Charles Epstein

Massachusetts Institute of Technology

When Charles Epstein was a boy, his parents often put him the back seat with a screwdriver and a broken or outdated appliance when the family went on long car trips. As the miles went by, he would disassemble the gadget, learning something about how it worked. “Years later, we would find screws tucked into the folds of the seats,” he says.

Epstein, a Department of Energy National Nuclear Security Administration Stewardship Science Graduate Fellowship (DOE NNSA SSGF) recipient, still wants to know how things work. He focuses on the fundamental physics of electrons and other subatomic particles, but he also likes building and tuning hardware designed to test those concepts. His doctoral research with Massachusetts Institute of Technology Physics Professor Richard Milner lets him do both.

Epstein directs an experiment designed to better understand and measure what happens when electrons collide and scatter, generating radiation – often a photon of light. “This is really simple and it should be relatively straightforward to understand, but it turns out it gets really complicated really fast,” Epstein says.

The process, called Møller scattering, is key to understanding experiments that track how nuclei react when an electron beam strikes them. Physicists want to measure how the electron interacts with the nucleus, but it also interacts with other electrons in the atom. Researchers compensate for this, but “we really don’t understand that normalization process as well as we should.”

The problem is the electron mass. Since it’s among the lightest subatomic particles, physicists can ignore mass in experiments that accelerate electrons to high energies. But mass becomes more important in high-precision, low-energy experiments.

Such trials are coming into play as physicists probe previously ignored regimes. Machines like one at Virginia’s Thomas Jefferson National Accelerator Facility produce beams with energies of around 100 megaelectron volts (MeV), compared to the 12 gigaelectron volts (GeV) produced at the lab’s main accelerator. “It’s a hundred times less energy,” Epstein says. “You get a lot of electrons at relatively low energy and you try to measure things really precisely.” Experiments there, such as MIT’s DarkLight project, could probe dark matter’s characteristics, a proton’s size, or other fundamental properties.

Epstein first built event-generator software that simulates electron-electron and positron-electron scattering while accounting for electron mass. It predicts what detectors will record from the dispersed particles. The program “combines different calculations. Some of them were really old and we had to research new ones, and some of them we had to recalculate ourselves.”

But the team needs an experiment to check the code’s results. Working at MIT’s High Voltage Research Laboratory, the group will send electrons into a carbon target. The researchers will use complex spectrometers to measure the energies and angles of electrons the collisions knock loose. The results will help them more precisely pin down the effects of the electron mass at low energies.

Epstein: DarkLight

Building and tuning the experimental apparatus has been arduous. With low-energy electrons, “if they hit any material, they’re going to scatter,” Epstein says, so the beam must connect to the target and detectors completely under vacuum. The team also had to design a new, highly sensitive detector, just a millimeter thick. The electrons “don’t deposit much energy because they don’t have much energy to begin with.”

The project’s demands have led Epstein to program his own software for data analysis and other purposes. He’s even written code to help estimate gas flow and pumping power required for the near-vacuums the experiment requires. The team uses a beam at the MIT lab, but eventually will run the experiment on the Jefferson facility’s low-energy device.

Collisions of a different sort occupied Epstein during his 2016 Lawrence Livermore National Laboratory practicum: ones between asteroids and Earth. With planetary defense researcher Paul Miller and postdoctoral fellow Tane Remington, he helped improve simulations of how a space rock might fragment when struck by an object designed to destroy or deflect it. “You don’t want to break it into two pieces and then hit two cities instead of just one,” Epstein adds.

Using data from rock impact experiments, Epstein studied parameters that describe the target rock – or asteroid – including its cracks, flaws or stresses. “We’re using what we saw in the (experimental) data to tune parameters in the simulation to make it more accurate, or at least to understand how they affect the results.”

Epstein expects to graduate in 2018. Beyond that, he’s only certain he wants to use the skills he’s developed, perhaps in fundamental nuclear physics research, stewardship science or industry.

Image caption: The Radiative Møller experiment, after construction at the MIT Bates Laboratory. Image courtesy of Charles Epstein.