Scientists are developing a revolutionary theory to calculate what happens inside a proton traveling at the speed of light.
For more than 2,000 years, scientists believed that the atom was the smallest particle possible. Then they discovered that it had a nucleus made up of protons and neutrons surrounded by electrons. After that, they discovered that the protons and neutrons themselves have a complex inner world filled with quarks and antiquarks held together by a superglue-like force created by the gluons.
“Protons along with neutrons make up over 99% of the visible universe, which means everything from galaxies and stars to us,” said Yong Zhao, a physicist at the US Department of Energy’s Argonne National Laboratory. (DOE). “Yet there is still a lot we don’t know about the rich inner life of protons or neutrons.”
Zhao co-wrote an article on an innovative method for calculating the structure of quarks and gluons of a proton moving at the speed of light. The name of the team’s creation is the Large Pulse Efficient Theory, LaMET for short, which works in conjunction with a theory called Lattice Quantum Chromodynamics (QCD).
The proton is tiny – about 100,000 times smaller than an atom, so physicists often model it as a point without dimensions. But these new theories can predict what happens in the proton by the speed of light as if it were a three-dimensional body.
The concept of momentum is vital not only for LaMET but also for physics in general. It is equal to the speed of an object multiplied by its mass.
More than half a century ago, says Zhao, a simple quark model by physicists Murray Gell-Mann and George Zweig discovered part of the internal structure of the proton at rest (no momentum). From this model, the scientists represented the proton as composed of three quarks and predicted their essential properties, such as electric charge and spin.
Subsequent experiments with protons accelerated to a speed close to the speed of light demonstrated that the proton is even more complex than originally thought. For example, it contains countless particles that interact with each other, not just three quarks linked by gluons. And gluons can briefly transform into quark-antiquark pairs before destroying each other and reverting to gluon. Particle accelerators like the one at DOE’s National Fermi Accelerator Laboratory produced most of these results.
“When you accelerate the proton and hit it with a target, that’s when the magic happens to reveal its many mysteries,” Zhao said.
About five years after the simple quark model rocked the physics community, a model proposed by Richard Feynman represented the proton moving at near light speed as a beam carrying an infinite number of quarks and gluons moving through the same direction. He called these particles “let’s go.” His parton model inspired physicists to define a set of quantities that describe the 3D structure of the proton. Researchers could then measure these amounts in experiments in particle accelerators.
Previous calculations with the best theory available at the time (lattice CQD) provided illuminating details on the distribution of quarks and gluons in the proton. But they had a serious flaw: they could not accurately distinguish between fast and slow partons.
The difficulty was that the lattice QCD could only calculate the properties of the proton that do not depend on its momentum. But applying Feynman’s parton model to lattice QCD requires knowing the properties of a proton with infinite momentum, which means that the proton particles all have to move at the speed of light. Partially filling this knowledge gap, LaMET provides a recipe for calculating parton physics from on-lattice QCD for large but finite momentum.
“We have developed and refined LaMET over the past eight years,” Zhao said. “Our article summarizes this work.”
Running on supercomputers, networked QCD calculations with LaMET generate new and improved predictions about the proton structure of the speed of light. These predictions can then be put to the test in a unique new facility called an electron-ion collider (EIC). This facility is under construction at the DOE’s Brookhaven National Laboratory.
“Our LaMET can also predict useful information about extremely difficult to measure quantities,” Zhao said. “And with sufficiently powerful supercomputers, in some cases our predictions might even be more accurate than can be measured at the EIC.”
With a better understanding of the 3D quark-gluon structure of matter using EIC theory and measurements, scientists are ready to get a much more detailed picture of the proton. We will then enter a new era of parton physics.