This might win an award for "most obvious statement ever," but the universe is big. And with its size comes big questions. Perhaps the biggest is "What makes the universe, well...the universe?"
Researchers have made a crucial step forward in their effort to build scientific equipment that will help us answer that fundamental question.
ProtoDUNE will validate the technology of the much larger DUNE experiment, which is designed to detect neutrinos, subatomic particles most often created in violent nuclear reactions like those that occur in nuclear power plants or the Sun. While they are prodigiously produced, they can pass, ghost-like, through ordinary matter. There are three distinct types of neutrinos, as different as the strawberry, vanilla, and chocolate flavors of Neapolitan ice cream.
Further, through the always-confusing rules of quantum mechanics, these three types of neutrinos experience a startling behavior -- they literally change their identity. Following the ice cream analogy, this would be like starting to eat a scoop of vanilla and, a few spoonfuls in, it magically changes to chocolate. It is through this morphing behavior that scientists hope to explain why our universe looks the way it does, rather than like a featureless void, full of energy and nothing else.
Large enough to encompass a three-story house, ProtoDUNE is located at the CERN laboratory, just outside Geneva, Switzerland. Years in the making, ProtoDUNE is filled with 800 tons of chilled liquid argon, which detects the passage of subatomic particles like neutrinos. Neutrinos hit the nuclei of the argon atoms in the ProtoDUNE detector, causing particles with electrical charge to be produced. Those particles then move through the detector, banging into argon atoms and knocking their electrons off. Scientists then detect the electrons.
It's similar to how you can know an airplane recently passed overhead because you observe contrails, the white streaks in the sky it briefly leaves behind. The ProtoDUNE detector has now observed particles coming from space -- what scientists call cosmic rays -- which has validated the effectiveness of the particle detector.
Though considerably large, ProtoDUNE pales in comparison to the size of the DUNE apparatus, which is still being developed. DUNE will be based at two locations: Fermi National Accelerator Laboratory (Fermilab), which is America's flagship particle physics laboratory located just outside Chicago, and the Sanford Underground Research Facility (SURF), located in Lead, South Dakota.
The biggest part of the DUNE experiment will ultimately consist of four large modules, each of which will be twenty times larger than ProtoDUNE. Because neutrinos interact very rarely with ordinary matter, bigger is better. And with an eighty times increase in volume, the DUNE detector will be able to detect eighty times as many neutrinos as ProtoDUNE.
These large modules will be located nearly a mile underground at SURF. That depth is required to protect them from the same cosmic rays seen by ProtoDUNE.
Fermilab will use its highest energy particle accelerator to generate a beam of neutrinos, which it will then shoot through the Earth to the waiting detectors over 800 miles away in western South Dakota.
This beam of neutrinos will pass through a ProtoDUNE-like detector located at Fermilab to establish their characteristics as they leave the site. When the neutrinos arrive in South Dakota, the much bigger detectors again measure the neutrinos and look to see how much they have changed their identity as they traveled. It's this identity-changing behavior that DUNE is designed to study. Scientists call this phenomenon "neutrino oscillations" because the neutrinos change from one type to another and then back again, over and over.
While investigating and characterizing neutrino oscillations is the direct goal of the DUNE experiment, the deeper goal is to use those studies to shed light on one of those fundamental questions of the universe. This will be made possible because the DUNE experiment not only will study the oscillation behavior of neutrinos, it can also study the oscillation of antimatter neutrinos.
A strong runner-up in the "most obvious statement ever" award is "our universe is made of matter." But researchers have long known of a cousin substance called "antimatter."
Antimatter is the opposite of ordinary matter and will annihilate into pure energy when combined with matter. Alternatively, energy can simultaneously convert into matter and antimatter in equal quantities. This has been established beyond any credible doubt.
Yet, with that observation, comes a mystery. Scientists generally accept that the universe came into existence through an event called the Big Bang. According to this theory, the universe was once much smaller, hotter, and full of energy. As the universe expanded, that energy should have converted into matter and antimatter in exactly equal quantities, which leads us to a very vexing question.
Where the heck is the antimatter?
Our universe consists only of matter, which means that something made the antimatter of the early universe disappear. Had this not happened, the matter and antimatter would have annihilated, and the universe would consist of nothing more than a bath of energy, without matter -- without us.
Which brings us back to the DUNE experiment. Fermilab will make not only neutrino beams, it will also make antimatter neutrino beams. The exact mix of neutrino "flavors" leaving the Fermilab campus will be established by the closer detector, and then again when they arrive at SURF, so that the changes due to neutrino oscillation can be measured. Then the same process will be done with antimatter neutrinos. If the matter and antimatter neutrinos oscillate differently, that will likely be a huge clue toward answering the question of why the universe exists as it does.
With the completion of the new ProtoDUNE technology that will be used in the DUNE detector, the race is on to build the full facility. The first of the detector modules is scheduled to begin operations in 2026.
While Fermilab has long made substantial contributions to the CERN research program, the DUNE experiment marks the first time that CERN has invested in scientific infrastructure in the United States. DUNE is a product of a unified international effort.
Modern science is truly staggering in its accomplishments. We can cure deadly diseases and we've put men on the moon. But perhaps the grandest accomplishment of all is our ability to innovate in our effort to study in detail some of the oldest and most mind-boggling questions of our universe. And, with the success of ProtoDUNE, we're that much closer to finding the answers.
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