The RNC also studies the quark-gluon structure of the conventional matter – protons, neutrons, and nuclei – that make up more than 99% of the visible mass in the universe. This program is focused on key measurements that provide information on the spatial, momentum, spin, and flavor structure of quarks in protons and neutrons, as well as studies of the modification of proton and neutron structure in dense nuclei.

The program began with studies of the spin structure of protons in polarized proton-proton collisions at RHIC, aimed at understanding the origin of the spin of the proton after earlier measurements showed that only a fraction was carried by the spin of its constituent quarks. There is now a much broader program making use of the precise, high-luminosity measurements that can be performed at Jefferson Lab (JLab). Current focus areas include using the “mirror nuclei” 3H and 3He to better understand the structure of neutrons, and understanding the nature of extremely energetic configurations inside of nuclei and the potential impact of these “short-range correlations” on the quark structure of protons and neutrons in nuclei.

A profound, long-standing problem in QCD is the nature of the spin of the nucleon. The quark and anti-quark spins are known to carry only a small fraction of the proton spin, whereas the distribution of the remaining fraction amongst gluon spin and orbital angular momenta is not well understood. RHIC is not only the world’s first and most flexible heavy ion collider, but is also the world’s only polarized proton collider, enabling unique studies of proton spin structure. Members of the RNC have contributed to world-leading measurements of nucleon spin structure and spin in QCD, through measurements with STAR of jets and other probes.