Holey Grail: A new experiment confirms that protons take a variety of different shapes (The Economist)

Holey Grail

A new experiment confirms that protons take a variety of different shapes.

PHILADELPHIA - Nearly 100 years ago, Ernest Rutherford acquired fame, and a Nobel prize, by shooting alpha particles (helium nuclei produced by radioactive decay) at gold foil. Unexpectedly, though, instead of passing through the gold foil, some of the particles bounced back. Rutherford realized that this is because atoms are made of a dense nucleus — later found to consist of protons and neutrons — surrounded by a diffuse cloud of orbiting electrons. The back scattering was caused by his "ammunition" bouncing off the nuclei.

Physicists who want to probe the structure of matter have since used more refined versions of Rutherford's method. The latest, meant to discover what shape protons are, was explained to the American Physical Society meeting in Philadelphia this week.

It has been known since the 1960s that protons are made of smaller particles called quarks and gluons. A proton contains three quarks. The gluons, as their name suggests, hold the quarks together. This being the world of quantum weirdness, the particles are also waves, so asking the question "what shape is a proton?" is not as straightforward as it may seem. Although the theory that describes the interaction of quarks and gluons is on a firm conceptual footing, it is so complicated in practice that it can take years to make a single calculation of something like the shape of a proton — even using the most powerful supercomputers. Instead physicists calculated the shape of the proton using many, very approximate, techniques. The simplest found the proton to be roughly spherical. But it would help the study of atomic nuclei to know what the correct shape of the proton is. Time, therefore, to do an actual experiment.

Vina Punjabi and her colleagues at the Thomas Jefferson National Accelerator Facility, in Virginia, used electrons, instead of alpha particles, to investigate. And, instead of gold foil, they threw electrons at liquid hydrogen. Since a hydrogen atom consists of a lone proton orbited by a lone electron, liquid hydrogen is a target-rich environment for those seeking to understand the structure of protons. Using a device called the Continuous Electron Beam Accelerator Facility (CEBAF), the researchers shot a stream of high-speed electrons into their liquid hydrogen, and studied the way that electrons scattered off the quarks inside the protons.

This sort of thing had been done before, with limited success. But the CEBAF has two features that allow far more precise results. First, it has a much higher "luminosity" than previous experiments — meaning that many more electrons are shot at the protons. Second, the electrons it produces all spin in the same direction. This allows much more precise data to be gathered.

And the result? Sometimes protons do, indeed, resemble spheres. But they are constantly changing shape, so they frequently look more like peanuts, or even doughnuts. For nuclear physicists, this is a particularly tasty finding.