The question may appear straightforward, but physicists have yet to find a solution. This is something that new measurements are attempting to change.
Neutrinos are the lightest particles in the universe. At least 500,000 times lighter than electrons, these incredibly abundant, infrequently interacting particles. They’re made by the sun, exploding stars, and disintegration processes on Earth, including ones in your own body. However, they interact with other matter so infrequently that you’d never guess there are so many of them.
For decades, physicists assumed these phantom particles had no mass. Experiments have shown, however, that neutrinos indeed have mass. In actuality, there are three separate types of neutrinos, each with its own mass.
The exact value of any of these masses has yet to be determined by scientists. Even determining which neutrino is the heaviest would be a significant step forward in our understanding of neutrinos and the physics that govern our universe. The answer to this puzzle, called as the “neutrino mass hierarchy” or “neutrino mass ordering,” has a lot riding on it.
The sun, the sky, and the earth
Neutrinos interact with matter as electron neutrinos, muon neutrinos, or tau neutrinos, named after their favorite partner particles. Neutrinos can also oscillate, which means they can switch between the three identities.
Many electron neutrinos are converted to muon and tau neutrinos by the time they reach Earth due to nuclear events in the sun’s core. Muon neutrinos are produced when high-energy particles collide with the Earth’s atmosphere; they may oscillate between electron and tau neutrinos before being discovered.
The three types of neutrinos, however, do not directly correlate to the three masses. Instead, there are three “neutrino mass states” numbered 1, 2, and 3, each with varying chances of interacting with matter as an electron neutrino, a muon neutrino, or a tau neutrino.
Scientists can make some judgments about the links between the three mass states by knowing the rates at which neutrinos oscillate from one type to another. Solar neutrinos have been carefully measured, and the second mass state is only slightly heavier than the first. The mass difference between the third mass state and the other two is substantial, according to measurements of atmospheric and accelerator-made muon neutrinos oscillations.
However, scientists have yet to discover if mass state 3 is much heavier or lighter than mass states 1 and 2.
In what are known as “long-baseline” neutrino studies, researchers fire neutrinos through hundreds of kilometers of solid rock to discriminate between the “regular mass hierarchy” (order 1, 2, 3) and the “inverted mass hierarchy” (3, 1, 2).
“When a neutrino travels, the electron neutrino half wants to interact with the electrons on Earth, whereas the muon and tau neutrino parts are unaffected,” says Zoya Vallari, a Caltech postdoc. “The amount of oscillation that occurs is influenced by this additional impact.”
Long-baseline experiments like the NOvA experiment in the United States and the T2K experiment in Japan have helped scientists better comprehend oscillation. However, their analyses of the mass hierarchy have thus far been unconvincing.
A crucial component of the puzzle
Whether the third neutrino is the lightest or the heaviest has huge ramifications for our understanding of these common particles (pun intended). For example, the mass of neutrinos has yet to be discovered. Determining if it is similar to the Higgs mechanism, which is responsible for the mass of other particles, requires working out the hierarchy.
Neutrinos might hypothetically be their own antimatter particles because they have no electric charge. Knowing the mass ordering will help scientists design tests to verify this idea, which will open the door to deeper questions about the universe.
The NOvA experiment delivers beams of neutrinos and antineutrinos about 500 miles from Fermilab in Illinois to a detector in Ash River, Minnesota, in search of a solution to the neutrino hierarchy conundrum. They’ll travel around 190 miles from J-PARC in Tokai, Japan, to a detector beneath Mount Ikeno as part of the T2K experiment.
The rate of neutrino oscillations is compared to the rate of antineutrino oscillations by scientists at the experiments. Any changes could aid scientists in determining what is going on with neutrino masses. It may also aid their understanding of why matter prevailed over antimatter in the early cosmos. We might be here because of neutrinos, but we don’t know for sure.
There is currently no major asymmetry between neutrino and antineutrino oscillations, according to NOvA. The T2K experiment revealed intriguing evidence that neutrinos and antineutrinos oscillate differently. The T2K system is now being upgraded, and NOvA will continue to collect data until the middle of the decade.
The inverted hierarchy, out of the two options, would make several future trials easier. “So, if I had a choice, I would select the inverted hierarchy,” says Pedro Machado, a theorist at the Fermi National Accelerator Laboratory of the US Department of Energy. “Theory cannot progress without experimental results.”
The inverted hierarchy would be more “fun” for Vallari as well, but “if I had to place a bet, I would do it on the conventional hierarchy,” she says.
A solution is within reach
The neutrino mass hierarchy, unlike many other mysteries in particle physics, offers a clear path to resolution. The answer is well within the experimental capabilities of the next generation.
The Deep Underground Neutrino Project, a Fermilab-hosted multinational experiment set to launch in the late 2020s, will transport neutrinos on a journey of about 800 miles from Illinois to South Dakota—60 percent further than NOvA, giving more matter for the neutrinos to interact with. The DOE Office of Science and other financial bodies are supporting both experiments.
According to Vallari, who is a member of the DUNE and NOvA collaborations, such a lengthy journey will magnify the Earth’s influence on neutrino oscillations, allowing researchers to tease out the mass hierarchy. The planned Hyper-Kamiokande improvement to the T2K experiment in Japan should similarly provide a response within a few years of data collection.
“I am fairly confident in predicting that we will obtain a definitive measurement of the mass hierarchy from at least one of the tests in the early 2030s,” Vallari says.
Even then, we’ll just know the differences between the three neutrino masses; the magnitude of the masses as a whole will remain unknown.