PSI researchers Bernhard Lauss (left) and Geza Zsigmond examined around 25 billion neutrons at PSI’s ultracold neutron source. They have largely ruled out the hypothesis that neutrons spontaneously turn into mirror neutrons.
Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer
Scientific Frontline: Extended "At a Glance" Summary: Ultracold Neutrons and the Mirror World Hypothesis
The Core Concept: The mirror world hypothesis postulates a hidden universe composed of corresponding mirror particles that interact with ordinary matter almost exclusively through gravity or rare neutral particle oscillations.
Key Distinction/Mechanism: Unlike normal matter, mirror particles are largely undetectable by electromagnetic forces; however, theoretical physics suggests neutral particles, such as neutrons, could oscillate—temporarily vanishing into the mirror world and reappearing—to explain discrepancies in measured neutron lifetimes.
Origin/History: While mirror matter theories have existed for decades as potential dark matter candidates, a high-precision study published on July 28, 2026, by the Paul Scherrer Institute (PSI) ruled out neutron-to-mirror-neutron oscillations with unprecedented certainty.
Major Frameworks/Components:
- Ultracold Neutrons: Neutrons produced by a high-intensity proton accelerator and significantly slowed to allow for extended observation inside a non-magnetic, stainless-steel vacuum container.
- Oscillation Hypothesis: The theoretical mechanism proposing that neutral particles can spontaneously transition back and forth between ordinary and mirror states.
- Dark Matter Candidates: The postulation that mirror matter, interacting primarily via gravitation, could account for the universe's unidentified mass.
- Magnetic Field Manipulation: The precise control and variation of surrounding magnetic fields to scan all theoretical regions where neutron oscillations might be triggered.
Branch of Science: Particle Physics, Theoretical Physics, and Quantum Mechanics.
Future Application: The highly sensitive experimental setup establishes a new global standard for ultracold neutron confinement and magnetic precision, driving advancements in particle detector technologies and guiding future high-intensity accelerator experiments.
Why It Matters: By measuring 25 billion ultracold neutrons and finding no evidence of oscillation, this experiment restricts previous theoretical speculations, forcing physicists to explore new explanations for both dark matter and the neutron lifetime anomaly.
The idea may sound like science fiction, but it has actually been put forward by renowned researchers in theoretical physics. A so-called mirror world might exist alongside our ordinary reality, in which every type of elementary particle has a corresponding mirror particle. There would then be mirror electrons, as well as mirror protons and mirror neutrons. However, according to this theory, normal particles are unlikely to interact much with mirror particles. “Essentially, the two types of particles sense each other’s presence through the force of gravity,” says Geza Zsigmond, a researcher at the PSI Center for Neutron and Muon Sciences.
Despite decades of searching, these mysterious mirror particles have never been detected. Although the results of recent experiments at the Institut Laue-Langevin in France rekindled speculation about mirror neutrons, a new experiment by PSI researchers has now shed further light on the question, achieving an unsurpassed level of precision. The results rule out, with very high probability, any transformation of neutrons into their mirror versions.
The fact that mirror particles scarcely interact with ordinary matter at all is precisely what makes the search for them so difficult. They can only interact with the matter we know in two ways: either through gravitation or through the rare oscillation of neutral particles. This is also why mirror particles may be candidates for what is known as dark matter. Even though dark matter accounts for considerably more of the total mass of the universe than ordinary matter, its nature remains a complete mystery. The weakness of the interaction between mirror particles and our own world is precisely the reason why they could account for dark matter.
Searching for Rare Oscillations
“There is no way of proving the existence of mirror particles using gravitational interaction alone,” says Bernhard Lauss, leader of the research group for ultracold neutron physics at the PSI Center for Neutron and Muon Sciences. “Because of this, we focused on the other property of mirror particles, namely that, based on this hypothesis, neutral particles might oscillate back and forth between our normal matter world and the mirror world.” This means that a neutral particle, such as the neutron, could—on very rare occasions—turn into a mirror particle and simply vanish from our world. After a while, it might then reappear out of nowhere.
“Neutrons are ideal for this experiment because, first, they are electrically neutral and, second, they possess a strange property,” adds Zsigmond. “Neutrons appear to have different lifetimes depending on how this is measured.” Part of this discrepancy could be due to the fact that some neutrons “disappear” into the mirror world while they are being measured.
To get to the bottom of this peculiar behavior, the PSI researchers joined forces with researchers from ETH Zurich and the Jagiellonian University in Kraków and devised a sophisticated experiment. They significantly slowed down neutrons—which are produced in large quantities at the high-intensity proton accelerator at PSI—thereby creating what are known as ultracold neutrons. They then trapped these in a special nonmagnetic stainless-steel vacuum container. This required, first, a very large number of ultracold neutrons—the PSI source is a world leader in producing these—and second, the magnetic field coils surrounding the container had to be controlled with the utmost precision. This is because the probability of neutrons crossing into the mirror world and back into our world again is extremely sensitive to the surrounding magnetic fields.
No Indication of a Mirror World—Despite World-Best Results
Every five minutes, the team stored around 1.5 million neutrons in a large stainless-steel tank. Each time, the researchers emptied it again after about 200 seconds and determined how many neutrons remained. They repeated the process over a period of several months, by which time they had measured around 25 billion neutrons in total.
“Over time, we gradually varied the strength and direction of the magnetic field so as to scan all the relevant regions in which one might expect oscillations between neutrons and mirror neutrons to occur,” says Lauss. “But we saw no evidence at all of such oscillations.” These findings mean that there is a very high probability that previous speculations about their transformation into mirror particles can be ruled out.
The PSI experiment is the best of its kind in the world and will set the standard for the foreseeable future. Further substantial improvements would require an extremely elaborate setup. “We are also proud that many young people were able to participate in this experiment,” notes Lauss. Two doctoral theses were completed in collaboration with ETH Zurich as part of the research, and in addition, many students were involved in the experiment.
Although the study found no evidence of a spectacular mirror world, its findings are nonetheless important for particle physics. “By restricting the scope for certain speculations, we are showing researchers in theoretical physics that they need to explore new avenues,” says Zsigmond. “Physics needs that kind of impetus if it is to continue to evolve.”
Published in journal: Physical Review Letters
Authors: N. J. Ayres, Z. Berezhiani, G. Bison, K. Bodek, V. Bondar, P.-J. Chiu, M. Daum, C. B. Doorenbos, S. Emmenegger, K. Kirch, V. Kletzl, J. Krempel, B. Lauss, D. Pais, I. Rienäcker, D. Ries, D. Rozpędzik, P. Schmidt-Wellenburg, K. S. Tanaka, J. Zejma, N. Ziehl, and G. Zsigmond
Source/Credit: Paul Scherrer Institute | Dirk Eidemüller
Edited by: Scientific Frontline
Reference Number: phy072826_01