Search for anomalously heavy isotopes of helium in the Earth's atmosphere

Our knowledge of the possible existence in nature of stable exotic particles depends solely upon experimental observation. Using a sensitive laser spectroscopy technique, we searched for a doubly charged particle accompanied by two electrons as an anomalously heavy isotope of helium in the Earth's atmosphere. The concentration of noble-gas-like atoms in the atmosphere and the subsequent very large depletion of the light He-3,He-4 isotopes allow stringent upper limits to be set on the abundance: 10(-12)-10(-17) per atom in the solar system over the mass range of 20-10 000 amu.