Exotic Atom

For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). [2][6] Hadronic atoms, such as pionic hydrogen and kaonic hydrogen, thus provide experimental probes of the theory of strong interactions, quantum chromodynamics. The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain mesons, the quarkonium states, which are made of a heavy quark such as the charm or bottom quark and its antiquark.

Source: en.wikipedia.org