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In physics, a bound state is a composite of two or more building blocks (particles or bodies) that behaves as a single object. In quantum mechanics (where the number of particles is conserved), a bound state is a state in the Hilbert space that corresponds to two or more particles whose interaction energy is negative, and therefore these particles cannot be separated unless energy is spent. The energy spectrum of a bound state is discrete, unlike the continuous spectrum of isolated particles. (Actually, it is possible to have unstable bound states with a positive interaction energy provided that there is an "energy barrier" that has to be tunnelled through in order to decay. This is true for some radioactive nuclei.)

In general, a stable bound state is said to exist in a given potential of some dimension if stationary wavefunctions exist (normalized in the range of the potential). The energies of these wavefunctions are negative.

In relativistic quantum field theory, a stable bound state of n particles with masses m₁, ..., m_n shows up as a pole in the S-matrix with a center of mass energy which is less than m₁+...+m_n. An unstable bound state (see resonance) shows up as a pole with a complex center of mass energy.

• A proton and an electron can move separately; the total center-of-mass energy is positive, and such a pair of particles can be described as an ionized atom. Once the electron starts to "orbit" the proton, the energy becomes negative, and bound states - namely the hydrogen atom - is formed. Only the lowest energy bound state, the ground state is stable. The other excited states are unstable and will decay into bound states with less energy by emitting a photon.
• A nucleus is a bound state of protons and neutrons (nucleons).
• A positronium "atom" is an unstable bound state of an electron and a positron. It decays into photons.
• The proton itself is a bound state of three quarks (two up and one down; one red, one green and one blue). However, unlike the case of the hydrogen atom, the individual quarks can never be isolated. See confinement.

• Composite field
• Resonance

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