Cloaking effect in atoms baffles scientists - physics-math - 04 November 2010 - New Scientist

Cloaking effect in atoms baffles scientists - physics-math - 04 November 2010 - New Scientist: "Atoms called positronium inexplicably scatter off gas particles as if they were lone electrons, even though they contain an anti-electron as well. The finding hints that engineers could use the well-known scattering properties of electrons as a rule of thumb in designing future medical scanners that employ positronium. It could also help interpret puzzling astronomical observations.

'Knowledge of how positronium interacts with the surrounding medium is important, whether this is human tissue or interstellar gas,' says experiment leader Gaetana Laricchia of University College London.

Positronium atoms are like hydrogen atoms, except the proton is replaced by a positron, the positively charged antiparticle of the electron. The atoms are unstable and their two constituent particles disappear in a puff of gamma rays within a microsecond.

When a beam of electrons or positrons flies through a gas, they scatter off the gas particles at predictable rates. Scientists guessed that positronium atoms, being twice as heavy as an electron and electrically neutral, would have very different scattering rates."