RUI: Hadron Structure and Interactions

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The determination of proton structure is important for understanding matter in the universe, and a central challenge for theoretical nuclear physics. The proton is constructed from basic building blocks of matter called quarks, antiquarks (antimatter counterparts of the quarks) and gluons, but the ways in which these constituents contribute to the properties of the proton are not completely understood. For example, the proton has a property called spin, which is used in medical applications such as MRI scans, but it is not known how the spins of the quarks and gluons combine to give the spin of the proton. In this project the PI will use new theoretical models of the fluctuation of protons into two-particle states to describe the contributions of quarks and antiquarks created (the proton sea) to the properties of the proton. The results will be compared to experimental measurements from Fermilab, Jefferson Lab, RHIC (the Relativistic Heavy Ion Collider) and other facilities. Undergraduate students will participate in the project, receive training in nuclear and particle physics, and gain experience in research methods and scientific communication. The students will present their work at professional meetings and to the broader public.The scientific goals of this project are: (i) to build a hybrid model for the momentum distributions of the antiquarks in the proton sea by combining a new model for pion-nucleon wave functions, derived by the PI from light-front perturbation theory, with a new statistical model for the momentum distributions of the quarks and antiquarks in the bare proton (no fluctuation) and in the pions and baryons created in the fluctuation states; (ii) to extend this framework to calculate spin distributions in the proton sea by including the contribution of vector mesons to the fluctuations; and (iii) to study the effects of using a new pion-nucleon form factors in calculations of nucleon elastic electromagnetic and axial form factors. The results of the Fermilab SeaQuest experiment will test a hybrid model, and thereby the role of the pion in contributing to the nucleon sea. The extension of the formalism to describe vector meson fluctuations will contribute a new meson cloud model to confront the RHIC experimental measurements of spin asymmetry. The form-factor study will increase the understanding of the role of the pion in pion-nucleon interactions.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.