# Particle Physics Fundamentals

The concept of fundamental, indivisible particles goes back to the ancient Greeks (a concept known as "atomism"). In the 20th century, physicists began exploring the goings on at the smallest levels of matter, and among their most startling modern discoveries was the amount of different particles in the universe. Quantum physics predicts 18 types of elementary particles, and 16 have already been experimentally detected. Elementary particle physics aims to find the remaining particles.

## The Standard Model

The Standard Model of particle physics, which classifies elementary particles into several groups, is at the core of modern physics. In this model, three of the four fundamental forces of physics are described, along with gauge bosons, the particles that mediate those forces. Although gravity isn't technically included in the Standard Model, theoretical physicists are working to extend the model to include and predict a quantum theory of gravity.

If there's one thing that particle physicists seem to enjoy, it's dividing up particles into groups. Elementary particles are the smallest constituents of matter and energy. As far as scientists can tell, they don't seem to be made from combinations of any smaller particles.

## Breaking Down Matter and Forces

All elementary particles in physics are classified as either fermions or bosons. Quantum physics demonstrates that 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 whole number or integer spin. These spins result in different mathematical applications in particular situations. Simple mathematics of adding integers and half-integers shows 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 results in an integer value.

## Fermions

Fermions 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. 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 the class of fermion that make up hadrons, such as protons and neutrons. 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. There are six distinct types of quark:

• Bottom Quark
• Strange Quark
• Down Quark
• Top Quark
• Charm Quark
• Up Quark

Leptons are a type of fundamental particle that do not experience strong interaction. There are six lepton varieties:

• Electron
• Electron Neutrino
• Muon
• Muon Neutrino
• Tau
• Tau Neutrino

Each of the three "flavors" of lepton (electron, muon, and tau) is composed of a "weak doublet," the aforementioned particle along with a virtually massless neutral particle called a neutrino. Thus, the electron lepton is the weak doublet of electron and electron-neutrino.

## Bosons

Bosons have a particle spin equal to an integer (whole numbers like 1, 2, 3, and so on). These particles mediate the fundamental forces of physics under quantum field theories.

## Composite Particles

Hadrons are particles made up of multiple bound together quarks such that their spin is a half-integer value. Hadrons are divided into mesons (which are bosons) and baryons (which are fermions).

• Mesons
• Baryons
• Nucleons
• Hyperons: short-lived particles composed of strange quarks

Molecules are complex structures composed of multiple atoms bonded together. The basic chemical building block of matter, atoms are composed of electrons, protons, and neutrons. Protons and neutrons are nucleons, the type of baryon which together form the composite particle that is the nucleus of an atom. The study of how atoms bond together to form various molecular structures is the foundation of modern chemistry.

## 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 nucleons, 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.

Edited by Anne Marie Helmenstine, Ph.D.

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