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 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.