The neutron is a particle that usually sits in the atomic nucleus and doesn't disappear. But some theories suggest that occasionally it might transform into its invisible twin — a particle that ordinary detectors can't catch. If this happened, it could explain what dark matter is made of — the mysterious substance that emits no light yet holds galaxies together.
To test this idea, physicists at Oak Ridge National Laboratory sent an intense beam of cold neutrons through a strong magnet. Inside the magnet was a thin cadmium plate that absorbed ordinary neutrons but would let invisible twins pass. A detector 20 meters away looked for signs that the twins had turned back into ordinary neutrons. But there was no signal.
The absence of a signal is also a result. Now we know that the probability of a neutron transforming into an invisible twin is ten times lower than previously thought. This helps rule out many theories about the nature of dark matter. The search continues: perhaps the twin hides in an even narrower energy range, and the next experiment might catch it.
The experiment used a magnet with a field of 6.6 tesla — about 130,000 times stronger than Earth's magnetic field.
The idea of invisible twin particles echoes stories about parallel worlds, where particles can cross from one world to another.