Scientists have been asking themselves the same question for 100 years: how long does a free neutron live before it decays? It seems that the answer to this mystery has finally arrived, but what seemed like a simple answer may have exposed a deep flaw in the way we understand the fundamental particles of the universe. And, well, this discovery could change all the laws of the universe, or at least, how we thought of them until now.
How did this mystery begin?
It all started in 1932, shortly after scientists discovered the neutron itself. Since then, they have been trying, at all costs, to measure how long a neutron, when not bound to an atomic nucleus, takes to decay. They called this measurement the neutron lifetime, and it is essential not only for nuclear physics but also for understanding how the first chemical elements were formed in the first minutes after the Big Bang, that is, for understanding our universe as a whole. From this, two experimental approaches were established as standard: the beam and bottle methods.
- Beam: Neutron beams are directed, and the protons resulting from the decay are counted.
- Bottle: Ultracold neutrons are stored in containers, and those that survive after a certain time are counted.
The big problem is that these two techniques provide different results, which has triggered one of the biggest puzzles in modern physics: the enigma of the neutron’s lifetime. In the beam method, the average measured is 888 seconds, while in the bottle method, the number drops to 878 seconds. Even though a difference of 10 seconds may seem like a small thing, in the context of particle physics, it is huge. 100 years later, physicist Eugene Oks came up with a new explanation.
What does hydrogen have to do with dark matter?
For Oks, in a small percentage of cases, the neutron does not disintegrate into three particles (proton, electron, and antineutrino), as we thought until now, but rather into just two: a neutrino and an exotic type of hydrogen atom (different from the green hydrogen found in America). He says this atom would be invisible, since it does not interact with light, and therefore completely escapes conventional detectors.
The physicist has even given this particle a name: “second flavor” of hydrogen. In this case, the electron would be very close to the proton, so close that the atom would not have an electric dipole moment, which would make it incapable of emitting or absorbing light. In less technical terms, this would be a dark atom, undetectable by instruments based on electromagnetic radiation.
This hypothesis by Oks ends up explaining why the beam method records a longer lifetime: it fails to account for these “invisible disintegrations”. Therefore, when recalculating the frequency of this type of decay using a modified solution of the Dirac equation, Oks concluded that it can occur in up to 1% of cases: enough to resolve the 10-second difference.
What impact does this discovery have on the laws of the universe?
Even though invisible hydrogen atoms don’t interact with light, they still exert gravitational force, fitting the profile of the mysterious dark matter perfectly. What Oks suggests is that this second flavor of hydrogen may be the main form of baryonic dark matter: the kind made up of familiar particles, such as protons and electrons, but arranged in unconventional ways.
And what does this mean for the current laws of the universe? Well, this whole explanation could solve one of the most enduring enigmas of modern physics and, in the process, offer an elegant explanation for dark matter, without having to resort to hypothetical particles. Perhaps we can finally understand the 85% of the universe that we’ve never seen, since it’s made of strange and indecipherable dark matter.
To learn more about this discovery, you can check the full study here: Oks, Eugene. (2025). Resolution of the neutron lifetime puzzle and the conceptual design of its experimental confirmation. Nuclear Physics, B(1014), 116879.
