RUI: Hadronic Structure from Spin Observables in pQCD

The nucleus of an atom is made up of protons and neutrons, which are a part of a broader category of particles called hadrons. However, these hadrons are not a fundamental form of matter since they are composed of other particles, namely, quarks and gluons (collectively called partons). The partons form a dynamical system inside of hadrons that is governed by the strong nuclear force. A goal of nuclear physics research is to understand this internal structure through the interactions of these partons, the elementary pieces of visible matter. In particular, the analysis of high-energy collisions sensitive to an intrinsic property, called spin, of hadrons and/or partons is especially useful. These observables allow the exploration of the 3-dimensional motion of partons inside of hadrons and how the proton's spin arises from its constituent partons. This research gives better insight into these aspects of hadronic structure as well as supports the science of the future Electron-Ion Collider to be built in the United States. Undergraduate students are included in substantive parts of the project in order to foster the interests of young scientists in this subfield of nuclear physics and train them for future careers in research. This research project furthers our knowledge of observables sensitive to parton intrinsic transverse motion and correlations, which allow for a 3-dimensional imaging of hadrons in momentum space, as well as the origin of the proton's spin. This includes performing Bayesian Monte Carlo global analyses of data from various experiments, including electron-positron, electron-proton, and proton-proton collisions for single-hadron and dihadron finals states, sensitive to the spin of partons and/or hadrons, with a rigorous incorporation of gluon radiation effects. Predictions are also made for future measurements at different experimental facilities. In addition, one needs a robust theoretical formalism in order to interpret experimental data. This project solidifies the framework used in dihadron spin observables through next-to-leading order calculations of some reactions. Lastly, this project develops the machinery to determine how much spin arises from partons that carry a small fraction x of the proton’s momentum, which is a crucial, yet poorly-constrained, piece of how the proton’s spin dynamically arises from its constituent partons. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.