Hitoshi Murayama lights path forward for dark matter research

From the solar systems in the Milky Way to the international collaborations down on Earth, Hitoshi Murayama believes that dark matter—and the quest to better understand it—holds us together.

“I actually rather consider dark matter to be my mom,” Murayama, a theoretical physicist at the University of California, Berkeley, told an audience of science writers Nov. 11 in Raleigh, N.C., during the Council for the Advancement of Science Writing’s New Horizons in Science briefing at the ScienceWriters2024 conference. “She made us. I’d like to see her and thank her at some point, but nobody has managed to do that.”

As chair of the 2023 Particle Physics Project Prioritization Panel (P5), an advisory group to the U.S. Department of Energy and National Science Foundation, Murayama worked with leading researchers to lay out a plan for the field for the next two decades, highlights of which he presented at ScienceWriters. The panel had spent over a year speaking to and parsing proposals from more than 1,500 researchers to identify the most pressing issues in particle physics. Chief among them: the nature of dark matter.

A hidden majority

Our solar system is 28,000 light-years away from the center of the Milky Way and travels roughly 140 miles in one second—the equivalent of crossing the width of the continental United States in 20 seconds. Murayama explained that, based on our solar system’s visible mass alone, it should have flung off the galaxy billions of years ago. “Gravity is too weak to keep us inside if the only source of gravity is stars we can observe,” he said. “There must be something we can’t see that’s still pulling us…. That’s what we call dark matter.”

The challenge for physicists is to find this dark matter and determine what it’s made of. Telescope images of galaxy clusters routinely include galaxies that appear to be elongated, closer to a line than a sphere. As Murayama explained, this is simply a “trick” performed by the dark matter between these galaxies and the telescope: It bends the fabric of spacetime and the light along with it. “Stretched” galaxies that move out from behind the dark matter return to their original shapes.

Using these “tricks” as a proxy for locating dark matter, physicists have found that the distribution of dark matter across the universe closely overlaps with the distribution of galaxies. “That is the picture of galaxies today,” Murayama said. “The stars are actually [a] minority. The majority is actually dark matter.”

The current model for dark matter treats it as an atom-like particle that passes through most matter without being detected. Millions stream through the human body every second. To study them, physicists have built experiments such as the Large Underground Xenon (LUX) detector, where tons of unreactive xenon atoms are stored miles underground, sheltered from any light, heat, or radiation. The idea is that only dark matter could penetrate into the chambers; any type of energy change or movement in the xenon atoms would have to come from dark matter.

So far, none of these detectors have picked up a signal. That means physicists may need a detector with at least 10 times more xenon, or 10 times as many “targets for dark matter to hit”, Murayama elaborated in an interview after the talk.

But this could still be nowhere near enough. For all physicists know, the mass of dark matter can be as small as that of a wave or as large as that of the sun. “It’s really surprising that we know so little,” he said.

A uniting force

A full interrogation into the nature of dark matter will require even more powerful versions of what are already particle physicists’ most powerful tools: telescopes that can detect microwaves from billions of light-years away, or particle accelerators that drive electron-positron collisions. Such collisions would be the equivalent of throwing “cherry pits” in an attempt to smash them together, compared to the “cherry pies” being tossed together at the world-leading Large Hadron Collider, Murayama told the audience.

Each of these will require international efforts, Murayama stressed. Building the “cherry pit” collider on U.S. soil would require more money than the federal budget can provide; the P5 report included a proposal to work with Europe and Japan to build one offshore. The type of LUX experiment Murayama proposed would require more collaboration between countries that independently built previous iterations—the U.S., United Kingdom, Japan, and China, as well as Europe. “This is the way science really brings people together,” he said.

It’s this human side of the work that Murayama tries to emphasize in his talks, as he explained in the interview. “When people think about physics, people think of it as a very dry and technical subject,” he said. “But it’s very personal. It’s talking about where I come from. It’s about my roots and my family.”

Toward that end, Murayama was also the first P5 chair to include review criteria about diversity, equity, inclusion, and accessibility when evaluating proposals. He led discussions about underrepresented groups within the field and public outreach to minority communities. After ScienceWriters2024, he planned to speak next at the 2024 National Society of Black Physicists and National Society of Hispanic Physicists Joint Conference in Houston, Texas.

Asked what he was personally most excited about regarding the future of dark matter research, Murayama simply laughed. “It’s my mom. I want to meet her! That’s part of who we are!”