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Ghostly Particles, Real Physics

Why is the universe not empty and intangible?

By Leto Sapunar

A rickety wooden elevator clatters and clunks its way 4,800 feet down a dark 19th-century mineshaft in Lead, South Dakota. The occupants — tense particle physicists — descend toward the future site of four house-sized underground pools of liquid argon. The pools are destined to become the largest particle detector of its kind.

Detectors like this would catch particles known as neutrinos, sometimes called ghost particles due to their elusive nature. The study of neutrinos might reveal something significant: Why wasn’t the universe annihilated long ago. To understand the necessity of both the terrifying elevator trip into the bowels of the Earth and the massive detectors, we need to go back to the very beginning of time.

The full-scale DUNE layout. (Image: Fermilab DUNE)

The Big Bang

Physicists think the Big Bang formed equal amounts of matter and antimatter. Matter is any solid “stuff” that exists in our universe and antimatter is its almost-identical opposite. The two opposing forms self-destruct on contact. Scientists postulate that if matter and antimatter were exact mirror opposites they would have annihilated each other in a kind of final battle in the very early universe. This would have left an empty, intangible cosmos.

Evidently that wasn’t the case.

The relatively tiny fraction of surviving matter makes up everything we can see and touch in our universe, and points to an asymmetry, or a slight discrepancy in the way the two opposing types of matter behave. Which might explain two of the great mysteries in physics: First, why is there any matter left at all? And second, why matter instead of antimatter?

Annihilation in the early universe can be thought of as a battle between equally matched armies containing equally matched soldiers. On one side are the “blue knights,” or particles, and on the other, “red knights,” or antiparticles. Each knight can only defeat one of the opposing kind, and both knights always perish in the duel. Only by having an excess of one type of knights can any knight survive the battle.

Where do neutrinos come in?

Neutrinos are a product of radioactive decay and are created in large numbers in stars and supernovae. They weigh almost nothing and travel near light speed. We can’t see them, but they are abundant, and interestingly, are constantly passing through us in the billions.

Neutrinos and antineutrinos — their relevant antiparticles — are just one type of matter-antimatter pair, but neutrino study can act as a microcosm for the early universe as a whole, giving scientists a chance to compare particle interactions in controlled circumstances. Physicists hope to shed light on the mystery of matter-antimatter asymmetry by using the site in Lead to detect neutrinos and antineutrinos and to discover the key differences between them. In other words, if red knights behave a little differently from blue knights, it might explain why every knight didn’t find an opponent and disappear.

The DUNE Project

Heidi Schellman, professor in OSU College of Science.

Heidi Schellman is the computational coordinator for the Deep Underground Neutrino Experiment (DUNE) project, as well as chair of the Oregon State University Physics Department. Her lab has two grants from the National Science Foundation and collaborates remotely on a range of studies. DUNE is a huge, international collaboration, destined to become the first in a new generation of enormous neutrino detectors.

“You start out at the top with this question, ‘Are there the same number of particles and antiparticles in the universe?’” says Schellman, “and then you wonder if neutrinos and antineutrinos operate in the same way.”

Schellman says the discrepancy between neutrinos and antineutrinos may be the reason the universe is made up of matter rather than antimatter. “We think we’ve found an asymmetry between matter and antimatter in the universe, which is small, but it’s there,” she says.

Scientists had no idea neutrinos existed until 1911 when, after carefully studying radioactivity, Lise Meitner and Otto Hahn found a problem with beta decay — one of the processes by which radioactive elements naturally break down. The reactants before the decay didn’t add up exactly to the final products — somehow energy was missing. It wasn’t until 1934 that Enrico Fermi provided the full theoretical explanation of the process and named the neutrino as the additional theoretical byproduct, which balanced the books.

Neutrinos are often called ghost particles because of how effortlessly they pass through matter.

“They’re very shy particles,” says Maggie Greenwood, a graduate student in Schellman’s lab, pursuing a Ph.D. in physics at OSU. “They don’t like to interact, which is why we have to build these huge detectors to find them,” she says, referencing the detectors to be built in Lead.

Due to their lack of electrical charge, neutrinos can fly right through the empty spaces between atoms in solid objects without being deterred by the atom’s electrical fields. The massive tanks of argon are necessary to catch the neutrinos’ very rare collisions with solid atomic cores.

The supporting infrastructure for ProtoDUNE. (Photo: CERN)

Schellman recently returned from the European Laboratory for Particle Physics on the Franco-Swiss border and home to the Large Hadron Collider, the largest particle accelerator in the world. However, Shellman’s current interests lie in the protoDUNE detector — a smaller scale culmination of the technologies to be used in DUNE which is also housed on-site.

“Some people like to smash things together and look for new particles. I don’t do that. I just do very precise measurements,” says Schellman.

Such close scrutinization of neutrino interactions could lead to a breakthrough in our understanding of matter-antimatter asymmetry, just as careful measurements of beta decay led to the discovery of neutrinos.

The DUNE project seeks to provide this closer look by creating a beam of neutrinos at the Fermi National Accelerator Laboratory in Batavia, Illinois and sending it straight through 800 miles of the Earth’s crust to the four massive detectors at the Sanford Research Facility in Lead.

Neutrinos come in three varieties or ‘flavors’ — electron neutrinos, muon neutrinos and tau neutrinos. The strange thing is, neutrinos can change type midflight, occasionally transforming into a different flavor. Envision the knights on both sides wearing hats which occasionally change from fedora to baseball to beret and back again.

DUNE project designers chose the beam-detector distance of 800 miles to give the neutrinos created in Fermilab a chance to change flavors before they reach the liquid argon detectors in Lead. Neutrinos will start as muon flavor, but some will oscillate to electron flavor by the time they arrive. Scientists are studying these flavor changes as a possible key difference in neutrino and antineutrino behavior.

Researchers perform tests on ProtoDUNE’s electric field cage. (Photo: CERN)

A 343 square feet prototype of the DUNE detector has been built at the European Organization for Nuclear Research in Switzerland and exposed to beams of charged particles to understand the technology and its performance. During the initial four-week period of testing, researchers, including Greenwood, recorded about 7 million particle interactions. The test was successful and makes the team confident that the technology performs well before starting to build the enormous underground version in South Dakota. The large version will be completed in stages over the next decade.

According to Greenwood, intense collaboration is one of the particle physics research community’s most distinct features. She notes, “You’re never alone, there’s always a bunch of people you can talk to and get help from.” Over 1,000 scientists from more than 175 different institutions are involved in DUNE and the work is, by nature, mutual rather than competitive.

Schellman enjoys the fact there can be huge cultural differences between collaborating researchers. She has advised students from all over the world. “Going to the European Laboratory for Particle Physics is also like going to your high school reunion,” she says.

This high school reunion sentiment is central to discovering the answer to a question that has perplexed humanity for the last century – why does the universe exist? By scrutinizing these ghost-like particles in order to find the subtle but crucial differences between the “blue” and “red” knights, the scientists hope to resolve the mystery of asymmetry.