Quasiparticle

A quasiparticle is an emergent excitation in a many-body system that behaves like a particle, with effective properties such as energy, momentum, and lifetime.

Many-body problem: A problem involving the interacting degrees of freedom of multiple particles. Quasiparticles simplify many interacting degrees of freedom into effective excitations.
Many-body quantum mechanics: Quantum mechanics applied to systems containing many interacting particles. Quasiparticles describe useful effective degrees of freedom within such systems.
Electron: A negatively charged elementary fermion with spin one-half. In a solid, an electron can acquire effective properties and become an electron quasiparticle.
Bare particle: An idealized particle considered without the surrounding medium's interaction-induced dressing. A quasiparticle's effective properties differ from those of its bare constituent.
Green's function: A function that describes how disturbances propagate through a system. Its poles and spectral features reveal quasiparticle energies and lifetimes.
Elementary excitation: A basic disturbance above a system's ground state that can carry energy and momentum. Quasiparticles are particle-like descriptions of excitations above a many-body ground state.
Phonon: A quantum of vibrational energy in a periodic solid or other elastic medium. It is a quasiparticle used to describe quantized lattice vibrations.
Virtual particle: An internal, non-observable element in a perturbative calculation of quantum interactions. Unlike virtual particles, quasiparticles can correspond to observable excitations with measurable lifetimes.
Spectral function: A frequency- and momentum-resolved measure of the states available to a system. Sharp spectral peaks signal well-defined quasiparticle excitations.
Collective mode: A coordinated oscillation involving many degrees of freedom in a physical system. Some quasiparticles arise from collective motion rather than one constituent particle.

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