Physicists at Manchester are firing beams of unstable atomic nuclei at detectors to map how nuclear structure changes in neutron-rich matter far from the stable isotopes found on Earth. This matters because the rules that govern atomic nuclei—how protons and neutrons arrange themselves, how they vibrate and rotate—are well understood only for the few hundred stable nuclei. Most of the thousands of possible nuclei are short-lived and barely studied. The group wants to know whether entirely new forms of collective motion emerge in these exotic nuclei, and how the ordering of quantum states shifts as matter becomes extremely neutron-rich or dilute. Answering those questions tests the fundamental theory of the strong force, Quantum Chromodynamics, which cannot be solved directly for nuclei. The research is fundamental science with no immediate practical application. But the same techniques—radioactive beam production and precision nuclear spectroscopy—underpin medical isotope production for cancer therapy and security screening for illicit nuclear materials. A deeper understanding of nuclear forces could also sharpen models of element formation in stars and supernovae, which in turn informs how we think about the origin of the elements that make up everything around us.
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Nuclear Physics aims to understand the structure and dynamics of nuclear systems. It is key to understanding the Universe from the first microseconds of its inception through its history of star and galaxy formation where nuclear reactions play a key role in the generation of energy and the creation of elements. The field has applications that benefit society in areas from medicine and security to power production, and a strong impact on other fields of science. The Manchester group makes leading edge contributions at an international level. Experimental work is performed at specific overseas facilities with focussed investment in the necessary instrumentation to carry out this work. Experiment: Atomic nuclei are a unique quantal laboratory in which microscopic as well as mesoscopic features, driven by effective two-body and three-body forces, can be studied. They are complex many-body systems, but often display unexpected regularities and simple excitation patterns that arise from underlying shell structure, pairing and collective modes of excitation. Such properties are also exhibited by simpler mesoscopic systems (for example, metallic clusters, quantum dots, and atomic condensates) the understanding of which draws heavily on techniques developed and honed in nuclear physics. A fundamental challenge is to understand nuclear properties ab-initio from the interplay of the strong, weak, and electromagnetic forces between individual nucleons. Recently, enormous progress has been made with such programmes for light nuclei. For heavier nuclei, shell, cluster and other beyond mean field many-body techniques, based on effective interactions, provide essential frameworks for correlating experimental data, yet still lack the refinement to reliably predict nuclear properties as one moves more than a few nucleons from well-studied stable nuclei. Uniquely, radioactive beam facilities allow an exploration of nuclear properties using both approaches over a wide range of N (neutron number), Z (proton number), T (temperature or excitation energy) and I (angular momentum). The key open questions the Manchester group will address include: * Do new forms of collective motion occur far from the valley of nuclear stability? * How does the ordering of quantum states, with all of its consequent implications for nuclear structure and reactions, alter in highly dilute or neutron-rich matter? Theory: To make precise connections between the structure and properties of the lightest, simple atomic nuclei and the underlying theory that describes the strong forces between nucleons, Quantum Chromodynamics. This is difficult because of the strength of the interaction. Even its fundamental degrees of freedom, quarks and gluons, are not the ones we see in nuclei. As the theory cannot be solved for situations relevant to nuclear physics, we work with "effective" field theories, built out of the appropriate degrees of freedom but incorporating robust constraints from the underlying theory, in particular its symmetries. An effective theory shouldn't be used at short distances where the underlying physics starts to be resolved. We apply a cut-off to mask the short-distance behaviour; a renormalisation approach is then used to ensures that predictions do not depend on cut-off scale. This approach is well-developed for couplings of photons and pions to single nucleons and for the forces between a pair of nucleons. We now wish to extend the approach to larger nuclei, initially looking at ones with 3-4 nucleons. We will use a powerful variational method to determine the nuclear wave functions, starting from forces provided by the effective theories. With these wave functions, we will then calculate interactions of photons with these nuclei to learn more about how nucleons respond to external fields. Experiments of these nuclei can tell us about the properties of the neutron, which are not accessible by other means since it is an unstable particle.
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