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Why did discovering the top quark complete the set of six quark types?
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subatomic particle
The Large Hadron Collider (LHC), the world's most powerful particle accelerator. At the LHC, located underground in Switzerland, physicists study subatomic particles.
The discovery of the top quark in 1995 was significant because it completed the set of three pairs of quarks required by the Standard Model, the prevailing theoretical framework for understanding fundamental particles.12
Prior to this discovery, physicists had identified the up and down, charm and strange, and bottom quarks. Because the previously discovered charm and bottom quarks appeared to occur in pairs, theoretical predictions suggested the existence of a sixth partner for the bottom quark—the top quark. Its confirmation at the Fermi National Accelerator Laboratory validated this organizational structure, though it remains notable for being significantly more massive than the other elementary particles.12