Birmingham Nuclear Physics Consolidated Grant 2020

Our proposed research has two broad themes that build upon our world leading areas of expertise.

The first of these involves the study of high-energy nuclear collisions at the Large Hadron Collider, the world's highest energy particle accelerator. The aim of the ALICE experiment is to study nuclear matter, as it would have existed about a millionth of a second after the Big Bang when the Universe was so hot and so dense that nuclei did not exist.

In its primordial state nuclear matter consists of its fundamental constituents (quarks and gluons) in a plasma state. We recreate this novel state of matter in our experiment and we are developing ways of studying these high-energy nuclear collisions to discover the properties of the quark-gluon plasma. This is technically challenging and the group has developed a sophisticated electronic trigger system that controls the experiment and is entirely responsible for it.

The quark-gluon plasma has remarkable properties, such as an abundance of strange quarks and near-perfect fluidity. In this proposal, we are trying to determine whether size matters by finding the smallest drop of plasma that still retains these properties. We are using grazing collisions to explore the internal structure of nuclei at high energy.

And we are looking at the debris of quarks and gluons that are sometimes scattered out of the collision, producing a shower of particles in our detector known as a jet, to study the conditions inside the plasma. We are also performing R&D into new detector technologies based on silicon pixel detectors in which the readout electronics is contained within the pixel.

The second strand extends beyond the quarks to the scale of nuclei. Here the challenge is to understand how the nature of the strong interaction plays out on the nuclear, rather than the sub-nucleon scale. Here the strong force is highly complex, which makes theoretical predictions formidable.

Our approach is to make extremely precise measurements of light nuclei to reveal aspects of the 'nuclear force' in systems with relatively few protons and neutrons. These properties can then be used to discriminate between theories and help identify the important interactions. One manifestation of the nuclear force is the observation of clustering in which protons and neutrons clump together inside larger nuclei, for example into alpha particles (two protons and two neutrons).

Watching how such nuclei fall apart enables their structure to be unveiled. The Birmingham group has made the world's most sensitive measurements of how the Hoyle state in carbon-12 falls apart into three alpha particles. (The Hoyle state is an excited form of carbon-12 which governs how much carbon and other elements are made in stars, so understanding its structure in detail is of great importance.) We propose to extend this research a number of systems that will provide a deeper insight into phenomena such as nuclear molecules (clustered of nuclear matter bound together by sharing neutrons) and nuclei important for nucleosynthesis.

Finally, we are developing an experimental programme to exploit gamma-ray beams to probe with great precision the structure of clustered nuclei via their electromagnetic properties. To-date this tool has provided us with some of the best insights into the structure of light nuclei and we plan to extend these studies to exotic cluster states above the cluster-decay energy threshold. This programme will produce measurements to constrain state-of-the-art theory.