Antiproton found going ‘round in strange circles

To the list of particles that have been Coulomb captured by a nucleus, add antiprotons and sigma minus particles. Gerhard Backenstoss and his collaborators from CERN, Karlsruhe and Heidelberg have observed x rays from atomic transitions of antiprotons bound in the Coulomb field of a thallium nucleus. Similarly they observed x rays from atomic transitions of ${\Sigma }^{-}$ particles orbiting sulfur, chlorine and zinc nuclei. Only very high values of angular momentum are observed; at lower levels the orbit is within the nucleus and the ${\Sigma }^{-}$ or antiproton is captured by a nucleon. The level at which transitions disappear is very sensitive to the arrangement of nuclear matter. As the particle works its way down to the nucleus its orbit becomes circular; so interaction with the nuclear surface is favored. The other exotic atoms previously observed contain pions, muons, and most recently, kaons.

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