a critical gap in the field by providing a concrete conceptual blueprint to hunt for the universe's most elusive building blocks under controlled conditions on Earth.

Limitations & Issues

While these findings present a breakthrough, the researchers acknowledge several limitations. 

First, their estimates rely on abstracted mathematical models rather than full three-dimensional simulations. Precise nuclear measurements for these hypothetical particles do not yet exist, requiring rough baseline calculations.

Second, commercial fusion designs remain active in development. Current experimental facilities lack the complete protective wall linings and physical space needed to house the massive detector. Consequently, this experiment must wait for next-generation, fully operational fusion plants.

Finally, the detector faces unavoidable background interference from solar neutrinos. While this solar noise can be mapped and subtracted, it remains a persistent hurdle for future experimentalists.

Why does it Matter?

This research establishes a groundbreaking link between two historically separate fields: clean energy development and fundamental particle physics. As the world invests heavily in commercializing nuclear fusion for sustainable power, this study proves that these future energy plants can simultaneously serve as unprecedented laboratories for exploring physics beyond the Standard Model. 

By highlighting the untapped scientific potential of fusion reactors, the researchers offer a blueprint that could influence how future facilities are constructed. Instead of building isolated, costly physics experiments from scratch, scientists and engineers can collaborate to integrate massive particle detectors directly into the designs of next-generation fusion plants. Ultimately, this approach opens an entirely new, complementary frontier in the search for the universe's most elusive particles, ensuring that the energy infrastructure of tomorrow expands our fundamental understanding of the cosmos.

Glossary – Important Terms & Their Definitions

Thermonuclear Fusion: A high-heat reaction fusing heavy hydrogen atoms into helium, releasing immense kinetic energy. 

Dark Sector Particles (Exotic Scalars / ALPs): Lightweight, hypothetical subatomic particles that interact so weakly they pass entirely undetected through solid matter. 

Neutron Flux: A massive, continuous stream of high-energy subatomic particles released during a nuclear reaction. 

Breeding Blanket: Specialised lithium-lined reactor walls that absorb neutrons to continuously generate new tritium fuel. 

Deuterium Dissociation: A process where an incoming particle splits a heavy hydrogen atom into a proton and neutron. 

Solar Neutrinos: Naturally occurring particles emitted by the sun that cause background interference in sensitive earthbound detectors. 

Bibliography

Baruch, C., Fitzpatrick, P.J., Menzo, T. et al. "Searching for exotic scalars at fusion reactors." Journal of High Energy Physics 10, 215 (2025). https://doi.org/10.1007/JHEP10(2025)215

University of Cincinnati. "Fusion reactors may create dark matter particles." ScienceDaily. ScienceDaily, 28 December 2025. <www.sciencedaily.com/releases/2025/12/251228020014.htm>.

Shaunak Wasker | Writer | The STEM Review