Pear-shaped nuclei help search for antimatter

Physicists have found an isotope of the element barium with an extremely rare pear-shaped nucleus – and it’s unlike anything predicted by our current nuclear models.

Besides its importance for understanding fundamental nuclear physics, such asymmetric nuclei could prove a useful tool for explaining one of the other great asymmetries in physics: why there is vastly more matter than antimatter in the universe.

The work, led by Brian Bucher from the Lawrence Livermore National Laboratory in the US, was published in Physical Review Letters.

You are made of several billion billion billion atoms, and at the core of each is a positively charged nucleus. Most nuclei are spherical or squashed like a rugby ball or American football. These shapes arise from the quantum mechanical interactions between protons and neutrons that make up nuclei.

As early as 1982, physicist George Leander and colleagues predicted that some nuclei might bulge in one direction, like a pear.

At first glance, this odd shape might seem impossible. But what Leander and co realised is that complex interactions involving the weak force can cause the neutrons and protons to cram together more on one side than the other, causing a bulge.

In 1993 physicists in Darmstadt, Germany confirmed this when they discovered the first ever pear-shaped nucleus, radium-226.

Some 20 years later CERN physicists found a second pear in the atomic fruit basket – radium-224. Surprisingly, its shape did not fit with theory, hinting at problems with our best models of the nucleus.

Now physicists have found a third pear-shaped nucleus in barium-144 (the isotope of barium including 88 neutrons along with its typical 56 protons). Theorists had predicted that this isotope would be pear-shaped. The problem is it’s so short lived, typically only lasting a few seconds before decaying, that confirming its bottom-heavy shape was a huge challenge.

Particular signals in this radiation indicate if the nuclei are symmetrical or not – a bit like how you might infer a baseball has a loose stitch if you hear a buzzing sound as it flies through the air.

So Bucher and his international team of UK, French and US scientists had to create and test it in a rapid-fire physics experiment.

First, they used the Argonne National Lab at the University of Chicago to produce a tailor-made beam of barium-144 atoms. Electric fields tore electrons off the atoms, exposing their nucleus. These nuclei were smashed into lead foil in a particle accelerator to kick them into an excited state and emit gamma rays.

Particular signals in this radiation indicate if the nuclei are symmetrical or not – a bit like how you might infer a baseball has a loose stitch if you hear a buzzing sound as it flies through the air.

The researchers found that the pear-shaped distortion was more than twice that predicted by nuclear structure models – a flag that our current models of the nucleus need a rethink.

On top of that, pear-shaped nuclei could also become important tools in the search for antimatter.

One of the big mysteries in physics is why the universe is made up almost entirely of matter when everything we know about physics says matter and antimatter should have been created in equal amounts.

Theorists have thrown around a lot of ideas to explain it, and some of these predict that the positive and negative charges in atomic nuclei should be slightly offset – creating what’s called a “permanent electric dipole moment”. Yet nothing like this has ever been discovered in any atom, despite a 60-year search.

So far these searches have only been made using regular shaped nuclei, such as that of mercury.

But pear-shaped nuclei would amplify the electric dipole, meaning odd-shaped nuclei, such as barium-144, should be a great place to look. They will be a better proving ground for ruling out – or in – some of these theories.