Friday, Sept. 4, the College of William and Mary’s School of Computing, Data Sciences and Physics hosted Dave Mack for its first colloquium this semester, titled “A New (Sideways) Look at Bhabha Scattering.” The event addressed developments in particle accelerator research.
Mack currently studies experimental nuclear physics at the Thomas Jefferson National Accelerator Facility, or JLab. He will be joining the College’s physics department as an external affiliate.
Wilson & Martha Claiborne Stephens Associate Professor of Physics Justin Stevens introduced Mack.
“He’s been a collaborator of mine for many years, even before I came to William and Mary,” Stevens said. “We’re looking forward to having him.”
JLab is currently home to the Continuous Electron Beam Accelerator Facility, or CEBAF. The massive racetrack-shaped accelerator lets scientists shoot electrons at near the speed of light and study their collisions with tiny atomic targets. The facility provides scientists with highly precise electron beams for experiments exploring the structure of matter.
Mack hopes JLab can move beyond electrons and supplement this research with a polarized positron beam. If built, it will be the first in the United States.
“A lot of my colleagues are looking for interesting effects that you can only see by comparing electron beams scattering with positron beams scattering,” Mack said.
Unlike an electron, which carries a negative electric charge, a positron is its antimatter counterpart and carries a positive charge. Producing these in large enough quantities, and with the desired polarization, is a significant challenge.
The positron beam would, in theory, start with a high-energy polarized electron beam, send those electrons into a tungsten target, produce polarized photons through electromagnetic interactions, use those photons to create electron-positron pairs, select the polarized positrons and finally accelerate them into a usable beam.
The beam, Mack explained, would allow JLab to study Bhabha scattering, the process that occurs when positrons and electrons scatter off of each other. The reaction is one of the simplest interactions physicists can study involving an electron and a positron. Its simplicity makes it particularly useful for searching for subtle effects that do not fit the predictions of the Standard Model of particle physics.
These deviations could lead to discoveries in dark matter.
Dark matter is, in theory, invisible, but could be observed through small irregularities in gravity.
Mack pointed to galactic rotation curves as an example of these irregularities.
“They looked at a complex system,” he said. “They applied known physical laws to make sure that they understood, and nothing made sense.”
Mack briefly covered alternative explanations for these gravitational irregularities, including modified theories of gravity. Regardless, he plans to continue assuming that dark matter exists.
“Experimental science is a long game,” said Mack, pointing to papers and theories published before he was born, which still provide “breadcrumbs” for his research. Many of these theories were unprovable before. Now, technology is catching up.
To Mack, these discoveries are as nebulous as they are exciting.
“Once you’re a senior physicist, like after age 50, 55, then every few weeks you wake up in the morning and you pinch yourself, realizing the universe is so weird that we have quarks of fractional charge thirds or two-thirds — but only where you can never see them,” he said. “We are clearly being trolled by the gods here.”
Mack was candid about the timeline. He estimated that the injector alone would take at least several years of research and development and that a full experimental program could be at least a decade away.
JLab faces several engineering challenges. Mack said the beam would generate enormous amounts of heat when it strikes the target. Even after producing the particles, researchers would still need to overcome the obstacle of getting them into the accelerator.
Despite this, Mack believes the beam would create the opportunity for completely new physics, as well as an exciting new life for JLab.
JLab’s hurdles extend beyond engineering difficulties, however. According to Mack, conducting science today is extraordinarily difficult.
“There are funding cuts in the research grants, there’s RIFs at national laboratories, there’s potential for final political appointee review of grants, legal challenges, scholarships, difficulties and impossibility of renewing visas, the $100K fee for H-1B visas,” he said. “How can universities survive that? And recently, the banning of our treasured mainland Chinese colleagues from the national lab, which is just going to be devastating for some of the Jefferson Lab process.”
Mack also expressed concern about the U.S. Department of Energy’s goal to make the lab a “multi-purpose laboratory.” He said he believes this is the wrong direction.
Derek Holmberg, a graduate student working at JLab, has similar concerns. He said he worries that the DOE and the Trump administration will want the lab to focus on commercial applications.
“It’s just kind of a backwards way of approaching it,” Holmberg said.
He added that most practical applications are found along the road to pure scientific discovery and that most technical applications of technology cannot be predicted from the outset.
Despite the uncertainty, Mack is hopeful.
“It’s a great idea,” he said. “It’s good physics. I haven’t heard of a better idea for Jefferson Lab’s future.”
