The international LHCb collaboration at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new type of ‘pentaquark state’ and the first pair of ‘tetraquark states’ including a new type of tetraquark state . The findings, presented at a workshop at CERN, add three new members to the LHC’s growing list of new hadrons. They will help physicists better understand how quarks combine to form these composite particles. Quarks are elementary particles and come in six types: up quarks, down quarks, charm quarks, strange quarks, top quarks, and bottom quarks. They usually combine in groups of three or two to form hadrons, such as the protons and neutrons that make up atomic nuclei. Even rarer, they can also combine into particles of tetraquarks and pentaquarks, known as “tetraquarks” and “pentaquarks.” About 60 years ago, theorists predicted both regular hadrons and these exotic hadrons, but only recently (in the past 20 years) have they been observed by the LHCb and other experiments. Most of the exotic hadrons discovered in the past two decades are tetraquark or pentaquark state, including a charm quark and a charm antiquark, and the remaining two or three quarks are up quarks, down quarks or strange quarks, or their antiquarks. But over the past two years, the LHCb has discovered a different kind of exotic hadron. Two years ago, the collaboration discovered a tetraquark state consisting of two charm quarks and two charm antiquarks, and two charm antiquarks, one up quark, one down quark, and one strange antiquark. “Open Charm” tetraquark state. Last year, it discovered the first instance of a “double-open charm” tetraquark with two charm quarks, an up antiquark, and a down antiquark. Charm means that the particle contains a charm quark without a corresponding antiquark.
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