A proton’s response to an electric field is a surprisingly stubborn number to pin down, and that uncertainty ripples through everything from the size of the atom to the predictions of fundamental physics. The Manchester Nuclear Physics group is building theoretical tools to extract this number—the proton’s polarisability—from experiments that scatter photons off hydrogen and other light nuclei. The same approach will also reveal the neutron’s polarisability, a quantity that is otherwise inaccessible because the neutron decays in minutes. These numbers are not abstract curiosities. They are essential inputs for calculating the Lamb shift in muonic atoms, a measurement that has recently thrown up a puzzle about the proton’s charge radius. Resolving that puzzle could tighten constraints on quantum electrodynamics and, in turn, on the Standard Model. This is fundamental science. It will not generate a new battery or a medical scanner next year. But the techniques developed here—linking few-body nuclear reactions to the underlying strong force—will sharpen the theoretical framework that underpins all nuclear physics, from stellar nucleosynthesis to the design of next-generation reactors.
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Nuclear Physics aims to understand the structure and dynamics of nuclear systems. It is the key to understanding the Universe from the first microseconds of its inception when the quark-gluon plasma prevailed, through its history of star and galaxy formation where nuclear reactions play an essential role both in the generation of energy and the creation of elements. The field also has applications that benefit society in diverse areas, from medicine and security to power production, and a strong impact on other fields of science. The Manchester group is part of the UK nuclear community which has devised a mode of operation that enables it to make 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. Theoretical projects are undertaken using models incorporating forces inspired by the underlying quantum chromodynamics. 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. In recent years, 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. Experimental measurements are made to test modern theories using the techniques of transfer reactions, gamma-ray spectroscopy and measurements of hyperfine atomic effects using lasers. We also aim to make connections between the interactions of nucleons and the underlying theory that describes the strong force, Quantum Chromodynamics. Key quantities are the polarisabilities that describe how the structures of nucleons respond to external electric and magnetic fields. We are developing theoretical tools to determine these from experiments on the scattering of photons from hydrogen and other light nuclei. The latter are needed to learn about the the properties of the neutron since it is an unstable particle, and are also interesting for the testing of nuclear forces in few-body systems and for the calculation of muonic atom Lamb shifts.
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