Fundamental Studies of the Proton Spin in QCD

The protons and neutrons that serve as building blocks for our visible universe are constructed from smaller, more fundamental particles called quarks and gluons. These point-like particles, or partons, are confined inside nucleons due to the nature of the strong force which governs their interactions. Consequently, the abundant, stable and easily manipulated proton has served as an experimental “partonic laboratory” for over half a century. The underlying mechanisms of Quantum Chromodynamics (QCD), the formal theory of strong interactions, are illuminated via investigations into how the proton mass, charge and spin manifest from partonic degrees of freedom. Spin, a type of angular momentum inherent to all fundamental particles, is a purely quantum mechanical dynamic and therefore serves as a particularly sharp probe into the nature of QCD. The “spin puzzle” is an outstanding question in QCD physics and can be stated simply: How does the spin and orbital angular momentum of the quarks and gluons combine to form the total proton spin of 1/2? Two decades of experiments have constrained the total quark spin contribution to be 1/3 of the sum. This surprisingly small value motivates experiments designed to measure the quark and gluon spin distributions and partonic orbital angular momentum pieces of the spin puzzle. This award will fund experimental efforts to probe the quark transversity distributions via measurements of Collins asymmetries in mid-rapdiity jets reconstructed in the STAR detector at the Relativistic Heavy Ion Collider. It will support efforts to further constrain the gluon contribution to the spin puzzle via measurements of dijet asymmetries in ps = 500 GeV proton-proton collision data. These funds will also support manpower for the installation and commissioning of the Forward Gem Tracker, the STAR detector upgrade designed to measure the parity violating asymmetries of W production that are sensitive to the flavor separated anti-up and anti-down quark spin distributions in the proton. This award provides the unique opportunity for University of Kentucky graduate and undergraduate students to learn and develop the software tools and analysis techniques necessary for jet reconstruction, while simultaneously contributing to the installation and commissioning of a state-of-the-art detector at the worlds only high-energy polarized proton collider.