Standard Model

One of the most startling discoveries over the 20th century was how many elementary particles exist in the universe. Though the concept of fundamental, indivisible particles goes back to the ancient Greeks (a concept known as atomism), it wasn’t really until the 1900s that physicists began to explore what was going inside matter at the smallest levels.

In fact, quantum physics predicts that there are just 18 types of elementary particles (16 of which have been detected by experiment already). It is the goal of elementary particle physics to continue searching for the remaining particles.

The Standard Model of Particle Physics

standard model

The Standard Model of Particle Physics is at the core of modern physics. In this model, three of the four fundamental forces of physics are described, along with the particles that mediate these forces – gauge bosons. (Technically, gravity isn’t included in the Standard Model, though theoretical physicists are working to extend the model to include a quantum theory of gravity.)

Groups of Particles

If there’s one thing that particle physicists seem to enjoy, it’s dividing up particles into groups. Here are a few of the groups which particles exist in:

Elementary Particles - The smallest constituents of matter and energy, these particles which don’t seem to be made from combinations of smaller particles.

  • Fermions - Fermions are particle that have a particle spin equal to a half-integer value (-1/2, 1/2, 3/2, etc.). These particles make up the matter that we observe in our universe.
    • Quarks - A class of fermion. Quarks are the particles that make up hadrons, such as protons and neutrons. There are 6 distinct types of quarks:
      • Up Quark
      • Charm Quark
      • Top Quark
      • Down Quark
      • Strange Quark
      • Bottom Quark
    • Leptons - A class of fermion. There are 6 types of leptons:
      • Electron
      • Electron Neutrino
      • Muon
      • Muon Neutrino
      • Tau
      • Tau Neutrino
  • Bosons - Bosons are particles that have a particle spin that is equal to an integer (1, 2, 3, etc.). These particles are what mediate the fundamental forces of physics under the quantum field theories.
    • Photon
    • W Boson
    • Z Boson
    • Gluon
    • Higgs Boson - part of the Standard Model, but not yet detected experimentally
    • Graviton - theoretically predicted as part of quantum gravity, but not actually part of the Standard Model

Composite Particles

  • Hadrons - Particles made up of multiple quarks bound together.
    • Baryons (fermions)
      • Nucleons - protons & neutrons
      • Hyperons – short-lived particles composed of strange quarks
    • Mesons (bosons)
  • Atomic Nuclei – protons and neutrons form together to create the atomic nucleus.
  • Atoms – The basic chemical building block of matter, atoms are composed of electrons, protons, and neutrons.
  • Molecules – A complex structure composed of multiple atoms bonded together. The study of how atoms bond together to form various molecular structures is the foundation of modern chemistry.

A Note on Particle Classification

It can be hard to keep all the names straight in particle physics, so it might be helpful to think of the animal world, where such structured naming might be more familiar and intuitive. Humans are primates, mammals, and also vertebrates. Similarly, protons are baryons, hadrons, and also fermions.

The unfortunate difference is that the terms tend to sound similar to each other. Confusing bosons and baryons, for example, is far easier than confusing primates and invertebrates. The only way to really keep these different particle groups separate is to just carefully study them and try to be careful about which name is being used.

Matter & Forces: Fermions & Bosons

All elementary particles in physics are classified as either fermions or bosons. Quantum physics demonstrates the particles may have an intrinsic non-zero “spin,” or angular momentum, associated with them.

A fermion (named after Enrico Fermi) is a particle with a half-integer spin, while a boson (named after Satyendra Nath Bose) is a particle with an integer spin. These spins result in different mathematical applications in certain situations, which is far beyond the scope of this article. For now, just know that the two types of particles exist.

Simple mathematics of adding integers and half-integers show the following:

  • Combining an odd number of fermions results in a fermion (because the total spin will still be a half-integer value)
  • Combining an even number of fermions results in a boson (because the total spin will now be an integer value)

Breaking Down Matter: Quarks & Leptons

The two basic constituents of matter are quarks and leptons. Both of these subatomic particles are fermions, so all bosons are created from an even combination of these particles.

Quarks are fundamental particles which interact through all four of the fundamental forces of physics: gravity, electromagnetism, weak interaction, and strong interaction. Quarks always exist in combination to form subatomic particles known as hadrons. Hadrons, just to make things even more complicated, are divided into mesons (which are bosons) and baryons (which are fermions). Protons & neutrons are baryons. In other words, they are composed of quarks such that their spin is a half-integer value.

Leptons, on the other hand, are fundamental particles that do not experience strong interaction. There are three “flavors” of leptons: the electron, the muon, and the tau. Each flavor is composed of a “weak doublet,” which is the aforementioned particle along with a virtually massless neutral particle called a neutrino. Thus the electron lepton is the weak doublet of electron & electron-neutrino.