Deep inside what we perceive as solid matter, the landscape is anything but stationary. The inside of the constructing blocks of the atom’s nucleus — particles called hadrons that a highschool student would recognize as protons and neutrons — are made up of a seething mixture of interacting quarks and gluons, known collectively as partons.
A gaggle of physicists has now come together to map out these partons and disentangle how they interact to form hadrons. Based on the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and often known as the HadStruc Collaboration, these nuclear physicists have been working on a mathematical description of the interactions of partons. Their latest findings were recently published within the Journal of High Energy Physics.
“The HadStruc Collaboration is a bunch based out of the Jefferson Lab Theory Center and among the nearby universities,” said HadStruc member Joseph Karpie, a postdoctoral researcher in Jefferson Lab’s Center for Theoretical and Computational Physics. “We now have some people at William & Mary and Old Dominion University.”
Other collaboration members who’re co-authors on the paper are Jefferson Lab scientists Robert Edwards, Colin Egerer, Eloy Romero and David Richards. The William & Mary Department of Physics is represented by Hervé Dutrieux, Christopher Monahan and Kostas Orginos, who also has a joint position at Jefferson Lab. Anatoly Radyushkin can also be a Jefferson Lab joint faculty member affiliated with Old Dominion University, while Savvas Zafeiropoulos is at Université de Toulon in France.
A Strong Theory
The components of hadrons, called partons, are sure together by the strong interaction, one among the 4 fundamental forces of nature, together with gravity, electromagnetism and the weak force, which is observed in particle decay.
Karpie explained that the members of the HadStruc Collaboration, like many theoretical physicists worldwide, try to find out where and the way the quarks and gluons are distributed inside the proton. The group uses a mathematical approach often known as lattice quantum chromodynamics (QCD) to calculate how the proton is constructed.
Dutrieux, a post-doctoral researcher at William & Mary, explained that the group’s paper outlines a three-dimensional approach to understanding the hadronic structure through the QCD lens. This approach was then carried out via supercomputer calculations.
The 3D concept is predicated on the notion of generalized parton distributions (GPDs). GPDs offer theoretical benefits over the structures as visualized through one-dimensional parton distribution functions (PDFs), an older QCD approach.
“Well, the GPD is significantly better within the sense that it permits you to elucidate one among the massive questions we now have in regards to the proton, which is how its spin arises,” Dutrieux said. “The one-dimensional PDF gives you a really, very limited picture about that.”
He explained that the proton consists in a primary approximation of two up quarks and one down quark — often known as valence quarks. The valence quarks are mediated by a variable roster of gluons spawned from strong force interactions, which act to attach the quarks together. These gluons, in addition to pairs of quarks-antiquarks — often denoted because the sea of quarks-antiquarks when distinguishing them from the valence quarks — are continually being created and dissolving back into the strong force.
Considered one of the stunning realizations on the proton’s spin got here in 1987, when experimental measurements demonstrated that the spin of quarks contributes to lower than half of the general spin of the proton. Actually, a variety of the proton’s spin could arise from the gluon spin and the motion of partons in the shape of orbital angular momentum. Numerous experimental and computational effort continues to be needed to make clear this example.
“GPDs represent a promising opportunity to access this orbital angular part and produce a firmly grounded explanation of how the proton’s spin is distributed amongst quarks and gluons,” Dutrieux noted.
He went on to say that one other aspect that the collaboration hopes to handle through GPDs is an idea often known as the energy momentum tensor.
“The energy momentum tensor really tells you the way energy and momentum are distributed inside your proton,” Dutrieux said. “They inform you how your proton interacts with gravity as well. But straight away, we’re just studying its distribution of matter.”
Simulating the Data
As mentioned, accessing this information requires some sophisticated calculations on supercomputers. After developing their latest approach, the theorists then conducted 65,000 simulations of the speculation and its assumptions to check it out.
This tremendous variety of calculations was performed on Frontera on the Texas Advanced Computer Center and the Frontier supercomputer at Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility at Oak Ridge National Laboratory. This number included 186 simulations of protons moving with different momenta carried out within the background of 350 randomly generated collections of gluons. This calculation required the processors at these facilities to collectively run for tens of millions of hours. Final evaluation of those results is accomplished on the smaller supercomputers at Jefferson Lab.
The upshot of this work was a strong test of the 3D approach developed by the theorists. This test is a crucial milestone result for DOE’s Quark-Gluon Tomography (QGT) Topical Collaboration.
“This was our proof of principle. We desired to know if the outcomes from these simulations would look reasonable based on what we already learn about these particles,” said Karpie. “Our next step is to enhance the approximations we utilized in these calculations. That is computationally 100 times dearer by way of computing time,” Karpie said.
Recent Data on the Horizon
Karpie identified that the HadStruc Collaboration’s GPD theory is already being examined in experiments at high-energy facilities worldwide. Two processes for examining hadron structure through GPDs, deeply virtual Compton scattering (DVCS) and deeply virtual meson production (DVMP), are being conducted at Jefferson Lab and other facilities.
Karpie and Dutrieux expect the group’s work to be on the slate of experiments on the Electron-Ion Collider (EIC), a particle accelerator being built at DOE’s Brookhaven National Laboratory on Long Island. Jefferson Lab has partnered with Brookhaven National Laboratory on the project.
It’s expected that the EIC might be powerful enough to probe hadrons beyond the purpose at which today’s instruments begin to lose signal, however the exploration of the structure of how hadrons are assembled won’t be waiting for the EIC to return online.
“We now have some latest experiments at Jefferson Lab. They’re collecting data now and giving us information for comparing to our calculations,” Karpie said. “After which we hope to find a way to accumulate and get even higher information on the EIC. It’s all a part of this progress chain.”
The HadStruc Collaboration members are looking toward additional experimental applications of their QCD theory work at Jefferson Lab and other facilities. An example of that is using supercomputers to calculate more precise results of knowledge which were in hand for many years.
Karpie added that he hopes to get a few steps ahead of the experimenters.
“QCD has all the time lagged behind experiments. We were often post-dicting as an alternative of pre-dicting what things are happening,” Karpie said. “So, now if we are able to actually get ahead — if we are able to do something that the experimenters cannot do yet — that may be pretty cool.”
A portion of this work was supported by Jefferson Lab’s Lab Directed Research & Development program. The LDRD program devotes a small portion of the lab’s research effort to supporting initiatives which are on the forefront of science and technology relevant to the DOE mission.
