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Shining Light on Dark Matter

Stefania Gori visited CERN, one of the world’s most sophisticated laboratories, as a high school student when its Large Hadron Collider was just a hole in the ground hundreds of feet below Geneva, Switzerland.

Today, the University of Cincinnati theoretical physicist is inspiring experiments there with her own ideas about her lifelong research subject: dark matter.

“Visiting CERN played a big role for me in deciding to push for particle physics as my career,” she said. “It’s a dream to imagine a future involved in these big experiments and active research.”

Most of the known universe is made up of dark matter. Nobody has observed it (at least, not yet) but scientists are confident dark matter is out there influencing planets, stars and galaxies.

Gori, an assistant professor of physics in UC's McMicken College of Arts & Sciences, is shaping how researchers study dark matter at some of the most sophisticated labs on Earth. Gori’s ideas have informed scientists at cutting-edge institutions such as Fermilab outside Chicago and the SLAC National Accelerator Laboratory at Stanford University.

Physicists are divided into two camps: theoretical and experimental. But Gori, who has published more than 30 papers on dark matter and elementary particles, tries to bridge both worlds.

“My field is in the middle. I’m a theorist. But I’m developing these theories and trying to design experiments in such a way to test them,” she said.

The National Science Foundation acknowledged her efforts by awarding Gori a $400,000 grant this year to pursue her research.

“Particle physics faces two fundamental problems that point to ‘new physics’ lying beyond our current understanding,” the foundation’s grant award said.

The first is how to explain the origin of the mass of the recently discovered Higgs boson particle. The second question is about the nature and origin of dark matter, which occupies more than five times as much mass as the remainder of the known universe.

“As part of her proposed research, Gori will pursue an ambitious research program focused on studying these two fundamental problems at the interface between high-energy and high-intensity experiments,” the foundation award said.

Dark matter is called “dark” because it does not absorb, reflect or create light. But the name might as well be an allusion to the mystery surrounding it.

“We know very little about it. There are lots of candidate theories for dark matter. And we don’t know which, if any of them, could be right,” UC physics professor Richard Gass said. “They could all be wrong.”

Scientists face a challenge in answering some of the big questions posed by dark matter. But Gass said Gori’s work certainly will raise tantalizing new ones.

“Answering them will depend on how the experiments go,” Gass said.

Scientists say dark energy is the theoretical reason the expansion of the universe is accelerating rather than slowing down. Intuitively, scientists expect to find the universe expanding at a slower and slower rate because of the forces of gravity. But observations by the Hubble space telescope found the opposite was true – the universe is expanding faster now than it was long ago.

“There is evidence of dark matter from looking at the location of galaxies,” Gori said. “The location of galaxies is different from what we would have expected. The reason is dark matter.”

Dark energy is responsible for 68 percent of the energy in the universe. Dark matter occupies 27 percent more. The measly remainder – less than 5 percent – is composed of planets, moons, suns, you, me and all other observable matter.

“What we have discovered from physics is there is much more matter than what we have observed,” Gori said.

Gori’s quest is taking her back to CERN, where she works side by side with experimental physicists from around the world who test her hypotheses using instruments such as the atom-smashing Large Hadron Collider.

The collider uses superconducting magnets to send protons hurtling at the speed of light in opposite directions around a ring the size of Cincinnati. These protons collide, producing 14 trillion electron volts of energy. That sounds impressive – even scary. But CERN says it’s equivalent to the energy a mosquito exerts to fly. The difference is this energy is created by matter a trillion times smaller than the buzzing insect.

Gori is trying to understand the properties of the Higgs boson, an elementary particle discovered at CERN in 2012. It’s sometimes called “the God particle” for its fundamental role in understanding matter. And she is investigating the nature of dark matter and helping to come up with novel experiments to test those ideas.

This collaboration with experimental physicists from every corner of the globe is the most rewarding aspect of her career, she said.

“The part of research I find super-interesting is to be the interface between theory and experiment,” she said. “This is a back and forth between theorists and experimental physicists. I find this very exciting. We’re trying to define this new phenomena together.”

Physics studies often involve international teamwork. But this is especially true of particle physics, UC’s Gass said.

“Particle accelerators are so expensive and there are only a handful in the world,” Gass said. “The cutting-edge ones have to be built through international collaboration.”

But Gori is accustomed to working with foreign colleagues.

She is a native of Italy, where she earned undergraduate and graduate degrees from the University of Pisa before obtaining her PhD in theoretical particle physics from Technical University Munich. She has traveled the world, working with and speaking to physicists at public and private universities from Bonn to Boston to Beijing.

Gori gave a lecture on physics last year to an all-girls Catholic high school in Cincinnati. She is a regular contributor to Women in Science and Engineering and advocates for women in science, technology, engineering and math (also called STEM).

“It has been demonstrated that the main reason women are discouraged from pursuing an academic career in physics is they lack role models,” she said. “I am trying to get more women involved in physics. It’s important to have role models that will inspire the next generation of women in STEM fields.”

UC’s Gass said Gori’s work at CERN and other labs brings esteem to UC and its Physics Department.

“It helps to raise the visibility of the institution, place your students in good post-doctoral programs and recruit good graduate students,” he said.

Gass said theorists often must wait a long time for their blackboard ideas to be put to the test by experimental physicists. But theorists such as Gori often play a central role in experiments.

“That’s particularly important for dark-matter research. We don’t know much about it,” Gass said. “So if you’re designing an experiment to detect dark matter, it’s important to know what signal you’re looking for and have an idea about how dark matter interacts with normal matter.”

Gori is eager to find out. She suspects dark matter might contain its own photons – dark photons. CERN last year greenlighted a new experiment to study the question.

And in March, CERN’s Dark Matter Working Group on which Gori serves proposed a new way of studying the possible interaction between dark matter and normal matter using the Large Hadron Collider.

“That’s a big mystery. We don’t know how dark matter behaves. The question is: does dark matter interact with the Higgs boson?” she said. “There are ideas that tell us dark matter comes with additional force. We’re trying to understand these properties. Is it alone? Or does it bring a set of additional particles?”

Scientists at CERN this summer announced the discovery of a new subatomic particle, nicknamed Xi for short, that theoretical physicists hypothesized they could find. Gori is excited about the prospect of testing her theories about dark matter through similar experimentation.

“We don’t know when the next big discovery will happen,” Gori said. “It’s been an interesting journey.”

By Michael Miller
Photos by Andrew Higley/UC Creative Services

Sep 28, 2017