A Program of Medium Energy Nuclear Physics

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It is now a well-established fact that nucleons, protons and neutrons, particles that make up the vast majority of the visible matter in the Universe, 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. In this quark form of matter, energy and mass are traded back and forth on short timescales according to the famous E = mc2. Particular questions that will help us to understand this essential feature of quark matter include: How is the magnetism of the nucleon constructed? What is the role of anti-matter in the nucleon? What happens when the temperature inside a nucleon is increased to a high value? Does the motion of the quarks inside the nucleon look the same as one moves forward or backward in time? The goal of these experiments is to compare with theoretical predictions from the so-called Standard Model of Particle Physics, both to better understand it, as well as to look for signs of new phenomena that are not described by it.The prevailing theory of the strong force, quantum chromodynamics or QCD, is a generalization of the highly successful quantum electrodynamics QED, yet we are hard-pressed to provide truly QCD-based 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 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 sPHENIX. This QCD research will comprise four experiments: SeaQuest at Fermilab, COMPASS-II and ATLAS at the European Cernter for Nuclear Research (CERN) as well as, in the future, sPHENIX at Brookhaven. 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.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 research program also includes a number of outreach activities: summer programs for undergraduates from smaller institutions, and for local high school students from underrepresented groups, along with leadership of a program of weekly public presentations, a public course in energy and sustainability and a popular website exploring the physics of baseball.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.