Astronomers estimate that more than 80 percent of the universe’s mass is made up of dark matter, which consists of invisible particles that give off no measurable energy except when they collide with one another in exceedingly weak explosions. What are these particles? That’s a big unanswered question, and one that has captivated scientists around the world for decades. One of these scientists is Harvey Mudd College’s Brian Shuve, assistant professor of physics. He recently received a National Science Foundation grant to explore the vast mysteries of dark matter particles, and he is taking several students along for the ride.
Shuve is particularly interested in hidden sectors, an intriguing area of dark matter research. Hidden sectors fall outside of the established ingredients of nature, such the well-known particles protons, electrons and neutrons. The theory of these known particles is called the Standard Model and describes how these particles bounce off one another due to forces. Hidden sectors are something else entirely: They contain particles and forces, like dark matter, that aren’t found in the Standard Model. Shuve says, “This suggests that dark matter may hold clues about new types of particles and forces that exist in our world.”
Finding the Hidden
Part of Shuve’s research focuses on idea generation, what he describes as “playing around with what these dark-matter particles might be doing and how they might fit into the big picture of how the universe works,” he says. “For instance, if you postulate a new particle or a new force, you have to also do the calculations and simulations to see whether that would disrupt the way that galaxies are formed or would change the universe to something that would be unrecognizable.”
The other part of his project involves taking the most promising of the dark particle possibilities and figuring out how to find evidence of those particles. That’s tricky. To study them, Shuve explains, scientists actually try to make dark particles by using particle accelerators. These are huge machines that hurl protons at one another at very high energies. Every now and again, the high-energy collisions yield new particles, some of which just might be dark-matter particles.
Unfortunately, scientists cannot simply scoop out and study these accelerator-made particles, because they are not only hard to detect but they can also disappear as fast as they’re made. That means the only way to tell they were ever even there is to look for imprints they leave in the debris from these collisions. It’s a difficult task considering that a single accelerator can cause trillions of collisions per year, but only a handful of them may result in hidden-sector particles. “We’re talking about a needle-in-the-haystack, but to the extreme,” Shuve says. To that end, he worked with three summer students to use new ideas to hunt for evidence for hidden-sector particles in real experimental data at the Stanford Linear Accelerator and the Large Hadron Collider. “I had two freshmen and one sophomore working with me this past summer, and they all made really impressive progress,” he says. Two of the students developed and used software to conduct a preliminary search for hidden particles in a recently released public dataset from the CMS Experiment at the LHC, while another finalized aspects of a study to search for hidden particles in data from the BABAR Experiment at SLAC.
For now, the search continues, and Shuve, a 2018 Kavli Institute for Theoretical Physics Scholar, is thrilled to be a part of it.
“We’re at a turning point in particle physics, where we’re really blowing open our ideas about what dark matter could be,” he says. “This is really the modern equivalent of being an explorer and going to uncharted land: We’re asking questions about why we exist the way we do and why the universe looks the way it does, and that’s very exciting.”