A detector buried 1.5 km underground in an old gold mine, waiting to catch neutrinos shot through 1,300 km of solid rock from a particle accelerator in Illinois. The DUNE experiment will help explain why the universe contains matter instead of nothing, one of physics' deepest mysteries.
Sanford Lab, CC BY-SA 4.0, via Wikimedia Commons
A beam of ghost particles
The Deep Underground Neutrino Experiment (DUNE) starts at Fermi National Accelerator Laboratory (Fermilab) in Illinois. Here, the Long-Baseline Neutrino Facility (LBNF) will create the most intense neutrino beam in the world. The process begins by accelerating protons to 120 billion electron-volts, approaching the speed of light. This 1.2-megawatt beam, designed to be upgraded to 2.4 megawatts, smashes into a graphite target. The collisions produce short-lived particles that are focused by magnetic "horns" into a 194-meter-long pipe where they decay, releasing a torrent of neutrinos.
This beam of trillions of neutrinos is aimed downward at an angle of 5.8 degrees, beginning a 1,300-kilometer path straight through the Earth's crust and mantle, no tunnel required. Neutrinos interact so weakly with matter that solid rock is no obstacle. Their destination is the Sanford Underground Research Facility (SURF) in Lead, South Dakota, where they will arrive at a target 1.5 kilometers (4,890 feet) below the surface.
A trap deep underground
The Sanford Lab is in the former Homestake Gold Mine, which operated for 126 years before closing in 2001. The site's depth is important, as the nearly one mile of overlying rock shields the sensitive detectors from cosmic rays, creating a radioactively quiet environment. To house the DUNE detectors, crews are excavating huge caverns, removing approximately 800,000 tons of rock.
Inside these caverns will sit four massive detector modules. Each will be filled with 10,000 tons of liquid argon, for a total of 40,000 tons of active detector material. The argon is kept at a frigid -184 degrees Celsius (-300 degrees Fahrenheit). When a rare neutrino interacts with an argon nucleus, it creates a shower of charged particles. These particles drift through the liquid argon in a strong electric field, some as high as 600,000 volts, allowing their tracks to be recorded in three dimensions by wire planes. This technology is called a Liquid Argon Time Projection Chamber (LArTPC).
The goal is to understand why matter exists. The Big Bang created equal amounts of matter and antimatter, which would have annihilated each other, leaving only energy. DUNE will precisely measure how neutrinos and their antimatter counterparts, antineutrinos, change "flavor" as they travel from Illinois. A difference in their behavior, a phenomenon called CP violation, could explain the matter-antimatter imbalance that allowed our universe to form. DUNE will also watch for neutrinos from supernova explosions and search for proton decay, which could help explain the unification of nature's forces.
💡Fun Facts
The same mine hosted the Nobel Prize-winning Homestake experiment, where in the 1960s Raymond Davis Jr. first detected neutrinos from the sun using a 100,000-gallon tank of dry-cleaning fluid.
When cooled to its operating temperature, each 58-meter-long detector module will shrink by about 16.5 centimeters (6.5 inches).
The thin wires that record particle tracks inside the detectors are just 150 microns thick, about the width of two human hairs.
On average, a supernova occurs in our galaxy only about once every 50 years; DUNE's detectors could record up to 2,000 neutrino interactions from such an event.
