Recent Experiments on Exotic Atoms

Exotic atoms are unique traps for elementary particles like muons, pions and antiprotons and allow physicists to test fundamental laws like the CPT theorem, quantum electrodynamics, the theory of strong interactions, and the properties of elementary particles. The trapped particle may also serve as a probe for the strong and electromagnetic forces exerted by the atomic nucleus, according to F. Joachim Hartmann of the Technical University of Munich in Germany, who spoke at the APS/AAPT Spring Meeting in Columbus, Ohio.

In an exotic atom an elementary particle of negative charge and sufficiently long lifetime orbits besides the electrons around the nucleus. The most common particles to form such atoms are the muon, the pion, and the antiproton. As the masses of the elementary particles forming exotic atoms are large compared to that of the electron, the dimensions of their orbits in these atoms are much smaller and the energies required to eject them from the atom are much larger than for electrons in ordinary atoms. This makes exotic atoms suitable for studying the properties of exotic particles such as their mass, charge, spin, magnetic moment and the strength of their interaction with nuclear matter, Hartmann reported.

Exotic atoms are formed when particles are shot into and slowed down in matter. When they reach energies comparable to the binding energies of the outer atomic electrons, they are captured by an atom to form highly excited exotic-atom states. Similar to ordinary atoms the system is de-excited by emission of exotic (muonic, pionic, antiprotonic) x rays with energies up to the gamma-energy region, or by the ejection of an electron from the host atom (the Auger effect). The whole de-excitation process, called a cascade, is finished within pico- to nanoseconds. Because it achieves this lowly excited state so rapidly, the exotic particle dumps an energy of up to 1 GeV into the nucleus, several times the energy released during a nuclear fission reaction. And, said Hartmann, "the energy of the emitted gamma radiation allows one to draw conclusions on important properties of the particle."

Antiprotonic helium is a very special exotic atom. In 1991 it was demonstrated that about 3% of all antiprotonic He atoms exist in long-lived, metastable states with a microsecond lifetime, orders of magnitude longer than in the ordinary exotic atom. Irradiation of this atom with laser light of the appropriate wavelength brings the exotic atom to a short-lived level. The usual fast cascade follows and leads to states from which annihilation occurs. Because the wave length and hence the energy of the laser light may be determined with a precision of parts per million, this resonant de-excitation provides a means of determining the energy levels in the simple three-body system with unprecedented accuracy.

In another recent experiment, heavy antiprotonic atoms were used to probe the difference between neutron and proton densities at the nuclear periphery. Antiprotons are shot into samples which contain only one isotope of an element. After exotic-atom formation and cascade, they annihilate with one nucleon from the nucleus. Because this annihilation occurs at some distance from the nucleus, all of the annihilation products (mostly pions) might miss the nucleus, producing a nucleus which has either one neutron (annihilation of the antiproton on a neutron) or one proton (annihilation on a proton) less than the original nucleus. If the nuclei thus generated are radioactive, they may be detected with high sensitivity via their decay products. If there are more neutrons at the place where annihilation occurs than one would expect, this would result in the enhanced generation of nuclei with one neutron less. According to Hartmann, such an effect has been observed for a number of nuclei and used as a rigid test for existing nuclear-structure theories.