Definitions

# Vector boson

A vector boson is a boson with spin equal to one unit of $hbar$ (Planck's constant divided by $2 pi$). In elementary particle physics, the vector bosons currently considered to be fundamental particles are all gauge bosons. The most familiar vector boson is the photon, or quantum of light, which is a gauge boson. For some time, through the 1970s and 80s, the search for intermediate vector bosons, vector bosons of "intermediate" mass, was a major topic in high energy physics.

There also exist composite particles that are vector bosons, such as the vector mesons, made of a quark and antiquark with a total angular momentum of one unit.

## Explanation

The name vector boson arises from quantum field theory. The component of such a particle's spin along any axis will always be measured to have one of three values: $+hbar$, 0, or $-hbar$ (this is, at least, true for massive vector bosons; the situation is a bit different for massless particles such as the photon, for reasons beyond the scope of this article). The space of spin states therefore has three degrees of freedom, the same as the number of components of a mathematical vector in three-dimensional space. Quantum superpositions of these states can be taken such that they transform under rotations just like the spatial components of a rotating vector. If the vector boson is taken to be the quantum of a field, the field is a vector field, hence the name.