Exotic atom

An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and no exotic atom observed so far can persist under normal conditions. In a muonic atom (previously called a mu-mesic atom, now known to be a misnomer as muons are not mesons), an electron is replaced by a muon, which, like the electron, is a lepton. Since leptons are only sensitive to weak, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction. Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics. Muon-catalyzed fusion is a technical application of muonic atoms. Muonic hydrogen is like normal hydrogen with the electron replaced by a negative muon - that is a proton orbited by a muon. It is important in addressing the proton radius puzzle. The symbol 4.1H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (4He-μ), which is like helium-4 in having 2 protons and 2 neutrons. However one of its electrons is replaced by a muon, which also has charge –1. Because the muon's orbital radius is less than 1/200th the electron's orbital radius (due to the mass ratio), the muon can be considered as a part of the nucleus. The atom then has a nucleus with 2 protons, 2 neutrons and 1 muon, with total nuclear charge +1 (from 2 protons and 1 muon) and only one electron outside, so that it is effectively an isotope of hydrogen instead of an isotope of helium.