work was followed by basic, exciting developments: the discovery of neutrons, of nuclear reactions and the transmutations of elements, of isotopes, of the detailed nature of radioactivity. These discoveries followed in quick succession, in parallel with the developing insight that a revolutionary new framework—quantum mechanics—was needed to describe phenomena or the scales of the atom and the nucleus. This period also initiated our understanding of how nuclear processes fuel the Sun.

The early sequence of discoveries led, during World War II, to the Manhattan Project, which was based on the prior investigation of basic nuclear properties and played a key role in the history of our nation and the world. Applications of nuclear techniques to benefit human health started early, with major developments in this field continuing to the present.

The 1950s and 1960s saw the conceptual development of basic models of the atomic nucleus that provided a successful, if approximate, phenomenological framework for describing nuclear structure and reactions. The roots of many of today's urgent questions in nuclear physics can be traced to this period: Why were such model descriptions of the nucleus so successful? How do they arise as low-energy, long-distance representations of the more fundamental theory of quarks and gluons? How do the symmetries that govern the strong interactions constrain this framework and influence the different temperature regimes? What are the limitations of the models as smaller length scales and higher energies are probed? Studies of nuclear beta decay undertaken in this period helped establish the form of the weak interaction and guided the formulation of a Standard Model that has been astoundingly successful in uniting the weak and electromagnetic interactions. Yet this model remains incomplete. How can we find hints of the missing physics with low-energy precision experiments in nuclei testing the limitations of the Standard Model?

Scope of the Field

The fundamental questions that confront nuclear physics today have inevitably led the field to extend its horizons, both in the reach of its frontiers and in the scope of its research enterprise. The size and energy scales of present-day nuclear physics extend from the world of atomic and condensed matter physics to the more microscopic domain of high-energy physics, and at the large end of the scale, to the stars and the cosmos. It is this broad reach that makes nuclear physics so interesting to many scientists (more than 3,000 in the United States alone) and so integrally connected to other sciences. Nuclear physics both contributes to and benefits from other fields—for instance, from atomic physics for intricate table-top experiments, to high-energy physics for hall-size collider detectors. High-energy physics is concerned with the elementary particles and their interactions; it is the goal of nuclear physics to understand and explain why and



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