Univeristy of Michigan Press Release on Pear-shaped Nuclei

Answers to FAQ’s:

I have received a number of questions about this work that will be addressed here. Since I am pursuing the EDM experiments, my answers will emphasize this aspect of the impact of the research.

1. How is the finding of a pear-shaped nucleus likely to affect nuclear physics research? Will it have any influence on our understanding of things like nuclear fusion?

This work does not address fusion in the sense that it is a possible future energy source. As for nuclear physics research, we will continue to use this technique to study radon and radium atoms at ISOLDE as well as to make use of other techniques at other labs. Perhaps the most significant effect on nuclear-physics research is that this provides increased confidence on the prospects of EDM experiments and their impact.

2. What dictated your choice of radium and radon?

These are two systems that are both accessible to EDM experiment for different reasons. Radon is a noble gas like helium or neon and thus has special features that allow a sensitive measurement. Radium has many attractive features, and it can be trapped by lasers for an EDM measurement. This work is underway at Argonne National Lab. From the nuclear-physics perspective, these are nuclei that were predicted to have strong pear-shaped effects.

3. Why hasn't the shape of nuclei been studied in such detail before?

Studies of the shapes of nuclei have been going on for a long time in particular for spherical and oval (rugby-ball or watermelon) shapes. The pear-shaped nuclei are not stable, that is they undergo radioactive decay to more symmetric shapes, so special technology is required to produce and accelerate these. This was developed at CERN by the REX-ISOLDE program. The ability to make the measurements of the pattern of gamma rays from the excited nuclei is a feature of the MINIBALL detector developed by a large team. The analysis of the data in terms of the nuclear shapes is also quite specialized with the greatest contributions coming from the University of Liverpool.

4. What role does the EDM play in a nucleus being pear-shaped? How is its study likely to lead to the finding of new physics phenomena?

The nuclear pear shape is a consequence of nuclear forces, while the EDM would arise due to much weaker, undiscovered forces. BUT the pear shape will make the effects of these undiscovered forces much stronger and easier to detect. The EDM is due to alignment of the axis of the pear along the spin of the nucleus, which arises due to the new forces that also generate the matter-antimatter asymmetry.

5. Could you explain the role of the strong nuclear force and the weak force in the octupole deformation of nuclei?

It is the strong-nuclear force that determines how the neutrons and protons move within the nucleus. This nuclear force on neutrons and protons has the important effect that it is not completely central and thus pushes protons and neutrons into unusual places (gravity is a central force that depends on the distance between two objects centers). The weak force has very, well, weak effect on the shape or structure of nuclei.