Exotic atom

Other muonic atoms can be formed when negative muons interact with ordinary matter.

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 two protons and two neutrons.

[14] Positronium has been studied since the 1950s to understand bound states in quantum field theory.

A recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.

Pionium, a bound state of two oppositely charged pions, is useful for exploring the strong interaction.

Kaonium, which is a bound state of two oppositely charged kaons, has not been observed experimentally yet.

(Top quarks are so heavy that they decay through the weak force before they can form bound states.)

[16] Atoms may be composed of electrons orbiting a hypernucleus that includes strange particles called hyperons.