The charm quark was the fourth quark to be discovered. The symmetry between the up and down quarks led to the expectation that there should also be a partner particle to the third quark, the strange quark. In addition, it was discovered that quarks with the same charge cannot transform into each other, therefore another quark, the charm, was needed to explain some observed decays.
Its discovery was made independently but at the same time in 1974 by two research groups located 5000 km apart, at the Brookhaven National Laboratory (BNL) in New York and at SLAC in California. They discover a bound state of a charm and an anti-charm quarks, which they called J and ψ, respectively. The discovered particle was named ψ (at SLAC) because the traces of its decay products in the detector formed the shape of this Greek letter. The meson now carries a double name, J/ψ meson.
Compared to the up, down and strange quarks, the charm quark is very heavy. Two consequences arise: first, its mass cannot be neglected in calculations involving it; secondly, it can form more stable bound states that, just like atoms, can appear as excited states with higher energy. We observe them as particles with higher mass and different properties. Studying them allows us to test the theory that describes interactions among quarks.
Theoretically, “exotic states”, namely bound states of quarks with more than three quarks, are also conceivable, for example tetraquarks (of four) or pentaquarks (of five). Currently, we have observed some particles that have not been clearly identified yet. New measurements will confirm if these are exotic particles.
