Getting up to speed on the proton

Artist rendering of the proton structure, showing its complex and dynamic system made up of quarks & gluons. Credit: Argonne National Laboratories
Scientists have created a revolutionary theory to calculate what happens inside a proton travelling at the speed light.

Scientists believed that the atom was the smallest possible particle for more than 2000 years. They discovered it had a nucleus composed of protons, neutrons and electrons. They discovered that protons and neutrons have complex inner worlds full of quarks, antiquarks, and a superglue-like force that is created by gluons.

"Protons and neutrons make up over 99 percent of visible universe, which means everything from galaxies to stars to us," stated Yong Zhao, a physicist with the U.S. Department of Energy (DOE), Argonne National Laboratory. "But, we still don't know much about the rich inner lives of protons and neutrons.

Zhao co-authored a paper about an innovative method to calculate the quark/gluon structure for a proton travelling at the speed of light. Large-momentum efficient theory (or LaMET) is the name of Zhao's invention. It works in conjunction with a theory called quantum chromodynamics lattice (QCD).

Because the proton is so tiny, it's 100,000 times smaller than an Atom, physicists often consider it a point without dimensions. These new theories are able to predict what's going on within the speed-of light proton, as though it were a three-dimensional body.

LaMET and physics are dependent on momentum. It is the speed of an object multiplied by its mass.

Zhao explained that physicists Murray Gell-Mann (and George Zweig) created a simple quark model of the proton in order to reveal some of its inner structure while it was at rest (no momentum) more than 50 years ago. Scientists pictured the proton consisting of three quarks, and predicted their essential properties such as spin and electric charge.

Later experiments using protons that were accelerated to speeds close to light speed showed that the proton was even more complex than initially thought. It contains many particles that interact with each other, not just three quarks bound to gluons. The gluons can briefly transform into quark-antiquark pair before they become gluons again. These results were mostly produced by particle accelerators such as the DOE Fermi National Accelerator Laboratory.

Zhao stated, "When you accelerate a proton and collide with it with a target that's when magic happens in terms revealing its many mysteries,"

Richard Feynman proposed a model that showed the proton moving at the speed of light, five years after the simple quark model was rejected by the physics community. It was a beam with an infinite number quarks and other gluons traveling in the same direction. These particles were called "partons" by Feynman. Parton's model inspired physicists and helped them to create a list of quantities that describes the 3D proton structure. These quantities could be measured in experiments at particle accelerators.

Some of the details regarding the distributions of quarks, gluons, and protons revealed by earlier calculations using the best theory available at the time (latticeQCD). However, they were unable to distinguish between fast-moving and slow-moving partons.

Lattice QCD was unable to calculate properties of protons that did not depend on their momentum. This created a problem. Feynman's parton method to lattice QCD is difficult because it requires knowledge of the properties of a proton that has infinite momentum. This means that all proton particles must be moving at the speed light. LaMET partially fills that knowledge gap by providing a recipe to calculate the parton physics for large, but finite momentum.

Zhao stated, "We have been refining LaMET for the past eight years." "Our paper summarizes all of this work."

Supercomputers are running lattice QCD calculations using LaMET, which produce new and improved predictions about structure of speed-of-light protons. These predictions will then be tested in a unique facility called the Electron-Ion Collider. The DOE Brookhaven National Laboratory is currently building this facility.

Zhao stated that "Our LaMET can also provide useful information regarding quantities that are extremely difficult to measure." Our predictions may be even more accurate than what is possible at the EIC if we have supercomputers that are powerful enough.

Scientists are now poised to get a better understanding of the 3D quarkgluon structure of matter by using theory and EIC measurements. The new age of parton Physics will soon be upon us.

The article entitled "Large-Momentum Effective Theory" was published in Reviews of Modern Physics. Xiangdong Ji, Yizhuang Liu and Yu-Sheng Liu from the Shanghai Jiao Tong University are also authors. Jian-Hui Zhang is a Beijing Normal University.

Further exploration of GEM simplifies protons' internal structure and their collisions