A Program of Medium Energy Nuclear Physics

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

It is now a well-established fact that nucleons, protons and neutrons, are made up of more elementary particles called quarks. The main physics program supported by this grant is to measure how quarks are distributed in nucleons and in particular how the magnetic moments of quarks are aligned compared with the nucleon?s overall magnetic moment. Measurements over the past decade have shown that the quarks contribute only about 1/3 of the total spin of the nucleon, which gives rise to prediction that a significant fraction comes from the orbital motion of the quarks and other sources. The goal of these experiments is to compare results on the quark distributions (and their orbital motion) with theoretical predictions from the so-called Standard Model of Particle Physics. Additional measurements to search for an electric dipole moment of the neutron, which would be a signal of physics beyond the Standard Model of particle physics, which describes the properties of the elementary particles making up most of the visible matter in the universe, are also part of this project. These research efforts will contribute to the education of postdocs, graduate students, and undergraduates in a broad array of skills needed in the advanced high tech workforce. The prevailing theory of the strong force, Quantum Chromodynamics or QCD, is a generalization of the highly successful QED, yet QCD is hard pressed to provide either quantitative or intuitive descriptions of nucleon structure. This project will focus on investigating key properties of the proton, targeting the poorly measured sea quarks and the unknown orbital motion of the quarks. The research will be performed at four experiments: SeaQuest at Fermilab, PHENIX at Brookhaven, and COMPASS-II and ATLAS at CERN. The strong force also played a role at the beginning of time. In the early stages of the big bang, the quark-gluon plasma (QGP) existed for an instant, before the strong force confined the quarks into bound states. This fleeting state of matter has been recreated in the laboratory and found to possess extraordinary properties, such as a viscosity so low that it may be at its theoretical minimum. This grant enables continued study of the QGP?s properties using fully reconstructed jets at ATLAS and correlations in small collision systems at PHENIX. This award also supports a search for a permanent electric dipole moment (EDM) of the neutron, a CP-violating quantity. The nEDM experiment aims at a 100-fold improvement over the present upper limit of the EDM set by previous experiments.