Nearly 14 billion years ago, a universe appeared in an unthinkably high-energy blast. Particles started to materialise out of that energy, as did their antiparticles, which are kind of like evil twins, a mirror image with the opposite electric charge. Each and every particle had an antiparticle, scientists believe, and they would annihilate each other in a pop of energy. Most particles met their end at the hands of their antiparticle in those earliest days.
Most particles met their end, I said”but not all. A small amount of matter persisted over the antimatter, and it condensed into galaxies, stars, planets, and eventually people. Some of those people wondered: What could possibly have been different between the matter and antimatter such that the matter dominated? Or, in short: Why are we here?
Today, many physicists think they’ve identified a signpost guiding them toward that question’s answer, thanks to the strange behaviour of the universe’s most abundant matter particle, the neutrino, sometimes called a “ghost particle.” The difficult-to-detect neutrino seems to undergo a strange identity-flipping process, and if this reaction occurs differently between neutrinos and antineutrinos, then this process, called neutrino oscillation, could help physicists explain why matter dominates over antimatter. Unlike the last few decades of successful particle hunts, neutrino physics is a trek into the unknown, one that the United States physics community has chosen to pursue full-on. A flagship, $US2 ($3) billion experiment called LBNF/DUNE will lead the search, in pursuit of answers that may take decades or more to find.
“I think we’re seeing a real change here in what particle physics is about,” Joe Lykken, Fermilab’s deputy director of research, told Gizmodo. “For a long time, it was about producing new particles, which people won Nobel prizes for. That was fine, but at the end of the day, our job as scientists is to understand the basic processes of the universe and how it works”¦ It’s not about adding more particles to a long list of particles; it’s about, how does the universe really operate in a fundamental way to produce what we see today?”
Particle physics experiments invariably look like giant, brightly coloured masses with pipes and wires coming out in all directions inside of Costco-sized warehouses. But the ProtoDUNE detectors on the CERN campus in Geneva stood out to me in their starkness; they were just a pair of hollow, red steel cubes, each the size of a house, that extended deep into the ground and dwarfed the engineers who stood atop them. After my 2017 visit, each empty container would be filled with 800 tons of liquid argon.
Neutrinos pass directly through most matter without so much as a bump, so they’re invisible to most experiments. Neutrino-hunting detectors all iterate on a similar concept: Fill the biggest container you can imagine with a detecting medium, like water or liquid argon, and wait for the rare neutrino interactions to happen. In water detectors, neutrinos interact with some of the water molecules, producing particles that in turn generate small flashes of detectable light as they travel faster than the speed light travels through water. In liquid argon detectors, some of the neutrinos will interact with the argon nuclei medium, knocking off particles that in turn knock electrons off of the atoms. The electrons drift in the direction of an electrically charged surface containing a particle-detecting element. The data and timing information gathered by these detectors tell researchers where the neutrino came from and about its energy and identity.
Standing on the platform overlooking ProtoDUNE with CERN physicist Stefania Bordoni, I realised that these enormous cubes were just a fraction of the final DUNE far detector, the larger of the experiment’s two detectors. They were prototypes to ensure that the production equipment would work and to test two different kinds of detecting technologies. The DUNE far detector will ultimately dwarf ProtoDUNE, with its 68,000 tons of liquid argon in four units built 4,850 feet underground in the Sanford Underground Research Facility, located at the retired Homestake precious metal mine in Lead, South Dakota.
But according to the most widely accepted theory of physics, it wasn’t supposed to be like this.
Physicist Wolfgang Pauli first theorised neutrinos in 1930 as a way to explain missing energy during a radioactive process called beta decay. When an atomic nucleus emits an electron, the atom must also spit out energy in the form of a massless, chargeless particle. He called them neutrons. Physicist James Chadwick discovered a much heavier neutral particle in the atomic nucleus two years later and called his particle the neutron as well, so physicists Edoardo Amaldi and Enrico Fermi began calling the smaller particle the neutrino, using the Italian -ino suffix meaning small. It wasn’t until 1956 that physicists Frederick Reines and Clyde Cowan discovered neutrinos in a nuclear reactor, where they observed neutrino’s antiparticle partners, the antineutrinos, interacting with a proton to produce a neutron and the antiparticle partner of the electron, the positron.
Experiments continued to expand our neutrino knowledge. Physicists at Brookhaven National Lab discovered a neutrino that would interact with muons (a heavier particle related to the electron), which they called the muon neutrino. Others theorised the presence of an even heavier neutrino flavour, the tau neutrino, which was eventually discovered in 2000 at Fermilab in Illinois.
Hints that there was something especially weird about neutrinos began to crop up in the 1970s. Physicist Raymond Davis Jr. led an experiment involving a 100,000-gallon tank filled with the dry-cleaning chemical perchloroethylene, built 1.4 kilometres beneath the surface at the Homestake Mine in order to shield it from particles from space. The experiment detected neutrinos from the Sun”but only around a third of how many they expected to find. Follow-up searches continued until 1998, when the Super-Kamiokande (Super-K) experiment in Japan discovered that neutrinos from the atmosphere could oscillate between flavours. In 2001, the Sudbury Neutrino Observatory in Canada discovered neutrino oscillations in solar neutrinos. In order for neutrino oscillations to make sense, neutrinos would need to have mass”contrary to the predictions of the Standard Model of particle physics, the theory that has predicted the existence of the other fundamental particles, such as the Higgs boson discovered in 2012.
“The neutrino oscillation phenomenon, implying that the neutrino has mass, is the only phenomenon beyond the Standard Model seen in the laboratory venue,” Chang Kee Jung, distinguished professor of physics and astronomy at Stony Brook University, told Gizmodo. Particle physicists are always looking for holes in the Standard Model to explain the unexplained pieces of our universe, such as dark matter. Neutrinos having mass when the Standard Model predicts that they wouldn’t could therefore be an inroad toward solving some of these mysteries.
In 1999, the neutrino physics community convened at a three-day Next generation Nucleon Decay and Neutrino Detectors workshop at Stony Brook University, led by Jung, so theorists and experimentalists could discuss what would come next. Physicists began dreaming of detectors that would be even larger and more advanced than the 50,000-metric-ton Super-K. Ideas began to take shape in the following years, as physicists started to consider a “single-phase” liquid argon detector design proposed by physicist Carlo Rubbia, as well as another “dual-phase” design filled with both liquid and gaseous argon proposed by his son, Andre Rubbia. Several groups of physicists devised large underground neutrino detection projects at various facilities, and the National Science Foundation put out a call for proposals for a Deep Underground Science and Engineering Laboratory (DUSEL) that would include not just neutrino physics but other extreme science and engineering projects as well. They ultimately settled on the Homestake site”but the National Science Board in charge of the NSF’s policies decided not to move forward with the project.
Meanwhile, the U.S. physics community was mulling over building another enormous experiment, the International Linear Collider (ILC). But ultimately, the ILC was also rejected, leaving a void where the U.S. could have had a multibillion-dollar flagship physics experiment, Jung said. By then, the White House’s Office of Science and Technology Policy and the National Academy of Science began to recognise the importance of neutrino physics and of building an underground laboratory to study it, according to a 2003 report: “A deep underground laboratory can house a new generation of experiments that will advance our understanding of the fundamental properties of neutrinos and the forces that govern the elementary particles, as well as shed light on the nature of the dark matter that holds the universe together. Recent discoveries about neutrinos, new ideas and technologies, and the scientific leadership that exists in the United States make the time ripe to build such a unique facility.”
Recognising the void left by the ILC and without DUSEL, the U.S. Department of Energy decided to pick up the underground neutrino physics program. By 2007, physicists began presenting studies containing various designs for the Long Baseline Neutrino Experiment, an upgraded particle accelerator that would send a beam of neutrinos through around 800 miles of Earth before striking a detector deep underground. At that distance, physicists hoped that they’d be able to spot neutrinos as they swapped flavours between leaving the accelerator and arriving at the far detector.
This experimental setup would also provide a way to look for charge/parity (CP) symmetry violation”places where matter acts differently from antimatter. If a muon neutrino produced in the particle accelerator arrives as an electron neutrino at the far detector at a different rate than the same process in antineutrinos, then neutrino physicists would be able to confirm that neutrinos differ from their antiparticle.
The only problem was that physicists had no idea whether the electron neutrinos would actually show up in their detectors.
By 2012, physicists had built up some pretty robust theory surrounding neutrino oscillations, devising a host of parameters that together described the oscillating behaviour. But of all of these numbers, the least well-known was called “Î¸13,” or theta 1-3. The future of neutrino physics stood with this number.
“If that parameter would have been zero, then there would have been virtually no electron neutrino appearance,” Elizabeth Worcester, physics co-coordinator for DUNE, told me. “The probability for the muon neutrinos to oscillate to electron neutrinos would have been nearly zero, so all of these [proposed] experiments couldn’t have looked for neutrino oscillation at all.”
Beginning in late 2011, six detectors placed both near and far from the Daya Bay nuclear power plant in Shenzhen, China watched in wait for neutrinos to, well, disappear. These detectors were designed only to measure electron neutrinos. That means that, if neutrinos oscillated, then the close detector would detect more neutrinos than the far detector as the neutrinos changed identities. This disappearance would allow the physicists to calculate the value of the theta 1-3 parameter.
In April 2012, the Daya Bay team released results better than they could have hoped for. Not only did the electron neutrinos disappear, but the calculated value of theta 1-3 was surprisingly high. That meant that physicists would be able to see neutrinos oscillate over the 800-mile distance between a neutrino beam from Fermilab and a detector at the Homestake Mine and that such an experiment would be able to see whether or not neutrinos violated CP symmetry.
“It was just obvious by eye, and people were totally blown away,” Worcester said.
Other experiments, like T2K, a particle beam directing neutrinos toward the Super-K detector, measured the inverse effect: neutrinos suddenly appearing in detectors. The neutrino community was set to build the neutrino experiment of their dreams. The Particle Physics Project Prioritisation Panel advisory board highlighted the importance of a long-distance neutrino experiment in their 2014 report, and the Long Baseline Neutrino Experiment soon turned into an international collaboration based at Fermilab. That collaboration became the Deep Underground Neutrino Experiment (DUNE), now referred to as LBNF/DUNE (LBNF for the Long Baseline Neutrino Facility).
The pre-excavation phase has already begun to prepare the mine for such a massive project. Ultimately, they’ll excavate a mass of rock weighing more than two Empire State Buildings, which they’ll need to bring up a shaft and put along a conveyor belt in order to dump it into a former mining area. They’ll also need to upgrade the former mine’s ventilation system and infrastructure so that at least 144 people (or more) can fit in the lab at the same time, Jaret Heise, science liaison director at the Sanford Underground Research Facility in the former Homestake Mine told Gizmodo.
Meanwhile at Fermilab, accelerator upgrades are underway. In a warehouse behind concrete blocks, scientists have been working on what looks like a train of physics equipment”copper-coloured pipes and endless wires surrounding components like resonators and magnets tasked with accelerating pulses of protons. Ultimately, this piece will turn into the 820-foot-long Proton Improvement Plan II, a new straight-line particle accelerator that will feed protons into Fermilab’s existing suite of particle accelerators. Ultimately, the powerful proton beam will strike a target, producing particles that in turn produce the neutrinos. Those neutrinos will travel first through a near detector, which will characterise the neutrino makeup in the beam and measure how the neutrinos interact with matter. From there, the neutrinos will travel to the far detector. The core of the CP symmetry experiment is comparing the makeup of the neutrino beam in the near detector and the far detector for both neutrinos and antineutrinos.
When science begins at the facility (it’s scheduled to start taking data in the late 2020s), physicists hope that LBNF/DUNE will be the ultimate neutrino experiment and the premiere high-energy physics facility in the U.S., Paul Dabbar, under secretary for science at the Department of Energy, told Gizmodo. Not only will it compare oscillations between neutrinos and antineutrinos, but it will attempt to solve another mystery relating to the neutrino’s mass. Not only are there three neutrino flavours, but there are three masses as well, called m1, m2, and m3, and the three masses don’t cleanly line up with the three flavours (blame quantum mechanics).
Scientists hope to understand whether m3 is heavier or lighter than m1 and m2, which has implications for understanding how particles behaved in the early universe. Additionally, physicists have theorised a new kind of particle, called the right-handed neutrino, that would offer a mechanism to give neutrinos mass and perhaps be part of the story for why there’s more matter than antimatter. Plus, there are already hints of a fourth kind of neutrino, called a sterile neutrino, showing up in existing experiments. If physicists are lucky, a supernova will go off in our galaxy at some point this century, which DUNE will be able to detect neutrinos from.
Even with all this work, we won’t get a definitive answer to “why is there more matter than antimatter?” If DUNE measures CP asymmetry, it would at least offer a stronger case that scientists are on the right path, Edward Blucher, DUNE’s co-spokesperson and physics professor at the University of Chicago, told Gizmodo. DUNE will hopefully be extremely sensitive to differences between the way that neutrinos and antineutrinos oscillate and able to demonstrate whether or not the two processes are different.
Just recently, the T2K collaboration released a measurement showing hints that the two are indeed different. But back in 1967, physicist Andrei Sakharov proposed that there are in fact three conditions that must be met in order to generate the observable difference between the amount of matter and antimatter, and CP violation is just one of those three. Physicists are in the midst of hunting for other processes, such as those where some of protons’ and neutrinos’ core features change, like in the case of a proton decaying or a neutrino annihilating itself. These discoveries together must also fit into a model of the universe’s evolution that actually predicts the observed difference in abundances, physics professor Silvia Pascoli at Durham University in the United Kingdom previously told Gizmodo.
For all we know, actually answering the question “why are we here?” might take decades or longer. And maybe it’s impossible to ever know. Maybe the universe never wanted to create the same amount of matter and antimatter right from the start, for no reason at all.
But that’s part of what makes the LBNF/DUNE special; scientists are plunging headfirst into the unknown with a machine optimised to do it. And LBNF/DUNE’s physicists realise that they’re playing a long game that might not conclude in their lifetimes.
“I think that the big questions that tie to the origin of the universe is what drew a lot of us to physics, and it’s important and part of how we explain why we want to do these measurements,” Worcester told Gizmodo. “But the biggest part of our day-to-day is, “˜my student’s code won’t compile, how do we fix it to make it work.'” Big questions get more funding, but these undertakings are broken into smaller but still scientifically interesting problems that go into constructing parts of the detector and developing tools to answer smaller questions in order to chip away at the larger ones.
There are plenty of other reasons to build such a large project, of course. LBNF/DUNE is going to be an international experiment that fosters worldwide physics collaboration, like CERN does. It will be competing with Super-K’s successor, Hyper-Kamiokande, in order to produce two independent measurements of CP violation. And, like all major accelerator undertakings, new non-physics technology will inevitably come as a side effect. These advances can lead to better particle accelerators used for cancer treatment, and the radiofrequency cavities used to accelerate particles may one day be useful for quantum computing.
Ultimately, this experiment represents an attempt to probe unexplored cracks in the Standard Model of particle physics. The Higgs boson signified the end of an era when two teams at CERN discovered it in 2012: it was a particle already predicted by the known physical laws, and most physicists expected that it would be found.
But with neutrinos, it isn’t obvious that an answer will be found if only we can build a big-enough experiment. No matter what physicists find at LBNF/DUNE, it’s going to be new and could edge them ever closer to solving some of these outstanding cases that the Standard Model has failed to explain. Truly, neutrino science sits at the precipice of the unknown.