Excited protons give clues to building blocks of matter

An experiment to probe the resonances of protons and neutrons has given deeper insight into the building blocks of matter which formed in the early universe.

Physicists at the US Department of Energy’s Thomas Jefferson National Accelerator Facility, explored the three-dimensional structure of “resonating protons” and have just published the findings in Physical Review Letters.

Much is known about the 3D structure of protons in their ground (non-excited) state, but physicists don’t know much about their structure when resonating.

Protons and neutrons, which make up the nuclei of atoms, are called “nucleons.” Each nucleon is actually made up of 3 quarks – elementary particles – tightly bound together by gluons, the particles which transmit the strong force.

In the 1960’s it was discovered that nucleons resonated, like a ringing bell.  These resonances are “excited states” of the nucleons which usually emerge due to a flipping of the spin state of one quark within the nucleon, or a change in the nucleon’s orbit when it undergoes a decay process.

When quarks rotate and vibrate against each other, the nucleon enters its excited state and resonates.

“This is the first time we have some measurement, some observation, which is sensitive to the 3D characteristics of such an excited state,” says lead author and researcher Stefan Diehl, a postdoctoral researcher at the Physics Institute at Justus Liebig Universitat Giessen, Germany, and a research professor at the University of Connecticut. “In principle, this is just the beginning, and this measurement is opening a new field of research.”

The experiment, conducted at Jefferson Lab’s CLAS12 detector, involved sending a high-energy electron beam into a chamber of cooled hydrogen gas.

Electrons excited the quarks inside the protons of the hydrogen nuclei to produce nucleon resonance and a quark-antiquark state called a meson.

These resonances could be picked up by the detector in the form of the popping in and out of existence of exotic particles.

“In the beginning, the early cosmos only had some plasma consisting of quarks and gluons, which were all spinning around because the energy was so high,” Diehl explains. “Then, at some point, matter started to form, and the first things that formed were the excited nucleon states. When the universe expanded further, it cooled down and the ground state nucleons manifested.”

“With these studies, we can learn about the characteristics of these resonances. And this will tell us things about how matter was formed in the universe and why the universe exists in its present form.”