Fermions Overview

Fermions are particles that have a spin value equal to half an integer and are therefore constrained by the Pauli Exclusion Principle. They include the three generations of quark and lepton.

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Image from: https://en.wikipedia.org/wiki/Standard_Model

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Let's look at some of the differences between Quarks and Leptons. While quarks have color and interact with the strong nuclear force, leptons do not. This means that leptons can move around more freely than quarks, which we have never found not bound together.

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Also, quarks have electromagnetic charges in 1/3 values, while leptons have charges in integer values.

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The particles in the second and third generations have a much larger mass than their first-generation counterparts. These particles such as top and bottom quarks are very unstable and decay quickly into the more common particles. Charm, strange, and muons are found very rarely in nature, and top, bottom, and tau particles are only seen in the laboratory.

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Why do fermions come in three generations? This is a question that we don't know the answer to yet.  However, a lot of experimental evidence says that there are only three generations and no more.