The Standard Model of Particle Physics – The Building Blocks of the Physical Universe
The standard model of particle physics shows how the elementary particles interact via the four forces to create the physical universe.
Fermions and Bosons Overview
Fermions (quarks and leptons) form the basis of matter, and bosons “mediate the interactions” between fermions. Each particle comes in a variety of types, and has an anti-particle, or is its anti-particle. Unknowns like dark matter aside (which the standard model doesn’t explain), that is all there is to it.
Together Fermions, Bosons, and their anti-particles are the only known building blocks of larger physical systems in the universe (including composite particles like protons and nuclei). Given this, we can explain the makeup of all composite particles, atoms, elements, molecules, larger objects, and the forces they exhibit by looking at the elementary particles and their interactions.
CERN: The Standard Model Of Particle Physics.
TIP: If you feel like the information below is overwhelming, try checking out our simple standard model explainer here.
TIP: If you aren’t overwhelmed, it is worth noting that quantum particles are explained best as localized vibrations in their respective quantum field and all display wave-particle duality (Quantum Field Theory).
Quarks, Leptons, Gauge Bosons, and the Higgs Boson Overview
The are main types of elementary particles are quarks, leptons, gauge bosons, and the Higgs boson:
Quarks – Quarks are fermions that account for most of an object’s mass. There are different types of quarks (described as flavor, generation, color), each type of quark has properties that allow it to bind together with other quarks. When quarks bind via “strong force” from the gluon boson which are explained below’ they form the familiar composite particles. Composite particles include hadrons (like the Baryons the proton and neutron) and other atomic particles like the atomic nuclei. Quarks also form mesons briefly by combining with “anti-quarks“, this is followed by the annihilation of both and the release of photons as radiation.
Leptons – Leptons are fermions that account for some mass once bound to a nucleus. There are different types of leptons (also described by type and generation); specifically there are electrons and neutrinos. Neutrinos don’t interact with much, but electrons are the special sauce of atoms that allow atoms to bind to other atoms.
Quarks and leptons for beginners.
Bosons – There are four fundamental forces in the universe from which all other forces arise. Each force has one corresponding boson particle. Photons are electromagnetic energy that gives particles a charge. Gluons carry the strong force that binds quarks in a nucleus; there are eight colors of Gluons that correspond with quark arrangements. W+Z Bosons decay atomic bonds as radiation and, theoretically, Gravitons, which governs the interaction of gravitational force. Note: we’ve proved that gravity exists, but not the graviton. To learn more about the four forces go here. You should think of each force, aside gravity, as a field that exhibits an excited measurable state called “a particle.” You can learn more about quantum field theory here.
There is also a Higgs Boson, which has a field that gives massless particles mass. To learn more about the Higgs Boson go here.
Higgs, Graviton, Weak Bosons, Photon, Gluon, Neutron, Neutrino, Proton, Electron, Up Quark, Electron.
TIP: Particles can also exchange “virtual bosons” (including virtual photons) which are how we explain the transfer of force through bosons between fermions.
The Known Elementary Particles Explained
There are 17 known elementary particles (61 if you count versions of quark and gluon “colors”) that make up everything including all the fermions and bosons.
Fermions can be grouped into quarks (sic flavors, three generations) and leptons (six flavors, three generations, includes electrons and neutrinos), and bosons can be grouped into gauge bosons (force carriers, which include photons, gluons, and W+Z bosons) and the Higgs Boson. The chart below shows all known elementary particles and anti-particles.
|Total number of (known) elementary particles:||61|
The Standard Model. A look at the standard model in simple terms.
TIP: Flavor, color, and generation are all just terms that denote “type”. Color has nothing to do with actual color; it’s just a matching system that models how quarks fit together.
What are Fermions?
All quarks and leptons can be referred to as Fermions, the “holders” of the force (as opposed to bosons which mediate force).
Types of Fermions
The types of fermions are:
- Quarks are affected by all four forces. Quarks hold photons as a charge. When the right combination of quarks comes together, they are “bound” into a system via strong force (the gluons, “glue” them together). That binding of energy creates a lot of the intrinsic mass of systems (like atoms, elements, people, planets, stars). Quarks make up the nucleus of atoms and protons. The nucleus contains most of the mass of an atom.
- Leptons can hold electromagnetic force as a charge or be neutral (like neutrinos). We find electrons in the shells of atoms and they are responsible for the electronic fields of atoms as well as passing electricity between systems. Neutrinos meanwhile act on weak force and can decay the bonds of strong force.
These particles can be grouped as fermions (quarks and leptons). Fermions repel and attract each other by exchanging bosons.
What are Bosons?
Bosons mediate the force between fermions by “carrying” the force. Fermions are changed by this interaction (more on that below).
Types of Bosons
Bosons are particles that carry forces between fermions. Each boson carries one of the four forces. The types of bosons are:
- Gluon: Mediates strong force (and affects the flavors of quarks).
- Photon: Mediates electromagnetic force.
- W + Z Bosons: Mediates the weak force to decay strong force.
- Graviton: A theoretical (for now) particle that mediates gravitational force.
- Higgs: A type of boson that gives massless energy particles mass (Google “Higgs Mechanism”).
TIP: Fermions “have force” and gauge bosons mediate forces between fermions (AKA “carry” forces). This is done by the exchange of “virtual particles”.
Virtual particles are proven on paper, not by an experiment (and given this, some don’t like the term). That said, virtual particles could be thought of as the actual information that is exchanged between fermions via bosons. There is a “virtual” type of each boson: virtual photons (which carry electromagnetic force), virtual gluons (which carry strong force), virtual W+Z (which carry weak force), and one could assume perhaps virtual gravitons (but hold off because gravitons aren’t proven just yet).
TIP: Unlike real particles, virtual particles don’t contribute to the mass-energy of the system, they are already accounted for by the particles carrying and exchanging the forces.
TIP: The electromagnetic repulsion or attraction between two charges can be thought of as due to the exchange of many virtual photons between the charges. So, for example, we can think of them as massless electromagnetic force carrier photons that exist for such a short time that they don’t exist.
Anti-particles (or just antiparticles) are particles with the same mass and opposite charge (including electric charge) of the corresponding particle. So a up-quark has a anti-up-quark. An electron has a positron (not a proton).
Particle-antiparticle pairs can annihilate each other, producing photons; since the charges of the particle and antiparticle are opposite, the total charge is conserved.
NOTE: Only the photon boson is produced in annihilation. A photon is its own anti-particle. A photon is “pure massless energy”. Energy can’t be created or destroyed. Energy and mass have equivalence (mass-energy can’t be created or destroyed). Motion (like spin) can be explained with energy. Energy IS charge. All the forces except gravity can be united as “United Electroweak Force.” The Higgs field gives massless particles their mass. Almost everything can be explained with using photons. Using the phrase “everything is energy or electromagnetic force, or photons, in order to determine whether the chicken or the egg came first, is metaphysics.”
The Properties of Elementary Particles – Mass, Spin, Charge, Color
Each of the elementary particles has properties of mass, spin, and charge; each can “carry” or mediate one or more of the four fundamental forces. For example, quarks can hold all forces, while photons only mediate electromagnetic force. Particles also have properties like direction and frequency (the electromagnetic spectrum depends on frequency for instance, as does the gravitational wave spectrum).
We can also break down quarks into flavors and colors (which describe different layouts of these elements) and leptons into different types as well.
The properties of a particle determine which forces can act on it and which forces it can carry or mediate. This affects how particles interact and how they form systems.
When particles interact, their properties can change, and they can become different elementary particles or form together to create composite particles like a nucleus or proton, or even a composite electron.
Attraction and Repulsion
Generally, just like with an atom, all particles fit together like a magnet through types of attraction and repulsion.
Gluons essentially “glue” attracted quarks together with “strong force” creating a zero charge composite particle.
Once quarks are glued, charged electrons combine with quark-based nuclei, protons, and neutrons to form atoms. Then atoms, in turn, attract like magnets based on charge. Weak force decays they bonds back into elementary particles.
Importantly, as noted above, fermions specifically comprise all matter and give things the measurable property mass-energy (bosons like gluons, photons, and W+Z bosons just “mediate” the forces between fermions and otherwise “carry” them).
All bosons have a spin of one, all fermions have a spin of ½ . This can also be expressed by replacing the “1” with “ħ” the symbol for the reduced Planck constant (the smallest measure in the universe that quantum particles seem to quantize to).
- Fermions: Have intrinsic mass and carry properties called charges and spin “±1⁄2” (plus or minus one-half, a half spin) or “±ħ⁄2″(intrinsic angular momentum ±ħ⁄2, where ħ is the reduced Planck constant).
- Bosons: Gluons and Photons are massless, W+Z have mass, W has a has ±1 charge (unlike all other bosons which have zero charges). All bosons, very importantly, have a spin of exactly “1” or “ħ” (intrinsic angular momentum ±ħ, where ħ is the reduced Planck constant).
Two fermions go in → interaction by boson exchange → Two changed fermions go out.
TIP: Two massless particles can come together to form a system with mass. The Higgs Mechanism shows how massless particles can “get” mass and other experiments show even photons can interact in the right settings.
Fermions and Bosons – Angular Momentum (1 and ±1⁄2 Spins)
Spin can be one of three things +1⁄2 or -1⁄2 or exactly one. This angular momentum determines how particles will interact and what forces they can carry. When two 1⁄2 spin fermions interact with a 1 spin-boson, the fermions can change spin, charge, direction, momentum, and frequency.
Importantly, since bosons carry one unit of angular momentum, the fermion’s spin direction will flip from +1⁄2 to −1⁄2 (or vice versa) during such an exchange in units of the reduced Planck’s constant.
Pauli Exclusion Principle: A general rule of the universe is that two identical fermions can never occupy the same quantum state, but two bosons can. This can be simplified to say two of the same quarks with the same spin can’t be in the same place at the same time, so they have to change states with the help of bosons. This mechanic is why we have all the different fermion types. In atoms, this Pauli exclusion principle says two electrons with the same state can’t exist in the same orbital and must have opposite half-integer spins, 1/2 and -1/2. This is why negatively and positively charged electrons bounce around to “holes” in atoms. The ability of fermions like electrons and quarks to change properties via the forces, and the way they bond together, are central to everything.
NOTE: There are a few other momentum types related to spin (angular and orbital angular momentum), direction, frequency and more each is conserved (either as itself or as another type of momentum or force).
The Laws of Conservation
Mass, spin, charge, and the forces particles carry are all “conserved”. Angular momentum is conserved, mass is conserved, a charge is conserved, color charge is conserved, energy is conserved, etc. Most of these forces can be conserved as another force, but the exact laws of conservation do have some rules, for instance sometimes mass-energy is conserved by producing extra photons. We can overly simplify this by just knowing mass-energy is conserved (not created or destroyed in interactions). So a force, mass, spin, or charge is always conserved when particles interact.
This does things like make quarks flip their spin from positive to negative when they interact via gauge bosons.
Particle Field Fluctuations
Particles all have fields by nature, and those fields fluctuate naturally when the particle experience anything other than true theoretical empty space (which doesn’t exist in our universe). With that in mind, when particles carry forces between other particles (via “virtual particles”) it produces fields, and it produces measurable effects of particle fields like the releasing of photons, and their equivalent electromagnetic waves. 
If you think of a particle as a point in a pool of water, when it interacts with other particles a ripple is formed. The origin of the ripple is the particle; the ripple is the field.
Photons are particularly interesting because their field fluctuations produce the electromagnetic spectrum, which contains visible light. The speed of its fluctuation relates back to its place on the electromagnetic spectrum. Thus visible light is just a specific type of field fluctuation from photons. Remember photons being a gauge boson can be held by fermions as a charge.
Learn more about the nature of light here.