By Andrew Grant
Atomic nuclei come in many shapes and sizes, and scientists have now obtained precise measurements of an elusive form: pear-shaped. Studying these exotic nuclei, which are described in the May 9 Nature, could allow physicists to better understand subatomic structure and to find new particles and forces.
“It’s a beautiful, clear-cut result of a very careful experiment,” says Christopher Lister, a physicist at the University of Massachusetts in Lowell.
Diagrams in middle school textbooks depict atomic nuclei as spherical, but the real story is a lot more complex. Protons and neutrons are jam-packed into a space just 10-15 meters wide, held together by a crushing force that dwarfs that of gravity. At the same time, the subatomic particles constantly move, shifting around and sometimes warping the nucleus into the shape of a football or even a flattened disk.
All those shapes are symmetrical vertically and horizontally. Physicists want to find asymmetrical nuclei because some theories predict that such deformed nuclei could exhibit strange new physical properties. Experiments over the last few decades have hinted that certain arrangements of protons and neutrons result in a pear-shaped nucleus, narrow on one side and bulging on the other.