The “particle zoo” is a colloquial term that is sometimes used to describe the array of subatomic particles in the universe, because there are so many different tiny localized objects to which can be ascribed various physical properties, like mass and charge. In line with this, all the particles of the Standard Model have now been observed, however experimenters are still searching for the particles associated with things like gravity, dark energy, and dark matter. This leads to a very particular taxonomy, pun intended.
In spite of its vast complexity, the entire universe is really only composed of two different kinds of subatomic particles, the bosons and the fermions. As part of this, particles can be distinguished by their spin, based on either integer or half-integer values. That is to say, a boson has a spin of 0, 1, 2, 3,…, whereas a fermion has a spin of 1/2, 3/2, 5/2,… So, for instance, there are spin-0 scalar bosons, spin-1 vector bosons, and spin-2 tensor bosons. In this way, bosons are described by symmetric wave-functions, while fermions are described by anti-symmetric wave-functions.
The bosons are particles, which can be characterized by Bose-Einstein statistics. As part of this, the strong nuclear force is an interaction that is mediated by gluons. Moreover, there are gravitons to mediate the force of gravity, as well as levitons to produce levity, which is currently known as dark energy. So, along with the W and Z bosons that mediate the weak interaction, and the Higgs boson, these particles all make up the distinct types of force carriers in the various different gauge fields.
In contrast to this, fermions are particles that can be described using Fermi-Dirac statistics. Along with this, they have quantum numbers that are determined by the Pauli exclusion principle. So, unlike bosons which can infinitely fill a given point of existence, fermions of the same type cannot occupy the same space at the same time. This includes particles like leptons and quarks, which serve as the building blocks of the material world.
Higgs bosons are spin-0 particles with no charge and a mass of 126 GeV. So, unlike the other gauge fields with vector and tensor bosons, these scalar bosons have a non-zero constant value in empty space. These force carriers are also very unstable, causing them to rapidly decay into other particles. Nonetheless, by coupling with Higgs bosons other particles are able to gain mass, with the exception of neutrinos.
Quarks are a type of fermions which serve as a fundamental constituent of baryonic matter. They combine together to form hadrons, the most stable of which are protons and neutrons. These are the only elementary particles that interact with all the fundamental forces. Quarks are also the only particles whose electric charges are not integer multiples of the elementary charge.
Gluons are elementary objects within a boson field that act as the exchange particles for the strong force between quarks. This is what holds hadrons together. The gluon is a vector boson, however while massive spin-1 particles have three polarization states, the gluon is massless having only two polarization states because gauge invariance requires the polarization to be transverse.
Leptons are half-integer spin fermions that do not interact with gluons, such as electrons and muons. As such, no two leptons of the same species can be in exactly the same state at a given point. One of the most prominent properties of leptons is their electric charge, which determines the strength of their electromagnetic interaction. This is important because charged leptons are able to combine with hadrons to form atoms.
Levitons are tensor field bosons, like gravitons, making them spin-2 particles. These are the elusive force carriers responsible for the accelerating expansion of the universe. This is because, unlike gravity which is attractive, levity is a repulsive interaction. Unfortunately, levitons have proved harder to model than other bosons, which do not interact with other particles of their own type. Moreover, levitons have never been detected. Scientists have only observed their effect, but they presently make up about 68% of the universe.
Weakly interacting massive particles are a type of fermion that constitute dark matter, which is a non-baryonic substance. These particles have a mass in the range of 100 GeV, and do not interact with gluons or photons. They don’t even interact with themselves. They do, however, make up about 27% of the universe, because five-sixths of all the substance it contains is invisible cold dark matter. Unfortunately, although WIMP-like particles are predicted by supersymmetry theories and extra-dimensional models, they still technically haven’t been observed…not yet, at least.
In total, I count twenty different elementary particles, with seven bosons and thirteen fermions:
- Z Boson
- W Boson
- Higgs Boson
- Up Quark
- Down Quark
- Top Quark
- Bottom Quark
- Charmed Quark
- Strange Quark
- Electron Neutrino
- Muon Neutrino
- Tau Neutrino