A Goldmine for Particle Physicists

One of the most profound questions humans have always had is, "Why do we exist?" It turns out that physicists have the same question, and the theorists at the Deep Underground Neutrino Experiment (DUNE) are spending all their hours staring at screens and scribbling away on blackboards hoping to bring us closer to the answer.

You, the device you are reading this on right now, and literally everything you can see is made of matter. 13.6 billion years ago, in the event we know as the Big Bang, all of matter and an equal amount of antimatter was created. Matter is made of the commonly known protons, neutrons, and electrons and fourteen other fundamental particles. Each one of these particles is said to have an antiparticle that is identical to itself and carrying the opposite charge. These antiparticles are what make up antimatter.

When a particle and its antiparticle meet, they annihilate each other in an explosion of light. So, if the Big Bang created equal amounts of matter and antimatter, why hasn't everything been annihilated? Where did all the antimatter go? If things worked the way physics predicted them to, the entire Universe would just be made of light, and we wouldn't be here pondering these questions. So, why do we exist? Physicists believe the answer might lie with neutrinos.

Neutrinos are part of the 17 fundamental particles produced from nuclear reactions on Earth and in stars. One source of neutrinos we are all familiar with is the Sun. Every second, more than 100 trillion neutrinos created in the Sun are streaming through your body. Despite how threatening that sounds, we don't have to be worried since neutrinos rarely interact with matter. It is extremely unlikely that even one of those trillion particles would interact with your body. Neutrinos are remarkably abundant but very elusive, making them extremely difficult to detect.

In the early 1950s, some scientists proposed to detect neutrinos by detonating a nuclear bomb with a detector below it. That’s not that surprising—For 20th-century physicists, dropping a nuclear bomb often seemed like a “reasonable” way to approach any problem. When this obviously unreasonable idea was rejected in 1956, Cowan and Reines, two particle physicists, placed detectors around a nuclear reactor to detect the very first neutrino. "I was blown away by the amount of human ingenuity required just to find that this particle exists," said Michael Wagman, a theoretical physicist at Fermilab.

Neutrinos can be of three flavors: electron neutrinos, muon neutrinos, and tao neutrinos. In the early 1970s, American physicist Ray Davis monitored a massive detector located deep in the Homestake Gold Mine in Lead, South Dakota. He was trying to detect neutrinos coming from the Sun. Davis was successful, but he detected precisely one-third of the expected number of neutrinos. Since the Sun produces electron neutrinos, Davis’s detector was set up to only detect those. On finding exactly one-third of the expected count, he also measured the other neutrino flavors. He found that the other two-thirds of the neutrinos were comprised equally of muon and tao neutrinos. Physicists then came to the only possible conclusion—neutrinos must oscillate between flavors while they travel. This means that a neutrino that was first observed to be an electron neutrino can, after traveling some distance, be observed as a muon neutrino or a tao neutrino. Davis won a Nobel Prize in 2002 for his work. Today, the same gold mine in Lead, SD, is being dug into and filled with gigantic particle detectors. It has turned into the Sanford Laboratory, the second home of the DUNE experiment.

"Imagine you bought strawberry ice cream from the store, and by the time you brought it home, it turned into chocolate ice cream," said Shirley Li, a theoretical physicist at UC Irvine. "This is what neutrino oscillation is like. It is extremely bizarre," she says. To be able to oscillate like this, neutrinos must have mass. Up until then, physicists believed neutrinos were massless. "I want to understand how neutrinos get masses," said Li. "If I could have the answer to one out of all the questions out there, it would be this one." Li believes that DUNE will help answer this question.

The Deep Underground Neutrino Experiment will consist of two neutrino detectors placed in the world's most intense beam of neutrinos. Fermilab will produce one neutrino beam in Batavia, Illinois, and is where the first detector will be placed. The second, much larger detector will be 1300 kilometers away in the Homestead Gold Mine. 

Scientists designed the DUNE experiment to study neutrino oscillation and mass. Knowing more about these could help them answer bigger questions, such as whether neutrinos played a role in the imbalance between matter and antimatter after the Big Bang that resulted in the growth of our Universe.

Construction work for DUNE is estimated to be finished by 2028. In 11 years, Fermilab will have upgraded their particle accelerator that will create the neutrino beam, dug caverns a mile into the ground at the Sanford Lab, and built four humungous particle detectors filled with liquid argon. Excavating 800,000 tons of rock requires a lot of physical labor. But another kind of mental labor is going on at Fermilab in preparation for DUNE.

To be ready to use the experiment, scientists must also complete a years-long process of developing the theoretical predictions they need to evaluate its data, work known as phenomenology.

Although perhaps not as well-known as other types of theory, phenomenology is an essential piece of the experimental process. Phenomenology uses fundamental theory to predict what the results of an experiment should look like. If the results stray very far from those predictions, either something is wrong with the experiment, or the experiment has discovered something that was not predicted by the theory.

Let’s say you and four friends want to make lasagna. One friend is more of a fundamental theorist, so she researches every ingredient that can possibly be used to make lasagna and gives you a 500-page report, as any good friend would.

A second friend is more of a model-building theorist. He takes that report and comes up with some possible combinations of ingredients to use, and in what order to use them. You, being a phenomenologist, use those models to make precise calculations, determining what amounts of ingredients to use, along with how long to bake the final product.

The fourth friend, being more of an experimentalist, actually gathers and measures all of your ingredients to your predicted values, sets them up appropriately, and bakes them for the prescribed amount of time.

If the lasagna turns out how it's supposed to, it’s a sign that your theoretical work was correct. If it doesn’t, it could be a sign that there’s something your theory didn’t account for, such as the effect that baking would have on a certain ingredient or how it would change one ingredient to combine it with another one.

The DUNE detectors will be full of liquid argon. When a neutrino collides with an argon atom in the detector, the collision will produce multiple additional particles, which will knock loose electrons from other argon atoms. A high voltage will draw these electrons to wire planes installed across the detector, which will collect the signals from those particles. Experimentalists will use these signals to learn about the particles that hit the wire planes and will use that information to infer the energy level, flavor and other properties of the original neutrino. Then they will compare those results to predictions from phenomenologists.

Theorists like Wagman and his team stand around blackboards and coffee machines every day, discussing simulation parameters and theoretical models that can be used to describe experimental predictions. They sit in front of computers, developing simulations, writing code, and running simulation jobs on big computing clusters. After this, they run statistical analyses on the simulation data to pull out the desired signals. "Understanding what theorists do is difficult," said Wagman. "People think we are just dreaming up crazy ideas that don't connect to the real world. As a phenomenologist, I say, blackboards, coffee machines, and computing clusters are what I do all day."

Making predictions for DUNE is a particularly difficult task, says Pedro Machado, a theorist at Fermilab. “Phenomenology for previous experiments such as those at the Large Hadron Collider (LHC) were relatively straightforward,” Machado says. “Since neutrinos are difficult to detect we need huge detectors, and every area in the detector is analyzed. This creates a lot of data and understanding what this data means is challenging compared to data taken from one single point of collision at the LHC.”

Multiple theorists are working on making predictions for what should happen in these collisions. One significant challenge is combining all of their work into one unified set of values.

An active community of scientists worldwide is working on this by building programs called event generators. Event generators combine predictions from different models in specified ratios to make final predictions for the experimental detectors.

“There's much work ahead, but the potential is promising,” says Noemi Rocco, a theorist at Fermilab who studies neutrinos. “We need substantial computing resources and collaboration across communities.”

In early 2024, phenomenologists for DUNE will gather at a workshop in Seattle. It is as if multiple theorist chefs have come up with their own lasagna recipes using different lasagna theories. Now, they want to condense all of those recipes into the one best lasagna recipe. 

Maybe one group has the best sauce, and one has the best pasta. Perhaps one group uses a chicken filling, and another uses mushrooms. “There are lots of variations,” Machado says. “They all have to discuss and come to one conclusion.” 

Theorists around the world will soon come together to determine the predictions that suit DUNE the best. Understanding neutrino oscillations and why they have mass may seem like extremely niche issues, but physicists suspect that the answers to these questions would help explain the answer to truly fundamental questions such as "Why do we exist?" or to be more specific, "Why is there more matter than antimatter?" They also hope to better understand other mysteries of the Universe, such as dark matter and black holes. With DUNE, scientists from all over the world are hoping to get closer to the answers that humanity has been seeking for over a century. If everything works out, the Homestake Gold Mine might just be the site for a second Nobel-Prize-winning discovery. A gold mine, both literally and figuratively, if you are a particle physicist.