Understanding the Functionality of the Deep Underground Neutrino Experiment

America's next major particle physics experiment, the Deep Underground Neutrino Experiment (DUNE), commenced construction this summer. This underground experiment will investigate elusive subatomic particles by sending a high-energy neutrino beam through Earth's mantle, reaching depths of up to 30 miles (48 kilometers), potentially uncovering profound cosmic secrets.

Managed and funded by a global collaboration, the experiment will stretch 800 miles (1,300 kilometers), starting at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, and concluding over a mile underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. Once completed, DUNE will integrate into the Long-Baseline Neutrino Facility (LBNF), a dual-site project linking Fermilab in Illinois to SURF in South Dakota.

Delving Deeper Underground

Neutrinos effortlessly traverse 800 miles (1,287 kilometers) of solid rock. These peculiar subatomic particles, classified as fermions, possess minimal mass and no electrical charge. Moving at nearly the speed of light—owing to their status as the lightest known particles—they barely interact with ordinary matter. Ubiquitous in the cosmos, they pass unimpeded through everything, be it living beings or vast layers of rock.

Given their elusive nature, how do researchers confirm the existence of neutrinos? The answer lies in colossal cryogenic detectors. DUNE will feature two underground detectors: one positioned near Fermilab's neutrino source (the "near detector") and another housed in a massive facility at SURF (the "far detector"). Following an upgrade to Fermilab's infrastructure, the most intense neutrino beam ever created will be aimed through the near detector and intercepted by the far detector, which consists of four enormous, cryogenically cooled liquid argon tanks. Each tank will stand six stories tall, span the length of a football field, and hold 18,739 tons (17,000 metric tons) of ultra-cold liquid argon.

Why argon? Neutrinos, though weakly interacting, very rarely collide directly with atomic nuclei in matter. By directing a high-intensity neutrino beam into large tanks of ultrapure argon, a tiny fraction of these elusive particles will randomly strike argon atoms. When collisions happen, highly sensitive detectors inside the tanks will detect a flash (called scintillation), allowing researchers to study the interaction. Due to the detectors' extreme sensitivity and the minuscule nature of these interactions, neutrino detectors are typically placed deep underground to protect them from cosmic rays and other surface-level radiation that could cause significant interference.

These rare interactions could reveal new physics and enhance our understanding of one of the most enigmatic particles in quantum physics.

Understanding Neutrinos

Neutrinos captivate scientists for numerous reasons, one being their unique ability to connect us directly to the sun's core. During nuclear fusion, both neutrinos and high-energy photons are generated. While photons are absorbed and re-emitted repeatedly within the dense solar plasma, taking up to a million years to escape as visible light, neutrinos travel unimpeded, reaching Earth in just minutes. This makes neutrinos invaluable for studying the sun's fusion processes in real-time.

However, solar neutrinos come with an intriguing mystery.

Neutrinos exist in three distinct "flavors"—electron, muon, and tau—along with their antiparticles. As they journey through space, they oscillate between these flavors, akin to a chameleon shifting colors based on its environment.

The sun exclusively produces electron neutrinos in its core. When scientists first detected these particles in the 1960s using highly sensitive detectors, they observed far fewer neutrinos than expected. In Nobel Prize-winning research, physicists discovered the explanation: electron neutrinos oscillate into muon and tau neutrinos during their journey. Since early detectors could only identify electron neutrinos, the others remained undetected, resolving the apparent shortage.

This brings us back to DUNE. Controlled experiments like DUNE are essential to unraveling neutrino flavor oscillations. By producing neutrinos at Fermilab and measuring their flavors upon arrival at the South Dakota facility, scientists can compare the sent and received neutrinos. This could lead to breakthroughs in understanding neutrino quantum behavior, precise mass measurements, and even the discovery of additional neutrino flavors beyond the known three.

But There’s More to Explore

DUNE’s scope extends far beyond neutrino oscillations. It aims to tackle the profound mystery of why our universe exists. While this may seem philosophical, the dominance of matter over antimatter in the universe remains one of the greatest unsolved questions in science.

Around 13.8 billion years ago, the Big Bang should have created equal amounts of matter and antimatter. When matter and antimatter collide, they annihilate, leaving only energy. If the Big Bang had truly produced equal quantities, nothing would remain today.

Our existence proves that the universe generated slightly more matter than antimatter. This imbalance allowed matter to prevail after the initial annihilation, making antimatter exceedingly rare. This suggests a violation of fundamental physical laws during the Big Bang, known as a "CP violation." While particle accelerators like the Large Hadron Collider investigate this asymmetry, DUNE will also contribute by studying neutrinos and their antimatter counterparts, antineutrinos.

Fermilab’s neutrino beam is slated to be operational by 2026, with the final DUNE detector expected to be completed by 2027. Scientists are optimistic that this could lead to groundbreaking discoveries, potentially rivaling the significance of the Higgs boson.