For many years, physicists believed neutrinos were completely compatible with the Standard Model. They don’t, however. Scientists hope to gain a deeper knowledge of the workings of the entire cosmos by better understanding these weird, elusive particles, one discovery at a time.
Neutrinos are both mysterious and all-pervasive. They are one of the most abundant particles in the universe, passing through most matter unnoticed; billions of them are currently flowing through your body harmlessly. Their masses are so little that no experiment has been able to determine them. They travel at nearly the speed of light—so close, in fact, that a bad cable connection at a neutrino experiment at Italy’s Gran Sasso National Laboratory in 2011 led to suspicion that they might be the universe’s sole known faster-than-light particle.
Physicists have spent a lot of work trying to figure out what these invisible particles are like. They discovered that neutrinos come in a variety of flavors in 1962. Scientists had identified three flavors of neutrinos by the end of the century—the electron neutrino, muon neutrino, and tau neutrino—and made the strange finding that neutrinos could change flavor through a process known as oscillation. This unexpected finding marks a watershed moment in physics, as it is the first evidence of physics beyond the highly successful Standard Model, the theoretical framework that physicists have built over decades to explain particles and their interactions.
Scientists are already preparing for new neutrino research that may provide solutions to some important questions:
How much would neutrinos weigh if they could be weighed?
Is it true that neutrinos have their own antiparticles?
Is it true that neutrinos come in more than three varieties?
Do neutrinos have the same mass as other fundamental particles?
Why is there more matter in the cosmos than antimatter?
The solutions to these issues may not only provide a window into physics beyond the Standard Model, but they may also provide answers to puzzles regarding the universe’s origins.
Machine vs. Nature
Scientists have three options when it comes to finding neutrinos to investigate.
They can catch naturally occurring neutrinos, such as those created by nuclear processes in stars like our sun, as well as neutrinos produced by cosmic particle collisions with Earth’s atmosphere and supernovae. The electron-flavor neutrinos are produced by stars like our sun, whereas cosmic particles and supernovae produce a mixture of all three neutrino flavors and their antineutrino opposites.
Scientists can also look into neutrinos produced in nuclear reactors that generate electricity for households and businesses. Antineutrinos with an electron flavor are produced in reactors. Experiments on neutrinos from this type of source necessitate the building of a particle detector near a nuclear power station and provide vital information on neutrinos and their interactions with matter.
Finally, scientists can create neutrinos for studies by firing protons from an accelerator at graphite or other targets, which release certain types of neutrinos. The advantage of accelerator experiments is that they can look at either neutrinos or antineutrinos. The powerful beams produced by these accelerator-created particles increase the likelihood of a neutrino interaction in detectors. Furthermore, accelerators can generate neutrinos with a higher energy than those produced by reactors or the sun. As a result, accelerator tests are particularly useful for identifying the nature of neutrinos.
Another advantage of the two types of man-made neutrino sources is that detectors can be placed at different distances from the source, depending on the science that needs to be done. For reactor experiments, the best distances can range from tens of meters to a few hundred kilometers, whereas for long-baseline oscillation experiments using neutrinos from accelerators, the optimal distances can range from hundreds to thousands of kilometers.
For example, the planned Long-Baseline Neutrino Experiment, which will use an existing accelerator at Fermi National Accelerator Laboratory, will have a detector placed at what former LBNE Spokesperson Bob Svoboda refers to as “the sweet spot”—a location just far enough away from the detector that neutrinos will have mixed their flavors to near maximum by the time they hit it. Svoboda, a professor at the University of California, Davis, said, “From this, we can learn a great deal about how neutrinos change.” Because LBNE produces both neutrinos and antineutrinos, scientists can investigate the distinctions between matter and antimatter interactions and what they mean for the universe’s matter-antimatter imbalance.
If you’re lucky, you’ll be able to catch them
Neutrino detectors come in a range of shapes and sizes. Because neutrinos are undetectable to detectors, scientists must use a roundabout approach: they record the charged particles and light flashes produced when a neutrino collides with an atom, and deduce the neutrino’s presence.
Because the tiny neutrino interacts with matter so seldom, putting a lot of matter in its way is the only way to detect it. The Japanese neutrino detector Super-Kamiokande is filled with 50,000 tons of water. Neutrinos, which originate from the sun and are produced by a 295-kilometer-away accelerator, combine with water molecules to form charged particles. As a result, these particles emit blue flashes known as Cherenkov radiation. The glow is captured and recorded by light sensors within the water tank.
SuperK’s technology is being advanced with the new NOvA detector, which is currently under construction in Ash River, Minnesota. NOvA will observe neutrinos launched at the detector from Fermilab, some 800 kilometers away, using liquid scintillator—a chemical that flashes as particles pass through—rather than water. NOvA will be one of the world’s largest plastic structures, measuring more than 60 meters long and 15 meters tall.
Rather than employing a single huge tank filled with liquid, the NOvA detector is highly segmented to obtain more information about the identification and energy of each incoming neutrino. According to Pat Lukens, project manager for the experiment at Fermilab, the 14,000 tons of liquid scintillator will be distributed among hundreds of thousands of PVC plastic tubes. Researchers will be able to pinpoint exactly where a neutrino collided with a nucleus in the detector, resulting in charged particles and light flashes, and which way the particles moved.
A grid of wires submerged in a detector liquid is another method for gathering more information about neutrino interactions. The wires attract charged particles that appear when neutrinos interact with the liquid when they are exposed to high voltage. The ICARUS neutrino detector in Italy uses this technology to reveal the precise tracks of the charged particles created when neutrinos interact in liquid argon. Scientists are designing the next generation of this type of detector for the much larger LBNE detector, which will be housed at the Sanford Lab in South Dakota.
The following steps are:
Recent neutrino experiments have revealed a wealth of new information regarding neutrinos and their behavior. Researchers at the Daya Bay Reactor Neutrino Experiment in southern China turned on the first set of detectors in 2011, aiming to make a vital measurement that would help them understand how one type of neutrino transforms into another.
The Daya Bay scientists reported success in March 2012, after only seven months of data collection: they had nailed the measurement of theta one-three, one of three so-called “mixing angles” that explain the oscillation of neutrinos between one flavor and another. Previous research had revealed that theta one-three had to be tiny, leading scientists to speculate that this mixing angle could be zero. When combined with other neutrino measurements from Japan, South Korea, France, and the United States, the Daya Bay finding revealed that the angle is tiny but not zero.
Neutrino scientists from all over the world celebrated when the size of that angle was revealed. The finding raised the possibility that neutrinos act differently than antineutrinos, which could explain the universe’s preponderance of matter over antimatter.
As a result, scientists are in a good position to learn more about one of the universe’s most numerous and pervasive particles. Boris Kayser, a Fermilab theorist, believes that new neutrino oscillation experiments have a good chance of meeting their objectives. They might identify the neutrino mass hierarchy and whether neutrino interactions violate matter-antimatter symmetry using the theta one-three finding. These are important steps in determining whether neutrinos are the cause of matter’s dominance over antimatter in our universe.
According to Kayser, the most difficult question to answer is, “What are the unknown unknowns?” While physicists have some expectations for what they will find, neutrinos have proven to be challenging to predict time and time again. It’s also likely that neutrinos may continue to surprise scientists in the future, given their strange nature.
Physicists are beginning to gain a better understanding of neutrino behavior through experiments that employ a variety of methodologies and technology. The findings could be crucial in resolving long-standing scientific puzzles.
WHAT ARE THE THREE KNOWN NEUTRINO TYPES’ MASSES?
Neutrinos have a modest, nonvanishing mass, according to experiments. The masses of neutrinos are unknown, despite the fact that they must be a million times lighter than an electron. Neutrinos could account for several percent of the universe’s mass and play a key role in the universe’s evolution due to their abundance.
The mass differential between the three neutrino kinds determines the frequency of neutrino oscillations. The NOvA experiment will shortly begin sending neutrinos 810 kilometers from Fermilab to Ash River, Minnesota. Scientists seek to identify which type of neutrino is the heaviest and which is the lightest by observing the oscillations that ensue.
The first step is to figure out what the mass hierarchy is. Scientists must also identify the absolute neutrino mass scale by measuring the mass of one of the neutrino kinds to complete their understanding of neutrino masses. In Germany, the KATRIN project will seek to do just that. The experiment will look into the nuclear decay of tritium, a radioactive type of hydrogen that is unstable. It will compare particle mass and kinetic energy before and after an electron antineutrino decay. Because the entire energy of all particles participating in the decay must be conserved, scientists can calculate the mass of the antineutrino if they can accurately measure the kinetic energy of the particles.
ARE NEUTRINOS ANTIPARTICLES IN THEIR OWN RIGHT?
Scientists have witnessed neutrinos and antineutrinos interacting with matter. However, whether a neutrino and its antiparticle are two different particles is unclear. Scientists can easily identify charged particles from their antiparticles based on their electric charge in the case of charged particles. A positron has a positive charge while an electron has a negative charge. Neutrinos, on the other hand, do not have an electric charge. As a result, a neutrino might be its own antiparticle. This instance is known as the Majorana neutrino, after Italian scientist Ettore Majorana, who was the first to recognize it. Alternatively, neutrinos and antineutrinos could be two independent particles that behave according to Paul Dirac’s equations.
The Majorana-vs.-Dirac neutrino debate is being settled by a number of nuclear experiments, notably the Enriched Xenon Observatory in New Mexico and the Majorana experiment in South Dakota. They’re looking at radioactive nuclei that have two neutrons decaying at the same time, a process called double beta decay that was originally discovered in 1986. Normally, this nuclear event produces two antineutrinos, which transfer energy away from the decay process.
The two antineutrinos would be neutrinos if the Majorana hypothesis is right, and they might “cancel each other out.” The consequence would be a neutrinoless double beta decay every now and again, with no neutrinos or antineutrinos emitted. If investigations support the Majorana hypothesis, it will pave the way for a slew of new hypotheses explaining how neutrinos acquire mass and why their mass is so much smaller than that of any other known particle of matter.
ARE THERE MORE THAN THREE FLAVORS OF NEUTRINO?
Only three neutrino flavors are described in the Standard Model, each of which is related to the electron or one of its heavier siblings via the weak nuclear interaction, which is responsible for radioactive decay and neutrino creation. However, a growing body of evidence implies that other neutrino flavors exist, with properties that differ from the three types of neutrinos now recognized. Experiments will continue to seek for these “sterile” neutrinos, which are so named because they do not interact with other matter via the weak interaction like other neutrinos do.
DO NEUTRINOS GET MASS FROM THE HIGGS?
The field associated with the Higgs boson, according to the Standard Model, supplies mass to quarks and charged leptons (a group of fundamental particles that includes the electron). Many scientists, on the other hand, believe that the masses of ultra-light neutrinos are derived in part from an unknown source. The Large Hadron Collider, which just identified a Higgs-like particle, will not be able to measure neutrino properties. Future nuclear and neutrino oscillation experiments, such as NOvA and the Long-Baseline Neutrino Experiment, may be able to shed light on the genesis of neutrino masses. Boris Kayser, a Fermilab theorist, argues that LBNE and NOvA could help interpret the outcomes of those nuclear experiments.
WHY DID MATTER BECOME THE WINNER INSTEAD OF ANTIMATTER?
Matter and antimatter generated in equal numbers when the universe began, according to physicists’ current understanding of the big bang. If that were the case, by now every speck of matter should have collided with every speck of antimatter. This would have released a great deal of energy and filled the universe with light and radiation, but it would have left the universe devoid of any matter. “Why isn’t the universe totally made up of energy?” Kayser wonders. “How come matter and antimatter didn’t annihilate each other as soon as they were created?”
The explanation lies in a phenomenon known as charge-parity symmetry violation. It’s a primary priority to find the correct form of CP violation to explain the preponderance of matter, and neutrinos are great candidates. Mark Messier, a co-spokesperson for the NOvA project and an Indiana University professor, adds, “It’s sometimes called the Holy Grail of neutrino physics.”
Previous research has discovered CP violation—a discrepancy in the behavior of particles and their antiparticles—among quarks, which are elementary particles. However, the overall matter-antimatter imbalance is not explained by this CP violation.
Because of their amazing lightness, neutrinos are thought to be the ultra-light relatives of very heavy particles that lived briefly in the early cosmos, according to the “see-saw picture” theory. The disintegration of these heavy particles may have disrupted CP symmetry, resulting in the current matter-antimatter imbalance. If this is the case, scientists should be able to detect CP violation in the oscillation of today’s neutrinos.