The basic objects in quantum field theory

The standard model is a quantum field theory. These theories are called so after the basic objects in them: Quantum fields.
In quantum field theory, particles are not the basic objects, but are derived features inherent in quantum fields: Particles are the localized excitations of quantum fields with well-define quantum numbers.
In standard quantum field theory, particles are asymptotic objects, existing only before and after collisions. Then they have definite properties, including a real mass -- one says they are on-shell. Decaying excitations can be treated in some approximation as such asymptotic objects if they are long-living enough; these are the unstable particles. Their mass is uncertain by an amount inversely proportional to their lifetime. Indeed, mass and lifetime are combined into a complex mass, which is the characterizing parameter as it is a pole of the S-matrix of the system. If particles do not live long enough to be observed directly, they still manifest themselves as resonances. (Sometimes, one says that unstable particles are off-shell. But this meaning of off-shell should not be confused with the off-shell notion for virtual particles, where mass m is real but momentum p is not restricted to satisfy the mass shell equation p^2=m^2 as for stable particles. Observable unstable particles are not virtual.)

During collisions, there is the quantum field, but there are no discernible particles. Depending on the ways the quantum field is analyzed, one may ascribe parts of the fields to certain particles. In schemes that do so, these particles are thought to be free (although they cannot be, which results in renormalization problems); they are called virtual particles, and correspond to the internal lines of corresponding Feynman diagrams. They are off-shell, and different schemes for analyzing the quantum field during a collision assign different portions of the field to different and differently interacting virtual particles. Nonperturbative schemes such as lattice gauge theory do not permit such an analysis in terms of virtual particles. Thus the presence and meaning of virtual particles is scheme-dependent, and one cannot ascribe any objective reality to them.
In a scattering experiment that does not change the number and type of particles, the in-going particles become virtual (and off-shell) in perturbative schemes until the interaction is completed, when they are recognizable again as a real, on-shell particle.

In a dense medium, collisions are so frequent that the asymptotic regime needed for the preceding interpretations to make sense is never achieved. However, using nonperturbative techniques such as the closed-time path integral, one can define quasiparticles with position-dependent masses (on-shell in Boltzmann-type equations, off-shell in Kadanoff-Baym-type equations) that satisfy a quantum kinetic equation with measurable consequences. These quasiparticles are real and measurable.
Indeed, from a fundamental point of view, most particles (all apart from leptons, quarks and gluons), and in particular the proton and the neutron, must be considered as quasiparticles.

Arnold Neumaier (
A theoretical physics FAQ