It’s going pear-shaped for atomic nuclei

According to the Standard Model, we shouldn’t even exist – but an international team of physicists have discovered that the reason we do may be due to pear-shaped nuclei.

The discovery, detailed in Nature, could advance the search for a new fundamental force in nature that might explain why the Big Bang created more matter than antimatter.

“If equal amounts of matter and antimatter were created at the Big Bang, everything would have annihilated, and there would be no galaxies, stars, planets or people,” said Tim Chupp, a University of Michigan Professor of Physics and Biomedical Engineering and co-author of the study.

Antimatter particles have the same mass but opposite charge from their matter counterparts. Antimatter is rare in the known universe, but when matter and antimatter particles collide, they mutually destruct.

Physicists have long been searching for signs of a new force of interaction that might explain the matter-antimatter discrepancy.

Most nuclei are shaped like rugby balls but pear-shaped nuclei are lop-sided because positive protons are pushed away from the centre of the nucleus by nuclear forces that are fundamentally different from spherically symmetric forces like gravity.

Until now, it has been hard to experimentally observe pear-shaped nuclei, but a technique at ISOLDE facility at CERN has been used successfully to study the shape of the short-lived isotopes Radon 220 and Radium 224.

The atom beams were accelerated and smashed into targets of nickel, cadmium and tin. In this method, the relative motion of the heavy accelerated nucleus and the target nucleus creates an electromagnetic impulse that excites the nuclei. Physicists can understand nuclear shape by studying the details of the excitation process.

In this study, the nuclei produced gamma rays that flew out in a distinctive pattern that revealed a pear-shape.

“Our expectation is that the data from our nuclear physics experiments can be combined with the results from atomic trapping experiments measuring EDMs to make the most stringent tests of the Standard Model, the best theory we have for understand the nature of the building blocks of the universe,” said University of Liverpool Professor Peter Butler who led the study.

Reference: Nuclear physics: Exotic pear-shaped nuclei