CLEO Collaboration Discovers Nine Baryons,

CLEO Collaboration Discovers Nine Baryons

The protons and neutrons that make up atomic nuclei are themselves composed of quarks. The proton contains two "up" quarks and one "down" quark, while the neutron contains one "up" and two "down" quarks. By combining different combinations of quarks, many particles, called "baryons," may be constructed, which have properties similar to the proton and the neutron. They form regular patterns, reminiscent of the periodic table that arises from the various combinations of protons and electrons that make up atoms.

Extensive studies have been made of baryons that contain the four lightest quarks (up, down, strange, and charm). The theory predicts 40 different baryons, 20 each having spin 1/2 and spin 3/2. The pattern is very pretty, as shown in the diagrams. The red and blue dots denote baryons that have been observed. Black dots are those that remain to be found.

The CLEO experiment at Cornell (of which our Illinois high-energy physics group is a member) has provided the most extensive study of baryons that contain one charmed quark (the second level of each diagram). We have discovered nine (marked in red) of the predicted fifteen baryons of this type and have made accurate measurements of their properties. Only one remains undiscovered. Although naive expectation would be that mesons (which contain only two quarks) are simpler than baryons, and thus would provide cleaner tests of the theory, in many cases the opposite is true. Baryons can tell us much about both the strong and the electroweak interactions.

More information about the CLEO discoveries is available from Professor Jon Thaler.