Boson

In the study of particle physics, all subatomic particles belong to one of two fundamental classes. These two classes are called fermions and bosons. Every single observed subatomic particle must fall into one of these two categories. While fermions are often described as the building blocks of ordinary matter, bosons play a very different role.

The main difference between these two groups is a property called spin. Spin is a quantum number that describes a particle's intrinsic angular momentum. For a boson, this spin quantum number is always an integer value, such as 0, 1, or 2. In contrast, fermions always have half odd-integer spin values like 1/2 or 3/2.

Some bosons are elementary particles, meaning they are not made of smaller components. According to the Standard Model of particle physics, there are five specific types of elementary bosons. The first is a scalar boson with a spin of 0, known as the Higgs boson. The Higgs boson is unique because it contributes to the phenomenon of mass through the Higgs mechanism. The other four are vector bosons, which have a spin of 1. These are known as gauge bosons because they act as force carriers.

They carry forces that act between other particles in the universe.

These gauge bosons are categorized by the specific forces they mediate. The photon is the force carrier for the electromagnetic field. There are also eight different types of gluons, which mediate the strong force. The weak force is carried by both a neutral weak boson and two types of charged weak bosons. Scientists have also hypothesized the existence of a second-order tensor boson called the graviton. The graviton would have a spin of 2 and would act as the force carrier for gravity. However, researchers have not yet successfully incorporated gravity into the Standard Model.

Not all bosons are elementary; many are composite particles. A composite particle is a larger structure made of smaller constituents, such as hadrons, nuclei, or atoms. Whether a composite particle is a boson or a fermion depends on its internal makeup. If a particle is made of an even number of fermions, the resulting composite particle will be a boson. For example, a hydrogen atom contains three quarks and one electron. Since these are four fermions, the total spin results in an integer, making the atom a boson.

Mesons are another common example of composite bosons.

Many stable atomic nuclei are also classified as bosons. This includes particles like deuterium, helium-4, carbon-12, and lead-208. In fact, even-mass-number nuclides make up 60% of all stable nuclides. Most of these are even-proton and even-neutron nuclides. Because of particle pairing, these specific nuclides have a spin of 0. There are also five stable odd-proton and odd-neutron nuclides. These five specific particles have integer but non-zero spins.

The concept of the boson is tied to a specific mathematical framework. The term was coined by Paul Dirac to classify particles that obey Bose–Einstein statistics. This framework was pioneered by the Indian physicist Satyendra Nath Bose. While working at the University of Dhaka, Bose achieved a major breakthrough. He treated photons as identical and indistinguishable particles. This allowed him to derive Planck's law without using classical physics. He sent his manuscript to Albert Einstein, who translated it and endorsed the work.

This collaboration led to the theoretical prediction of the Bose–Einstein condensate.

Bosons exhibit unique behaviors when they are at high densities or very low temperatures. Unlike fermions, there is no restriction on how many bosons can occupy the same quantum state. This allows them to behave in a characteristic manner described by Bose–Einstein statistics. For instance, if you cool a gas of helium-4 atoms to temperatures near absolute zero, the particles lose their kinetic energy. They then condense into a low-energy state and become a superfluid.

This state allows the liquid to flow without resistance. Similar behaviors are seen in other systems, such as Cooper pairs in superconductors or excitons in semiconductors. Certain quasiparticles, including plasmons and phonons, also behave as bosons in these environments.