Jiji Fan

Assistant Professor of Physics
Jiji Fan
Assistant Professor of Physics
Photo: Mike Cohea/Brown University
In its first run, the Large Hadron Collider found evidence for the Higgs boson. Now it’s running at almost twice the energy. Jiji Fan is developing theories that will suggest how to find answers to remaining fundamental questions about the universe.

As a particle theorist, Jiji Fan is working to address some of the deepest questions in nature.

“We have a theory called the standard model which pretty successfully describes most of the fundamental particles and how they interact in experiments, but it’s not sufficient to address many fundamental questions about the universe,” said Fan, assistant professor of physics. “So my research is to propose theories beyond the standard model to address those questions.”

And now is a great time to be in this particular line of work, she says.

The Large Hadron Collider (LHC), the world’s most powerful particle accelerator, restarted earlier this year at nearly twice the energy of its first run from 2010 to 2013. That first run turned up the elusive Higgs boson, the particle that gives mass to some elementary particles and the final missing piece of the standard model. The higher energy regime of this current LHC run will be better suited to peering behind the standard model’s curtain.

“The discovery of the Higgs boson completes the standard model, but we know there has to be something else beyond the standard model,” Fan said.

For one thing, she says, the standard model fails to account for dark matter, which is thought to make up as much as 80 percent of the matter in the universe. Scientists know dark matter is there because they see its gravity affecting the rotation of galaxies and the way light travels through the universe. But no one has ever detected it directly, and the standard model has no candidate for what a dark matter particle might be.

The standard model also fails to adequately explain the mass of the Higgs boson. While the Higgs helps explain why the building blocks of matter (quarks and leptons) have mass, the Higgs’ own mass is a conundrum. The Higgs should gain mass from all of the other particles with which it interacts. At high energies, that should make its mass practically infinite. Yet when detected at the LHC, its mass was 125 gigaelectronvolts (about 125 more massive than a proton).

Fan is particularly interested in supersymmetry, a theory that may help explain these questions and others. The idea behind supersymmetry is that the known particles each have shadowy, unseen cousins with slightly different properties. These “sparticles” partially cancel out the interaction of known particles with the Higgs, thus explaining its seemingly small mass. Supersymmetry also proposes candidates for a dark matter particle.

Supersymmetry may also help explain other fundamental questions, like why there’s more matter in the universe than antimatter and why gravity is so weak compared to the other forces that govern the universe.

Fan’s job as a theorist is to try to understand how these theoretical particles might manifest themselves at the LHC.

“Supersymmetry is a good theoretical signature generator,” she said. “Different supersymmetric scenarios could predict different experimental signatures. We think about how the supersymmetric particles would decay and what their final states might be. Then we think about how to search for them at the LHC.”

That means she’ll be working closely with Brown’s experimental physicists who work with the CMS experiment, one of two large-scale experiments going at the LHC. She’ll also be working with Brown physicists involved in the search for dark matter.

“Brown has a fantastic CMS group,” she said. “There are also people working on direct and indirect detection of dark matter. I’m quite excited to work with all of them.”

Fan comes to Brown from Syracuse University, where she was an assistant professor. She has a Ph.D. from Yale and worked at Princeton and Harvard as a postdoctoral researcher. She says she’s looking forward to her new academic home.

“I’ve heard a lot about the great academic environment in the physics department and throughout the University,” she said. “I’m looking forward to being part of that.”