Year in review: Collider creates pentaquarks

New evidence clinches case for particle quintet


HIGH FIVE  After analyzing 26,000 decays, a team at the Large Hadron Collider reported evidence for particles composed of five quarks (illustrated).


Among the haul of subatomic particles discovered in 2015 are two quark quintets. Until recently, quarks, one of the fundamental units of matter, had been known to come only in clumps of three (to form such particles as protons and neutrons) or sometimes two.

Researchers at the Large Hadron Collider near Geneva spotted signs of the “pentaquark” particles while studying the decay of another particle called bottom lambda. About 12 percent of the decays produced particles with properties that could be explained only by a five-quark composition (SN: 8/8/15, p. 8).

The discovery is the most convincing of several reported pentaquark sightings since the turn of the century, physicists say, and confirms theoretical research suggesting that quarks cluster in large groups. The new particles will help physicists better understand the strong nuclear force, which holds atomic nuclei together.

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