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At facilities known as “charm factories,” large numbers of charm-containing particles can be produced with little contamination from other types of particles. This allows scientists to observe subatomic processes with great precision, and may reveal subtle signs of new physics phenomena predicted by theorists. The particles composed of charm c and anti-charm c quarks, like J/ meson, have no charm property (quantum number). Many experiments have studied their properties. First predicted to exist in 1964, scientists have observed six kinds of quarks (and their antiquark counterparts) in the laboratory: up, down, charm, strange, top and bottom. (15.4) The weight diagrams for the ground-state pseudoscalar (0+) and vector (1. Nevertheless, in an SU(4) classication, the sixteenmesonsaregroupedintoa15-pletandasinglet: 44 151. However, SU(4) is badly broken owing to the much heavier cquark. Particles containing a charm quark are known as either “charmed particles” or “charmonia.” They have only a fleeting existence before decaying into more conventional particles. At a CERN seminar held virtually on 11 August, LHCb announced the first signs of an entirely new kind of tetraquark with a mass of 2.9 GeV/c²: the first such particle with only one charm quark. A fourth quark such as charm ccan be included by extending SU(3) to SU(4). When the charm quark did turn up, it was as a constituent of the J/psi particle, whose discovery in 1974 finally convinced physicists that quarks were real. Theorists had predicted the existence of the charm quark to explain the absence of an expected particle interaction. It is hundreds of times more massive than the up and down quarks that make up protons and neutrons. With a mass of 3100 MeV, over three times that of the proton, this particle was the first example of another quark, called the charm quark. The charm quark is one of six quarks that, along with leptons, form the basic building blocks of ordinary matter.