# Quasiparticles: Excitations and Interactions

In physics, quasiparticles are theoretical concepts used to describe the behavior of quantum systems, especially in solid state physics and quantum field theory. These concepts represent excitations and interactions in a system that can be treated mathematically and conceptually like particles, but do not correspond to the usual properties of real particles.

Even if physical properties (e.g. momentum, energy, spin or mass) can be assigned to quasiparticles, they are still purely mathematical constructs that are used to describe certain phenomena. Quasiparticles can only be introduced in many-particle systems because they represent a combination of real particles.

For example, to explain the fractional quantum Hall effect (to some extent), quasiparticles called *composite fermions* are introduced. A composite fermion is made up of an electron and an even number of magnetic flux quanta (smallest possible magnetic flux).

There are a variety of other quasiparticles such as Cooper pairs, Majorana fermions, phonons, anyons, skyrmions, defect electrons (holes) and so on.