Unparticle Physics - Background

Background

All particles exist in states that may be characterized by a certain energy, momentum and mass. In most of the Standard Model of particle physics, particles of the same type cannot exist in another state with all these properties scaled up or down by a common factor – electrons, for example, always have the same mass regardless of their energy or momentum. But this is not always the case: massless particles, such as photons, can exist with their properties scaled equally. This immunity to scaling is called "scale invariance".

The idea of unparticles comes from conjecturing that there may be "stuff" that does not necessarily have zero mass but is still scale-invariant, with the same physics regardless of a change of length (or equivalently energy). This stuff is unlike particles, and described as unparticle.

Such unparticle stuff has not been observed, which suggests that if it exists, it must couple with normal matter weakly at observable energies. Since the Large Hadron Collider (LHC) announced it will begin probing a higher energy frontier in 2009, some theoretical physicists have begun to consider the properties of unparticle stuff and how it may appear in LHC experiments. One of the great hopes for the LHC is that it might come up with some discoveries that will help us update or replace our best description of the particles that make up matter and the forces that glue them together.