Bosons

Bosons are force particles. All particles with integer (whole number) spin are considered to be bosons. This differentiates bosons from fermions (matter particles) which have half-integer spin. While fermions (leptons and quarks) are thought of as matter particles, bosons (gauge and scalar bosons) are force carriers that mediate the interactions between matter particles and hold matter together. All particles with integer (whole number) spin are considered bosons.

An important characteristic of bosons is that their statistics do not restrict the number of bosons that can occupy the same quantum state—In other words, they do not abide by the Pauli Exclusion principle. In contrast to fermions, which follow Fermi-Dirac statistics and the Pauli Exclusion Principle, bosons follow Bose–Einstein statistics, or, more colloquially- B–E statistics. B-E statistics describe how bosons behave collectively in large groups; because they do not abide by the Pauli exclusion principle, any number of identical bosons can occupy the same place. This property is exemplified in helium-4 when it is cooled to become a “Bose Einstein Condensate,” a unique form of superfluid matter.

Bose-Einstein Statistics: What is the Difference?

The reason that the statistics of classical physics fail to appropriately explain radiation and other phenomena where bosons are being examined in collective quantities, while Bose-Einstein Statistics produce accurate results that match experimental observations, is due to the fact that in collective quantities, individual bosons are indistinguishable from each other. They do not abide by the Pauli Exclusion Principle, meaning they can simultaneously occupy the same quantum state as other bosons; so one cannot treat any two bosons having equal energy as being two distinct identifiable particles.

Imagine flipping two coins in the universe we observe around us- what is the possibility of getting two heads? The possible combinations are: HH, HT, TH, TT, so there is a 1 in 4 (25%) chance of producing two heads. This is how the majority of what we observe in the everyday world works— in accordance with classical physics. But bosons violate the Pauli Exclusion Principle, meaning they can simultaneously occupy the same quantum state as other bosons; this means that two bosons occupying the same quantum state cannot be treated as two distinct particles— so if you visited an alternate universe were coins behaved not like fermions but like bosons, here you would observe HH, HT=TH, TT; In this universe HT and TH are one-in-the-same because the two coins are indistinguishable from one another, so here your probability of producing two heads is not 1 in 4, but rather 1 in 3 (33%). This is how Bose-Einstein Statistics are able to accurately reflect how bosons behave collectively.

Gauge Bosons

Gauge bosons are force carriers for the four fundamental forces of nature: electromagnetism, the strong force, the weak force, and gravitation*. Fermions that interact with the four fundamental forces of nature do so by exchanging gauge bosons. The Standard Model currently recognizes three kinds of gauge bosons: photons, which carry the electromagnetic force; gluons, which carry the strong force, and W and Z bosons, which carry the weak force. The fourth fundamental force— gravitation— is speculated to also be carried by a gauge boson, dubbed the graviton. In the absence of experimental evidence and a mathematically coherent theory of quantum gravity, it is unknown whether the graviton would be a gauge boson or not but it is typically hypothetically classified as a gauge boson.

*the force carrier for gravitation is hypothetical.

Scalar Boson

A scalar boson is a boson whose spin equals zero. ‘Boson’ refers to particles with integer-valued spin; 'scalar' fixes this value to include zero as an 'integer'.The name “scalar boson”; arises from quantum field theory and is a concept derived from transformation properties under Lorentz transformations; which essentially attempt to explain the apparent oddities and contradictions that can occur between two observers in space-time. These transformations account for the fact that observers moving at different velocities may measure different distances, elapsed times, and even different orderings of events. There is an interesting history to these concepts, but the basic point one really needs to know is that scalar bosons have zero spin.

There are various known existing composite particles (non-fundamental particles) that are scalar bosons- the alpha particle and the pi meson are two such examples- but until very recently, no instance of a fundamental spin-zero boson had been experimentally recorded. That all changed in March of 2013, upon the tentative confirmation that the Higgs Boson had in fact been discovered at the Large Hadron Collider. Predicted and incorporated into the Standard Model back in 60s, the Higgs Boson is currently the only fundamental scalar boson ever recorded and its discovery has proved a major victory in supporting the validity of the Standard Model.