Neutrinos

Neutrinos were first predicted to exist as the result of a puzzle related to the decay of neutrons. While neutrons are stable inside atomic nuclei, free neutrons are observed to decay, in an average time of about 10 minutes, into protons and electrons. The electric charge works out fine, because a neutron is electrically neutral, while a proton has a positive charge and an electron an equal and opposite negative charge. The mass of a proton plus an electron is almost as much as the mass of a ...

 How to Disguise a Helium Atom as a Hydrogen Atom

Donald Fleming of the University of British Columbia in Vancouver, Canada, and colleagues managed to disguise a helium atom as a hydrogen atom byreplacing one of its orbiting electrons with a muon, which is far heavier than an electron. Because it is so heavy, the muon sits 200 times closer to the helium nucleus than the electron it replaces and cancels out one of the nucleus's positive charges. The remaining electron then behaves as if it were orbiting a nucleus with just one positive ch...