Quarks are elementary particles and the basic unit of matter. Quarks combine with each other to form a composite particle called hadron. The most stable hadrons are protons and neutrons, which are the units that make up the nucleus.

Because of a phenomenon called quark confinement, quarks cannot be directly observed or separated; Quarks can only be found in hadrons. For this reason, most of human understanding of quarks comes from the observation of hadrons.

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Quarks have six flavors, namely, up, down, down, odd, bottom and top. The mass of the upper and lower quarks is the lowest among all quarks. Heavier quarks will quickly become upper quarks or lower quarks through a process called particle decay. Particle decay is a process from high mass state to low mass state.

It is for this reason that upper and lower quarks are generally stable, so they are common in the universe, while strangeness, charm, top and bottom can only be produced by the collision of high-energy particles (such as cosmic rays and particle accelerators).

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