A hydrogenlike system that consists of a strongly interacting particle (hadron) bound in the Coulomb field and in orbit around any ordinary nucleus. The kinds of hadronic atoms that have been made and the years in which they were first identified include pionic (1952), kaonic (1966), &Sgr;- hyperonic (1968), and antiprotonic (1970). They were made by stopping beams of negatively charged hadrons in suitable targets of various elements, for example, potassium, zinc, or lead. The lifetime of these atoms is of the order of 10-12 s, but this is long enough to identify them and study their characteristics by means of their x-ray spectra. They are available for study only in the beams of particle accelerators. Pionic atoms can be made by synchrocyclotrons and linear accelerators in the 500-MeV range. The others can be generated only at accelerators where the energies are greater than about 6 GeV. See Elementary particle, Hadron, Particle accelerator
The hadronic atoms are smaller in size than their electronic counterparts by the ratio of electron to hadron mass. For example, in pionic calcium, atomic number Z = 20, the Bohr radius of the ground state is about 10 fermis (1 fermi = 10-15 m), and in ordinary calcium it is about 2500 fermis. Thus the atomic electrons are practically not involved in the hadronic atoms, and the equations of the hydrogen atom are applicable. The close approach of the hadrons to their host nuclei suggests that hadron-nucleon and hadron-nucleus forces will be in evidence, and this is one of the motivations for studying these relatively new types of atoms.
Antiproton atoms are the latest in the series of hadronic atoms to be observed. The main research effort involving antiproton atoms has been dedicated to the investigation of the x-ray spectra of the antiprotonic hydrogen. The transitions to the ground state depend directly on the elementary antiproton-proton interaction at the threshold. If this interaction turns out to be simple enough, the antiprotonic atoms will be a future tool for measuring the matter distribution of the nuclear surface. Another source of low-energy antiprotons—the Low Energy Antiproton Ring (LEAR), which makes precision measurements on antiprotonic atoms feasible—was put into operation at CERN near Geneva, Switzerland.
There are two additional hadrons with lifetimes that are long enough to be candidates for hadronic atom formation: the negative xi (&Xgr;-) and the negative omega (&OHgr;-), but even at the largest accelerators, these particles are too scarce for their atoms to be detected.