Why is the universe full of matter? UT-Arlington physicist helps global team get closer to an answer

A massive experiment involving shooting trillions of particles 800 miles through the earth could help physicists fix a decades-old flaw in how they understand the nature of matter.

Why is there something rather than nothing?

For centuries, humans have grappled with this question by turning to religion or philosophy. But an international team of over 1,000 scientists think they can find the answer by shooting a beam of tiny particles called neutrinos 800 miles through solid ground from Illinois to South Dakota.

There, the neutrinos will encounter 70,000 tons of liquid argon buried a mile underground — and one of the rarest occurrences in the universe will be captured.

Thanks to a team led by Jaehoon Yu, physics professor at UT-Arlington, the "Deep Underground Neutrino Experiment," known as DUNE, is one step closer to actually pulling it off.

What’s the matter with matter?

The Standard Model — a wildly successful theoretical framework physicists use to describe the fundamental building blocks of nature — is in trouble. Why? Because there is something rather than nothing.

Physics tells us that every particle has a twin "antiparticle," identical in every way but oppositely charged. If they meet, the twins annihilate each other in a brilliant flash of energy. The Standard Model says that just after the Big Bang, there were equal parts matter and antimatter.

If that’s true, in the beginning matter and antimatter should have devoured each other, leaving ... nothing. If the very existence of the universe contradicts the Standard Model, physicists are clearly missing something. What tipped the primordial scales toward matter? The answer could bring about a quantum leap in our understanding of the universe.

“Here be Dragons”

Think of a particle and antiparticle as two sides of a coin. Each side essentially behaves identically. To understand why matter won out, physicists are searching for a “weighted” coin — a particle whose two sides behave differently, and so could have nudged the early universe toward matter. So far, physicists have only found normal coins.

Enter the neutrino.

“Neutrinos are the rebels of the Standard Model,” said Lindley Winslow, an assistant professor of physics at MIT. They just don’t fit. In the old-time map of particle physics, neutrinos would be in the “Here be dragons” area, hanging out with the mermaids.

Neutrinos were theorized by Wolfgang Pauli in the 1930s as neutral particles that would account for observations of radioactive decay that didn’t jive with current theory. “I have done something very bad today by proposing a particle that cannot be detected,” he later wrote. It was detected in nuclear reactors in the 1950s, but it has evaded more detailed detection ever since.

Here’s what we do know about neutrinos: They are lighter than we can currently measure, have no charge and outnumber all other particles in the universe. Trillions whiz through you every second, but they barely ever touch other matter. In fact, because of their size and charge, they can move through a light year of solid lead without touching a single atom.

It gets weirder. Neutrinos come in three distinct "flavors," and an individual neutrino can shapeshift between these flavors as it rockets through space. If neutrinos and antineutrinos change flavors differently, that means that they fundamentally behave differently and that they could be the weighted coin.

But how can scientists observe an event as rare as winning the lottery, and capture it with enough precision to weigh something that’s nearly weightless?

A mile underground

DUNE has a strategy: Buy lots of lottery tickets, and never miss a winning number.

Un-metaphorically speaking, this means producing an enormous number of neutrinos, and then capturing as many collisions as possible.

At Fermilab in Batavia, Ill., a particle accelerator will create an intense beam of neutrinos or antineutrinos. After measuring their initial flavor, they will be shot through the Earth toward the Sanford Lab in Lead, S.D. This distance maximizes how often neutrinos change flavors, ensuring that any differences observed between neutrino and antineutrino behavior are real.

Schematic of Deep Underground Neutrino Experiment
Schematic of Deep Underground Neutrino Experiment(Fermilab)

After a 4-millisecond trip, the beam will enter the largest and most sophisticated neutrino detector ever built. Four detectors the size of an Olympic swimming pool — but six times deeper and filled with liquid argon — will be buried nearly a mile underground.

Roughly 10 neutrinos a day, out of the trillions that pass unnoticed, will collide with an argon atom. The neutrino remains intact, but the argon atom breaks up into smaller, charged particles. These particles careen off through the liquid, and the detector captures their charged trajectories.

Collection plane view of a neutrino interaction in the MicroBooNE detector from data taken in March 2016. The view is from the side of the detector; the neutrino beam enters from the left.

Scientists can deduce the properties of the neutrino, like its flavor, from these trajectories. Once they measure enough collisions, they can compare the change in flavor ratios of neutrinos and antineutrinos across this vast distance. If the change in flavor ratios is different, that is good evidence that neutrinos are the weighted coin. That is, if detectors precise enough can be built at such a large scale.

A blank canvas

According to Yu, the biggest obstacle to building an enormous yet precise neutrino detector is maintaining a uniform electric field within it.

To see such a small collision, you need a totally blank canvas. A uniform electric field is like that empty canvas that will capture the neutrino collision. The slightest murmur in that field would mar the final picture of a collision, and by extension, the neutrino.

Earlier this spring, Yu and his colleagues completed a prototype detector. “They’ve ticked off one of the key engineering issues in making this whole thing work,” said MIT’s Winslow, who is not involved with DUNE. The final detector, 25 times larger than the prototype, will be installed in 2027.

Yu’s vision extends far beyond 2027. He hopes DUNE’s findings usher in a “new physics” that could revolutionize nuclear power. Currently, we rely on 19th-century steam engine technology to harness the energy generated from cracking open an atom. Yu thinks that understanding neutrinos and the dynamics of the early universe could allow us to skip the steam engine step and convert nuclear power directly to nearly unlimited electricity.

“That’s one of my 1,000-year dreams,” Yu said. “We’re doing this not just to satisfy curiosity. This is giving something to the next generation so that we can make our lives better ... and the first step is fixing the Standard Model.”

Jonathan Lambert. Jonathan is an American Association for the Advancement of Science (AAAS) Mass Media fellow this summer at the Dallas Morning News. @evolambert



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