In quantum mechanics, a boson (, ) is a particle that follows Bose–Einstein statistics. Bosons make up one of two classes of elementary particles, the other being fermions. The name boson was coined by Paul Dirac to commemorate the contribution of Satyendra Nath Bose, an Indian physicist and professor of physics at University of Calcutta and at University of Dhaka in developing, with Albert Einstein, Bose–Einstein statistics, which theorizes the characteristics of elementary particles.Examples of bosons are fundamental particles such as photons, gluons, and W and Z bosons (the four force-carrying gauge bosons of the Standard Model), the recently discovered Higgs boson, and the hypothetical graviton of quantum gravity. Some composite particles are also bosons, such as mesons and stable nuclei of even mass number such as deuterium (with one proton and one neutron, atomic mass number = 2), helium-4, and lead-208; as well as some quasiparticles (e.g. Cooper pairs, plasmons, and phonons).An important characteristic of bosons is that there is no restriction on the number of them that occupy the same quantum state. This property is exemplified by helium-4 when it is cooled to become a superfluid. Unlike bosons, two identical fermions cannot occupy the same quantum state. Whereas the elementary particles that make up matter (i.e. leptons and quarks) are fermions, the elementary bosons are force carriers that function as the 'glue' holding matter together. This property holds for all particles with integer spin (s = 0, 1, 2, etc.) as a consequence of the spin–statistics theorem.
When a gas of Bose particles is cooled down to temperatures very close to absolute zero, then the kinetic energy of the particles decreases to a negligible amount, and they condense into the lowest energy level state. This state is called a Bose–Einstein condensate. This property is also the explanation for superfluidity.