For several decades, the nuclear science community has been calling for a new type of particle collider to pursue – in the words of one report – “a new experimental quest to study the glue that binds us all.” This glue, the mediator of subatomic particle interactions within atomic nuclei, is responsible for most of the visible universe’s matter and mass.
To learn about this glue, scientists are proposing a unique, high-energy collider that smashes accelerated electrons into charged atomic nuclei (ions) or protons, which carry a positive charge.
While the world’s largest accelerator – the Large Hadron Collider (LHC) at CERN in Europe – is intensely focused on seeking discoveries beyond the standard model of particle physics, this electron-ion collider, or EIC, will help nuclear physicists better understand how standard particles create the constituent particles that, in turn, make up atomic nuclei – the building blocks of matter.
Teams of scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) can be counted among the more than 800 researchers around the world who have been engaged in laying the scientific groundwork for the collider from its earliest stages to the present.
They have drawn upon a long history of Berkeley Lab R&D for particle accelerators and colliders, including expertise in working with electron beams and ion beams. Berkeley Lab’s inception stems from founder Ernest Lawrence’s development of accelerators called cyclotrons, and the Lab continues to drive innovation in accelerator R&D today.
Last year, two longtime Berkeley Lab scientists participated in a study by the private, nonprofit National Academy of Science. The study, “An Assessment of U.S.-Based Electron-Ion Collider Science,” concluded that the EIC is “central to completing our understanding of atomic nuclei,” would build on existing expertise in accelerator science and technology, and “keep the U.S. at the forefront of new collider technologies.”
The EIC would function as the world’s largest electron microscope to peer within atomic nuclei. The proposed electron-ion collider would require very high luminosity (the ability to focus a large number of particles into a very small area repeatedly over time), a range of polarized (custom-oriented) beams, and a range of beam-collision energies. The collisions would allow scientists to piece together multidimensional images of the constituents of nucleons and nuclei, conceptually similar to medical scans known as MRIs (magnetic resonance imaging), the NAS report notes.
Such a research tool, the report stated, could reveal the mechanisms by which gluons create most of the visible matter in the universe. Gluons are particles in nuclei that are exchanged among other particles, called quarks, to form protons, neutrons, and atomic nuclei. The binding energy of the gluons is responsible for most of the mass of familiar matter, but it’s not clearly understood how this particle glue works.
Ernst Sichtermann, a senior scientist in Berkeley Lab’s Nuclear Science Division, participated in the NAS committee that prepared the report, as did Wick Haxton, a theoretical physicist at Berkeley Lab and a physics professor at UC Berkeley. Sichtermann was also involved in an EIC community working group that prepared a 2007 white paper, “A High-Luminosity, High-Energy Electron-Ion Collider,” and in other efforts that have studied the scientific merits of the proposed EIC.
“The science drivers have been clearly identified,” Sichtermann said. “This is a unique facility that will push the envelope as never before.”
Despite progress over the years in fine-tuning nuclear science experiments to tease out more and better data, and increasingly powerful and sophisticated computer simulations and analysis tools, scientists are still puzzled by what goes on in the hearts of atoms, Haxton said.
“We don’t know what we don’t know,” he said. “We’re just not satisfied. We need to look where we haven’t looked before, and that’s how we learn. Our overall goal is to understand the structure of real matter that we manipulate every day.”
There is still no formal commitment or chosen location to build an electron-ion collider, though Berkeley Lab researchers do contribute to ongoing research at the world’s leading particle collider experiments and via a list of in-house studies that are relevant to EIC science:
- In December 2018, the University of California announced support for a research program at Berkeley Lab titled, “The Science of Dense Gluon Plasma,” led by Berkeley Lab Nuclear Science Division Director Barbara Jacak. The effort is one of just 16 research programs approved from a pool of 179 applications.
- Also in December, Feng Yuan of the Lab’s Nuclear Science Division received support through a Laboratory Directed Research and Development (LDRD) program to study “Theoretical Challenges for Electron-Ion Collider Physics.” Yuan and Sichtermann will co-chair an EIC conference later this year.
- Spencer Klein, a scientist in the Lab’s Nuclear Science Division, earlier received an LDRD to pursue EIC R&D, including simulations and accelerator physics studies. And Jacak and Sichtermann are pursuing EIC detector R&D.
- GianLuca Sabbi, a scientist in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division, has also been involved in several EIC R&D efforts. One of these projects, initially supported in part by the LDRD program, focused on new accelerator ring concepts and particle-colliding regions and is now supported by the DOE.
- In a separate effort, Ji Qiang, a senior scientist in ATAP, has worked for the past two years on beam simulations to help validate a design for an electron-ion collider, and leads Berkeley Lab’s effort in a multi-laboratory collaboration to study colliding beam effects. “Working with collaborators, we have demonstrated that the high luminosity in the proposed EIC is achievable,” Qiang said, and the research team also discovered a potential problem with beam performance that required a workaround in the design. “In our next study, we will continue to improve the computational tools with a new algorithm to increase the simulation accuracy, add new capabilities, and help validate the EIC beam dynamics design,” he said.
Haxton said that theoretical expertise in nuclear science, accelerator and beam physics, detector development, data analysis, and in machine-learning methods could also aid the electron-ion collider effort. Berkeley Lab is among the institutions that maintain these capabilities. He noted that it will take a large collaboration to make the collider a reality.
“There are a lot of talented groups around the country, and their strong cooperation will be needed to construct such a machine and associated detectors optimally, and on time,” he said.