Quasiparticle
Formally, quasiparticles and collective excitations are closely related phenomena that arise when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in vacuum.
The electron behaves as though it has a different effective mass travelling unperturbed in vacuum.
[1] In another example, the aggregate motion of electrons in the valence band of a semiconductor or a hole band in a metal[2] behave as though the material instead contained positively charged quasiparticles called electron holes.
The quasiparticle concept is important in condensed matter physics because it can simplify the many-body problem in quantum mechanics.
Therefore, while it is quite possible to have a single particle (electron, proton, or neutron) floating in space, a quasiparticle can only exist inside interacting many-particle systems such as solids.
It is these strong interactions that make it very difficult to predict and understand the behavior of solids (see many-body problem).
On the other hand, the motion of a non-interacting classical particle is relatively simple; it would move in a straight line at constant velocity.
The principal motivation for quasiparticles is that it is almost impossible to directly describe every particle in a macroscopic system.
Therefore, as a starting point, they are treated as free, independent entities, and then corrections are included via interactions between the elementary excitations, such as "phonon-phonon scattering".
On the other hand, a collective excitation is usually imagined to be a reflection of the aggregate behavior of the system, with no single real particle at its "core".
As this example shows, the intuitive distinction between a quasiparticle and a collective excitation is not particularly important or fundamental.
For these systems a strong similarity exists between the notion of quasiparticle and dressed particles in quantum field theory.
When a kinetic equation of the mean-field type is a valid first-order description of a system, second-order corrections determine the entropy production, and generally take the form of a Boltzmann-type collision term, in which figure only "far collisions" between virtual particles.
