Frozen-planet states in exotic helium atoms

An atom of ordinary helium consists of a nucleus with two protons and two neutrons surrounded by two electrons. Experiments at CERN have involved colliding slow antiprotons with these helium atoms to form an exotic form of helium called antiprotonic helium, in which one of the electrons is replaced with an antiproton (a particle like a proton but with the negative charge of an electron). Thus, an atom of antiprotonic helium is uncharged, like ordinary helium, but includes one negatively-charged particle over 1800 times heavier than an electron.

Antiprotonic helium atoms can only survive in configurations in which the antiproton cannot 'fall' into the nucleus and annihilate. Until now, the only widely studied configuration involves antiproton making circular orbits around the nucleus, shielded by the remaining electron. Grozdanov and Solov'ev describe a different configuration, named a 'frozen planet' state, in which the electron rapidly circulates round the nucleus, generating a potential well that traps the antiproton. The period of time in which the antiproton can remain trapped in this well depends on its energy and the distance from the nucleus. The researchers plan to extend their studies to include similar configurations that rotate, which they suggest may be more amenable to experimental research.