Ultracold neutrons don’t disappear into the mirror world
For decades, a theory in physics has postulated the existence of a mirror world whose interaction with our own reality is extremely feeble. The particles in this mirror universe are also thought to be ...
For decades, a theory in physics has postulated the existence of a mirror world whose interaction with our own reality is extremely feeble. The particles in this mirror universe are also thought to be candidates for dark matter. Researchers at the Paul Scherrer Institute PSI have now examined some 25 billion neutrons—thereby ruling out, with a very high degree of certainty, their disappearance into the mirror world. The research is published in the journal Physical Review Letters.
The idea may sound like science fiction, but it has been put forward by 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 particle 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 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 version.
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 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 them 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, magnetic field coils surrounding the container that could be controlled with the utmost precision. This is because the probability of neutrons crossing into the mirror world and back into our world is extremely sensitive to the surrounding magnetic fields.
No indication of a mirror world—despite world-leading 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 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."
Publication details
N. J. Ayres et al, New High-Sensitivity Search for Neutron to Mirror-Neutron Oscillations at the PSI Ultracold Neutron Source, Physical Review Letters (2026). DOI: 10.1103/2qck-n6mb
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Citation: Ultracold neutrons don't disappear into the mirror world (2026, July 28) retrieved 28 July 2026 from https://phys.org/news/2026-07-ultracold-neutrons-dont-mirror-world.html
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