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The Missing Piece: a Physicist’s Search for Dark Matter

In Wright Laboratory, a half-buried concrete bunker on the campus of Yale University, a team of physicists finishes up their meeting in a dingy conference room. A slab of orange foam covers a hole in the ceiling above the conference table, and the scratchy mauve carpet underfoot bears decades worth of coffee stains. Dozens of framed posters from international physics conferences line the cinder block walls like so many rock stars. Outside, it rains.

These scientists don’t look much like the men of the Manhattan Project, with nary a moustache nor a three-piece suit to be found. Of the five, four are women, and jeans, t-shirts, and leggings are more in fashion than the white coats of bygone days. Sumita Ghosh, GRD ’21, a small, lithe woman with long dark hair cut close on the sides, looks perfectly at home.

As people begin to file out, Ghosh bounds to the room’s whiteboard and turns towards her boss and mentor, Reina Maruyama. “As you know, my saturated absorption signal is unacceptably low,” says Ghosh.

“Sumita, one second,” says Maruyama, moving quickly through some newly arrived emails. A lead investigator of huge, collaborative experiments in South Korea and the South Pole in addition to her work as an associate professor at Yale, Maruyama is planning a trip to Iceland to present some of her research. 

Ghosh’s head bounces slightly as she waits for Maruyama’s attention, her blue glasses reflecting lines from the humming lights overhead. Maruyama closes her laptop. On cue, Ghosh begins drawing a convoluted diagram of arrows, crescents, lasers, and mirrors, talking the whole time. Maruyama looks on, not so much guiding as passively absorbing the stream of information that rolls relentlessly from her student.

Ghosh puts her marker down at the end of her monologue and takes a needed breath. “So, that’s my plan,” she says. “That’s what I’m going to do.”

“I think that sounds good,” says Maruyama, eliciting a “Yay!” from Ghosh. Quickly bundling up her things, she heads off to her workspace, cordoned off in a corner of the lab by a heavy black curtain. Behind the barrier, among an intimidating array of gleaming steel and crossing wires, Ghosh spends most of her time alone looking for a particle that exists all around us. The particle is ubiquitous—it outnumbers the atoms that make up the visible universe, with all its planets and stars, by six to one—yet remains elusive. It forms a silent reflection of our material world which it surrounds. Physicists call this particle “dark matter.” And there’s a good chance they’ll never find it.

Dark matter is unlike anything else in the universe. It doesn’t interact with light, so we can’t see it. We only know it exists from its outsize effects on the things we can see. Astronomers were the first to be cued to its existence almost a century ago. They calculated that galaxy clusters would tear apart into specks of astral dust without some unseen halo of incredible mass holding them together. Since the 1970s, when strong evidence emerged proving the existence of this unseen mass, scientists have raced to find and isolate the missing particle that forms the dark side of the Universe. All experiments so far have come up empty, but that doesn’t discourage Ghosh. She was made for this moment.

“People have callings, right?” she says. Most people don’t care about the properties of some subatomic particle, even if it occupies some 85 percent of all physical space. But they also don’t feel preternaturally compelled to describe the laws of the Universe. “I do,” says Ghosh.

A practicing Hindu, Ghosh considers the search for dark matter a spiritual undertaking. “God created the Universe and all the laws of the Universe,” she said. In one telling oddly prescient of the modern Big Bang theory, the Hindu Creator, Brahma, is said to have first birthed himself from a golden egg, then expanded its contents to form the rest of the Universe. “We, as humans, don’t know the laws of the Universe right now,” said Ghosh. But if she could find dark matter, “that’s one step closer to understanding God.”

In her quest to understand dark matter, and perhaps trace it back to its beginnings, Ghosh’s principal weapon is math, “Integrals and algebra that suddenly just explain everything that’s happening in the Universe!”

The daughter of an engineer and a biophysicist, Ghosh inherited an affection for numbers. The demanding professions that required her parents’ quantitative skills, however, removed them from the home. Her father traveled often on behalf of a semiconductor firm and her mother worked long hours as a postdoctoral researcher. “I really spent most of my childhood by myself,” she said. Impromptu math lessons around the kitchen table at her California home became an important source of connection with her father and physicist grandfather. She mastered concepts quickly, learning long-division from her grandfather in first grade, when other students were just beginning to memorize simple multiplication tables. “He taught me how to divide eight-digit numbers by seven-digit numbers,” she said. “I was really proud of myself.”

Ghosh learned to love math at these family colloquiums, which her younger brother, Shayan, claims to have been lucky enough to avoid. There wasn’t any bothersome memorization, like in biology or history, just pure logic. But when it came time to choose a major in college at UC Berkeley, she opted for engineering. “My mom thought I was going to be a Math major,” she said with a smile, but “my dad didn’t want me to be Ted Kaczynski.”

After college, Ghosh declined opportunities to join the semiconductor industry, so she could apply to graduate school, not as an engineer, but as a physicist. “I like the way physicists think,” says Ghosh. “With engineers, it’s like, here’s a problem; how do we fix it? Whereas with physicists, it’s like, here’s a phenomenon; why is this happening?”

Early on in graduate school at Yale, Ghosh focused on applied physics. She spent time studying semiconductors, the material that computer chips are made from, and then moved on to a lab using quantum physics to boost the speed of information processing. Although she found the work fascinating, Ghosh felt compelled to move on. She joined the lab of a young professor, David Moore, and stumbled upon her future.

In Moore’s lab, Ghosh studied tiny peculiarities in charge interactions, not to build more efficient computers, but because she found it inherently interesting. She helped build a complex force detector, called a gravito-optical trap, made of a miniscule levitating sphere trapped in the path of a laser beam. Moore was thrilled to use the contraption for gravity and charge interaction experiments. As an afterthought, he mentioned to Ghosh that the device might be able to measure dark photons, a theorized type of dark matter. She was instantly hooked.

“What about this dark matter stuff?” she asked herself. “That’s really cool.” She began to explore the topic. The more she read, the more obsessed she became. In this single problem, there was historical significance, spiritual meaning, and so much math. Instead of spending time in Moore’s lab, she took classes that armed her with exotic concepts in math and theory, necessary tools for the hunt. At academic conferences, she trailed the dark matter researchers like a zealot. She even spent a week at a “dark matter summer school,” a kind of monastic retreat for like-minded young scientists.

With her enthusiasm, she brought Moore around to sponsor her search for dark photons. In October 2018, she received official approval from her advisory committee to chase dark photons for her doctoral thesis. With her calling finally discovered, Ghosh was eager to get to work. But her project languished behind other priorities in Moore’s lab. Delays built on delays. Months went by without an ounce of progress or hope of accomplishing anything soon. Ghosh began to feel anxious, as if her research was steadily being buried and forgotten. “She was in a very challenging situation,” said Emily Kuhn, GRD ’21, a friend of Ghosh’s.

“I lost my thesis project,” said Ghosh. “I didn’t know what to do.” She was drifting along aimlessly until Maruyama, a mentor and member of her thesis advisory committee, threw her a lifeline.

While Moore is tangentially interested in dark matter, Maruyama is the dark matter person at Yale. The search party she leads in New Haven is called HAYSTAC (the Haloscope at Yale Sensitive to Axion Cold dark matter).

HAYSTAC isn’t looking for dark photons. Rather, a different dark matter candidate has caught their eye: the axion. Physicists first posited the existence of axions to solve a problem in physics completely unrelated to dark matter. A vanishingly small particle assumed to rarely interact with normal matter, the axion was quickly adopted by dark matter researchers and targeted as a prime suspect in their search. “It’s a two-for-one,” said Liz Ruddy ’20, an undergraduate researcher on the HAYSTAC team. “It would patch up a lot of different holes in physics.”

In a strong magnetic field, axions are predicted to decay spontaneously into photons, the packets of energy that form light. Just don’t expect this to happen too often. With its teensy mass, axions don’t carry much energy to spark this transformation. The photons they’d birth would be rare and feeble. HAYSTAC is trying to trap and amplify these wimpy photons in a metal chamber called a haloscope. If the haloscope is manipulated just right, it might catch the axion-to-photon conversion in action and record a tiny power fluctuation inside. The actual mass of an axion isn’t known, so researchers aren’t quite sure exactly how small their power signature will be. Across the world, like a fleet of ocean trawlers fishing at different depths, similar experiments are being run each designed to explore a different slice of the hypothesized range of an axion’s mass.

Maruyama and HAYSTAC were already operating in uncharted waters by the time Ghosh joined the team. But Maruyama wanted to expand the search to higher masses where noisy deviations can muddy the power signal. To dampen the noise, she planned to directly count axion-to-photon conversions. She needed a detector sensitive enough to capture single photons to do so. Creating this detector, however, is no simple task. Only someone with experience in engineering, quantum physics, and optics would be up to the challenge. Most importantly, they’d have to be really good at math. Ghosh was a perfect fit.

Ghosh was at brunch when Maruyama emailed to offer her the spot. Wholly overwhelmed, she began to cry. “So, I come to HAYSTAC, and for the first time in my life, I’m genuinely—” she cuts herself off and takes a breath. “I’m the most excited about this experiment than I’ve ever been about anything that I’ve ever done.” Now, Ghosh is ready to embark.

Minutes after the lab meeting, she stands on a step stool, dark hair braided and pulled back. She peers through black safety glasses with thick candy green lenses, a precaution against the retina-searing lasers that pinball around the maze of mirrors, pinholes, and waveplates beneath her. The lasers blast a tube of potassium atoms, elevating the particles to a high energy state. These excited atoms are the critical photon detectors.

Particles at the quantum level exist at energy levels. If the right amount of energy is absorbed by a particle, it can move up one, several, or many levels to an elevated state. The energy difference that separates a high level (n) from the next one up (n+1) is very small. Only a tiny amount of energy will move a particle up these higher steps.

The laser-roasted potassium atoms occupy very high energy levels. Ghosh hopes to use them to capture the low-energy photons predicted to be released by axions in a strong magnetic field. If the excited potassium absorbs this energy, they will bump up to a higher energy level. At this even more excited state, the potassium atoms will shed a single electron if nudged by the right electric field. These freed electrons can then be counted by an electron detector to give an indirect count of the axion conversions that have occurred. By counting these conversions one-by-one, one electron to each photon, Ghosh will dramatically reduce her signal’s noise.

Ghosh is still working on preparing her potassium atoms, the detectors. Once they are ready, she will have to attach them to the haloscope contained within a super refrigerator kept below negative 450 degrees Fahrenheit. Attaching single-photon detectors to a haloscope has never been done before, but Ghosh prefers to focus on the problem at hand. “I don’t know how we’re going to do that,” said Ghosh. “Nobody’s done single-photon detection because it’s really hard.”

Of course, the whole experiment rests on the assumption that dark matter does interact to some degree with regular matter. Ghosh is happy to admit just how tenuous this assumption may be. “It’s totally possible that dark matter will just never be found on Earth,” she said. “But that doesn’t mean that people will ever stop looking.”

We certainly won’t know for years to come. The range of masses in which dark matter may hide is so vast that it takes single experiments years to rule out significant fractions. Ghosh imagines that even if her trawl comes up empty, she’ll still feel fulfilled. “That’s still a huge accomplishment that I can be proud of and say, ‘This little spot on the map, I did that.’”

With new experiments to run and lots of space to cover, ignorance feels like opportunity and success seems tantalizingly palpable. “We don’t know what’s possible with dark matter because we don’t know anything about it,” said Ghosh. “Once we find it? I don’t know. Space travel?” she added with a laugh. 

Turning for a moment away from her work bench, her voice drops and grows more serious. “All I want to do is learn about the Universe.”

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