From The DOE’s Thomas Jefferson National Accelerator Facility
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5.27.24
Researchers have developed a new method that merges experimental data with advanced calculations to explore how gluons contribute to proton spin, revealing complex dynamics and setting the stage for future three-dimensional proton imaging. Credit: SciTechDaily.com
New research combining experimental and computational approaches provides deeper insights into proton spin contributions from gluons.
Nuclear physicists have been tirelessly exploring the origins of proton spin. A novel approach, merging experimental data with cutting-edge calculations, has now illuminated the spin contributions from gluons—the particles that bind protons. This advancement also sets the stage for three-dimensional imaging of the proton structure.
Joseph Karpie, a postdoctoral associate at the Center for Theoretical and Computational Physics (Theory Center) at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility, led this groundbreaking research.
Historical Context
He said that this decades-old mystery began with measurements of the sources of the proton’s spin in 1987. Physicists originally thought that the proton’s building blocks, its quarks, would be the main source of the proton’s spin. But that’s not what they found. It turned out that the proton’s quarks only provide about 30% of the proton’s total measured spin. The rest comes from two other sources that have so far proven more difficult to measure.
One is the mysterious but powerful strong force. The strong force is one of the four fundamental forces in the universe. It’s what “glues” quarks together to make up other subatomic particles, such as protons or neutrons.
Manifestations of this strong force are called gluons, which are thought to contribute to the proton’s spin. The last bit of spin is thought to come from the movements of the proton’s quarks and gluons.
Experimental Challenges and Theoretical Advances
“This paper is sort of a bringing together of two groups in the Theory Center who have been working toward trying to understand the same bit of physics, which is how do the gluons that are inside of it contribute to how much the proton is spinning around,” he said.
He said this study was inspired by a puzzling result that came from initial experimental measurements of the gluons’ spin. The measurements were made at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility based at Brookhaven National Laboratory in New York.
The data at first seemed to indicate that the gluons may be contributing to the proton’s spin. They showed a positive result.
“When they improved their analysis, they started to get two sets of results that seemed quite different, one was positive and the other was negative,” Karpie explained.
While the earlier positive result indicated that the gluons’ spins are aligned with that of the proton, the improved analysis allowed for the possibility that the gluons’ spins have an overall negative contribution. In that case, more of the proton spin would come from the movement of the quarks and gluons, or from the spin of the quarks themselves.
This puzzling result was published by the Jefferson Lab Angular Momentum (JAM) collaboration.
Collaborative Efforts in Proton Spin Analysis
Meanwhile, the HadStruc collaboration had been addressing the same measurements in a different way. They were using supercomputers to calculate the underlying theory that describes the interactions among quarks and gluons in the proton, Quantum Chromodynamics (QCD).
Among the supercomputers used in this work were Bridges at the Pittsburgh Supercomputing Center, Frontera at Texas Advanced Computer Center at UT Austin, resources at NERSC at the DOE’s Lawrence Berkeley National Laboratory, resources of the DOE Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, and Marconi100 at CINECA (Consorzio Interuniversitario per il Calcolo Automatico dell’Italia Nord-orientale)
To equip supercomputers to make this intense calculation, theorists somewhat simplify some aspects of the theory. This somewhat simplified version for computers is called lattice QCD.
Karpie led the work to bring together the data from both groups. He started with the combined data from experiments taken in facilities around the world. He then added the results from the lattice QCD calculation into his analysis.
“This is putting everything together that we know about quark and gluon spin and how gluons contribute to the spin of the proton in one dimension,” said David Richards, a Jefferson Lab senior staff scientist who worked on the study.
“When we did, we saw that the negative things didn’t go away, but they changed dramatically. That meant that there’s something funny going on with those,” Karpie said.
Implications and Future Directions
Karpie is lead author on the study that was recently published in Physical Review D. He said the main takeaway is that combining the data from both approaches provided a more informed result.
“We’re combining both of our datasets together and getting a better result out than either of us could get independently. It’s really showing that we learn a lot more by combining lattice QCD and experiment together in one problem analysis,” said Karpie. “This is the first step, and we hope to keep doing this with more and more observables as well as we make more lattice data.”
The next step is to further improve the datasets. As more powerful experiments provide more detailed information on the proton, these data begin painting a picture that goes beyond one dimension. And as theorists learn how to improve their calculations on ever-more powerful supercomputers, their solutions also become more precise and inclusive.
The goal is to eventually produce a three-dimensional understanding of the proton’s structure.
“So, we learn our tools do work on the simpler one-dimension scenario. By testing our methods now, we hopefully will know what we need to do when we want to move up to do 3D structure,” Richards said. “This work will contribute to this 3D image of what a proton should look like. So it’s all about building our way up to the heart of the problem by doing this easier stuff now.”
See the full article here .
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The DOE’s Thomas Jefferson National Accelerator Facility is supported by The Office of Science of the U.S. Department of Energy. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE Applied Technologies, manages and operates the Thomas Jefferson National Accelerator Facility for the U.S. Department of Energy’s Office of Science.
History
The DOE’s Thomas Jefferson National Accelerator Facility was established in 1984 (first initial funding by the Department of Energy) as the Continuous Electron Beam Accelerator Facility (CEBAF); the name was changed to Thomas Jefferson National Accelerator Facility in 1996. The full funding for construction was appropriated by US Congress in 1986 and on February 13, 1987, the construction of the main component, the CEBAF accelerator begun. First beam was delivered to experimental area on 1 July 1994. The design energy of 4 GeV for the beam was achieved during the year 1995. The laboratory dedication took place 24 May 1996 (at this event the name was also changed). Full initial operations with all three initial experiment areas online at the design energy was achieved on June 19, 1998. On August 6, 2000 the CEBAF reached “enhanced design energy” of 6 GeV. In 2001, plans for an energy upgrade to 12 GeV electron beam and plans to construct a fourth experimental hall area started. The plans progressed through various DOE Critical Decision-stages in the 2000s decade, with the final DOE acceptance in 2008 and the construction on the 12 GeV upgrade beginning in 2009. May 18, 2012 the original 6 GeV CEBAF accelerator shut down for the replacement of the accelerator components for the 12 GeV upgrade. 178 experiments were completed with the original CEBAF.
In addition to the accelerator, the laboratory has housed and continues to house a free electron laser (FEL) instrument. The construction of the FEL started 11 June 1996. It achieved first light on June 17, 1998. Since then, the FEL has been upgraded numerous times, increasing its power and capabilities substantially.
Jefferson Lab was also involved in the construction of the Spallation Neutron Source (SNS) at DOE’s Oak Ridge National Laboratory . Jefferson built the SNS superconducting accelerator and helium refrigeration system. The accelerator components were designed and produced 2000–2005.
Accelerator
The laboratory’s main research facility is the CEBAF accelerator, which consists of a polarized electron source and injector and a pair of superconducting RF linear accelerators that are 7/8-mile (1400 m) in length and connected to each other by two arc sections that contain steering magnets.
As the electron beam makes up to five successive orbits, its energy is increased up to a maximum of 6 GeV (the original CEBAF machine worked first in 1995 at the design energy of 4 GeV before reaching “enhanced design energy” of 6 GeV in 2000; since then, the facility has been upgraded into 12 GeV energy). This leads to a design that appears similar to a racetrack when compared to the classical ring-shaped accelerators found at sites such as The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH)[CERN] or DOE’s Fermi National Accelerator Laboratory. Effectively, CEBAF is a linear accelerator, similar to The DOE’s SLAC National Accelerator Laboratory at Stanford University, that has been folded up to a tenth of its normal length.
The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring-shaped accelerators. (There is some beam structure, but the pulses are very much shorter and closer together.) The electron beam is directed onto three potential targets (see below). One of the distinguishing features of Jefferson Lab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is Jefferson Lab’s use of superconducting Radio Frequency (SRF) technology, which uses liquid helium to cool niobium to approximately 4 K (−452.5 °F), removing electrical resistance and allowing the most efficient transfer of energy to an electron. To achieve this, Jefferson Lab houses the world’s largest liquid helium refrigerator, and it was one of the first large-scale implementations of SRF technology. The accelerator is built 8 meters below the Earth’s surface, or approximately 25 feet, and the walls of the accelerator tunnels are 2 feet thick.
The beam ends in four experimental halls, labelled Hall A, Hall B, Hall C, and Hall D. Each hall contains specialized spectrometers to record the products of collisions between the electron beam or with real photons and a stationary target. This allows physicists to study the structure of the atomic nucleus, specifically the interaction of the quarks that make up protons and neutrons of the nucleus.
With each revolution around the accelerator, the beam passes through each of the two LINAC accelerators, but through a different set of bending magnets in semi-circular arcs at the ends of the linacs. The electrons make up to five passes through the linear accelerators.
When a nucleus in the target is hit by an electron from the beam, an “interaction”, or “event”, occurs, scattering particles into the hall. Each hall contains an array of particle detectors that track the physical properties of the particles produced by the event. The detectors generate electrical pulses that are converted into digital values by analog-to-digital converters (ADCs), time to digital converters (TDCs) and pulse counters (scalers).
This digital data is gathered and stored so that the physicist can later analyze the data and reconstruct the physics that occurred. The system of electronics and computers that perform this task is called a data acquisition system.
12 GeV upgrade
As of June 2010, construction began on a $338 million upgrade to add an end station, Hall D, on the opposite end of the accelerator from the other three halls, as well as to double beam energy to 12 GeV. Concurrently, an addition to the Test Lab, (where the SRF cavities used in CEBAF and other accelerators used worldwide are manufactured) was constructed.
As of May 2014, the upgrade achieved a new record for beam energy, at 10.5 GeV, delivering beam to Hall D.
As of December 2016, the CEBAF accelerator delivered full-energy electrons as part of commissioning activities for the ongoing 12 GeV Upgrade project. Operators of the Continuous Electron Beam Accelerator Facility delivered the first batch of 12 GeV electrons (12.065 Giga electron Volts) to its newest experimental hall complex, Hall D.
In September 2017, the official notification from the DOE of the formal approval of the 12 GeV upgrade project completion and start of operations was issued. By spring 2018, all fours research areas were successfully receiving beam and performing experiments. On 2 May 2018 the CEBAF 12 GeV Upgrade Dedication Ceremony took place.
As of December 2018, the CEBAF accelerator delivered electron beams to all four experimental halls simultaneously for physics-quality production running.