The Standard Model Part 1: What Are Particles?

An introduction to hadrons, leptons, and quarks

The Standard Model. Source: Department of Energy
Johannes Moreelse, Democritus, the Laughing Philosopher, c. 1630. Source: Mauritshuis
Classification of particles. Source: A-Level Physics

What are Particles?

A particle is a small bit of matter that can be assigned different properties such as mass, charge, or spin. All particles in the universe can be classified by mass and separated into two groups: leptons or hadrons. Hadrons themselves are not elementary particles, while leptons are. An elementary particle is a particle that can’t be broken into anything smaller.

Hadrons

All hadrons can be divided into two subgroups, mesons and baryons, and are composed of different combinations of quarks. Murray Gell-Mann and George Zweig taught us these combinations in 1964: intermix any three quarks to get a baryon. A proton is an example of a baryon: it is made of two up quarks and one down quark bound together. A neutron is also an example of a baryon. It has two down quarks and one up quark. Gell-Mann and Zweig also taught us that to get a meson, mix one quark and one antiquark. An example is a pion, made of one up quark and one anti-down quark. Pions are the lightest meson particle. Kaons are another example of a meson and are composed of a strange quark and an up or down antiquark.

Quarks

Quarks are the smallest things we know of and the smallest particle making up your object. They have six varieties that are referred to as “flavors.” Above were mentioned three flavors: up, down, and strange quarks. The six types of quarks are split into three generations and are as follows: the “up quark” and the “down quark” complete the first generation, seconded by the “charm quark” and “strange quark,” and lastly, the “top quark” and “bottom quark.” The first generation is stable, while the second and third aren’t. All stable matter in the universe is made from first generation particles.

Leptons

When classifying particles by mass the second group is called leptons. Unlike hadrons, all particles that fall under the category of leptons are believed to be elementary particles. Leptons are not composed of smaller particles.

Quark and lepton generations. Source: Quanta Magazine

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Simone Lilavois is a NYC high school student passionate about understanding the nature of existence in relation to the Cosmos.

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Simone Lilavois

Simone Lilavois is a NYC high school student passionate about understanding the nature of existence in relation to the Cosmos.