A couple months ago, scientists with IceCube, an Antarctica-based neutrino observatory, discovered two very high-energy neutrinos — named Bert and Ernie — that appeared to have originated from beyond our solar system. This is amazing news, and we talked to the researchers about what it means.
The IceCube team isn't sure yet sure where the neutrinos came from (possible sources: black holes and gamma-ray bursts), but they are 99 percent sure the particles weren't produced locally from cosmic rays slamming into the atmosphere — a common source of high-energy neutrinos on Earth. A month later, the same group of scientists announced that they detected over two-dozen more of the energetic, extraterrestrial particles.
We talked to two of IceCube's scientists to learn the ins-and-outs of these mysterious particles, including where they come from and how scientists go about studying them.
What are neutrinos?
In the most basic sense, neutrinos are elementary subatomic particles that have almost no mass and can travel vast distances without interacting with normal matter. "They can go through light-years of lead without stopping," says Nathan Whitehorn, an IceCube physicist at the University of Wisconsin–Madison. In fact, trillions of neutrinos are passing right through you each second.
Neutrinos owe their ghost-like powers to the facts that they have no electric charge (are unperturbed by electromagnetic forces) and are only affected by the weak nuclear force, which works at very short ranges, and gravity, which is quite weak at subatomic scales.
Famed physicist Wolfgang Pauli initially proposed the concept of the neutrino back in 1930 to explain what happens when tritium (an isotope of hydrogen) undergoes beta decay, Whitehorn tells io9. You see, when tritium decays into helium-3, an electron is also released, carrying away some of the energy. But scientists noticed that the equation was unbalanced — tritium had more energy than its resulting decay product, suggesting that an invisible particle must be carrying away additional energy.
Pauli initially thought to call this mysterious particle a "neutron," but this led to confusion when James Chadwick discovered a massive nuclear particle in 1932 and named it the neutron (a name that stuck). Soon after, Enrico Fermi developed a theory of beta decay, which included Pauli's invisible particle — he decided to call the new particle the neutrino, or "the neutral one."
Scientists didn't detect neutrinos until over thirty years later, Whitehorn says. A team led by Clyde Cowan and Fred Reines detected neutrinos coming from a nuclear reactor (Savannah River Plant in South Carolina) and published their results in the journal Science.
Since then, scientists have learned a few strange properties of neutrinos. For one: There are several different types, or flavors, of neutrinos, which are so named for the charged particle they're associated with. Cowan and Reines detected the electron neutrino (technically, the electron antineutrino), which is associated with the electron, but there are also muon neutrinos and tau neutrinos, which are associated with the muon and tau, respectively, Whitehorn says.
Interestingly, neutrinos can oscillate, or change flavors, mid-flight. This odd ability was first hinted at by the work of the late physicist Ray Davis Jr. in the 1960s. Scientists calculated how many neutrinos the sun should be producing, but Davis only detected a fraction of the particles he expected to find — this discrepancy was later called "the solar neutrino problem," and it lasted for about 40 years.
"[Davis] thought the sun was going out because there were too few of the neutrinos," Whitehorn says. "But they were just moving into a type that he couldn't see." That is, the neutrinos were oscillating into muon neutrinos and tau neutrinos, but Davis' detector was only sensitive to electron neutrinos.
Where do neutrinos come from?
There are numerous sources of neutrinos in the universe, both in space and here on Earth. "Wherever there is nuclear physics, there are neutrinos," says IceCube principle investigator Francis Halzen, a physicist with the University of Wisconsin–Madison. "Neutrinos are the liaisons that make nuclear physics possible."
For example, you already know that neutrinos are produced in the sun and other stars. They are the byproduct of nuclear fusion, which involves the merging of two protons (hydrogen atoms) to form a deuteron, releasing a positron (antielectron) and an electron neutrino at the same time.
High-mass stars end their lives in supernova explosions, which also produce neutrinos. Before the explosion, the star collapses in on itself, forcing protons to combine with electrons, forming neutrons and electron neutrinos. Eventually a neutron core will form in the center of the dying star — as it cools, it releases neutrino-antineutrino pairs of all flavors.
Scientists also believe that tons of neutrinos were created during the Big Bang, so there should currently be a "cosmic neutrino background" similar to the cosmic microwave background radiation. These particles, however, are thought to have energies too low to detect with current neutrino detectors. "Some other astrophysical neutrino sources could be black holes in the center of active galaxies, gamma ray bursts and potentially some star forming regions of galaxies," Whitehorn adds.
Several neutrino sources exist here on Earth. In terms of artificial sources, nuclear reactors produce electron antineutrinos via beta decay, while particle accelerators produce neutrinos of different flavors by firing protons into graphite or other targets. The natural decay of certain radioactive elements — potassium-40, uranium-238 and thorium-232 — also generates antineutrinos, which are sometimes called geoneutrinos. "The energy signature tells you about the composition of earth," Whitehorn says.
Halzen notes one surprising source of neutrinos on Earth: Our bodies. "You make them all the time, when the potassium in your body decays into neutrinos," he says.
The most intense source of local neutrinos, particularly high-energy neutrinos, is cosmic rays (which are actually high-energy particles) smashing into Earth's atmosphere, the researchers say. When high-energy protons interact with the molecules in the atmosphere, they shower the Earth in tiny particles, some of which are subatomic pions. The pions decay to muons and muon neutrinos; the muons, in turn, decay to electrons, muon neutrinos and electron neutrinos.
How do scientists study neutrinos?
Just as there are multiple sources of neutrinos on Earth, there are a number of projects seeking to detect and learn more about the elusive particles. Neutrinos are invisible, so scientists can only detect them indirectly when they interact with other matter. As you can imagine, this isn't easy considering that neutrinos are electrically neutral and pass through most matter unaffected.
The solution: Put a whole bunch of matter in its way and use a very large detector. Some neutrino detectors, such as those used by Ray Davis Jr., consist of large tanks filled with chlorine solutions, such as tetrachloroethylene (dry cleaning fluid). Scientists can detect a neutrino when it crashes into a chlorine nucleus, changing it into an argon nucleus.
Several other detectors use a different approach — they measure Cherenkov radiation, a kind of sonic boom for light. "When a neutrino interacts with matter, it makes a charged lepton that it's associated with," Whitehorn explains, meaning that an electron neutrino makes an electron, and so on. Though nothing can travel faster than the speed of light in a vacuum, these charged particles can travel faster than light in a medium, such as water or ice, creating a blue flash that scientists can detect. Our Esther Inglis-Arkell explains:
These particles still interact with the medium as they go through, exciting molecules which then release energy by releasing photons. Usually, this wouldn't create enough of a glow to be noticeable, but just like sound waves piling up at the tip of an aircraft, emissions of photons "pile up" as multiple photons are emitted at once. These photons move in phase with one another, and constructively interfere. Their peaks and valleys build on each other to the point where they are visible to the naked eye. This is Cherenkov (or Cerenkov) radiation, and it is emitted in a cone centered on the moving particles. It's a very beautiful glow, but probably not worth looking into a nuclear reactor for.
Of course, catching a glimpse of that blue flash is still difficult in the face of all the background noise around us, so scientists bury their light-sensitive photomultiplier tubes in water or ice. For example, IceCube is entombed in Antarctic ice. "We have a mile of ice that shields us from the background radiation," Halzen says. "If we didn't do that, we would just be swamped from all the stuff happening on the surface of the Earth." Even so, IceCube only picks up about 100,000 neutrino events from the Sun and atmosphere a year, a testament to just how elusive the particles are.
One of the most famous neutrino observatories, the Super-Kamiokande, or Super-K, is submerged in 50,000 tons of ultra-pure water 1,000 meters underground in Japan's Mozumi Mine. The ANTARES neutrino detector also uses water — it consists of 12 strings of photomultiplier tubes that are 2,500 meters beneath the Mediterranean Sea.
Instead of detecting neutrinos, the goal of some neutrino projects is to better understand neutrino oscillations, such as Fermilab's MINOS Experiment. In the experiment, two detectors observe a beam of neutrinos (generated by an accelerator), one at the source of the beam and another 450 miles away.
Why do scientists study neutrinos?
Halzen and Whitehorn identify two main reasons to study neutrinos. The first has to do with neutrino oscillations. Initially, scientists believed that neutrinos were massless — in fact, in the Standard Model of particle physics, neutrinos are massless. "But you can't have oscillations without mass," Whitehorn says.
"Neutrino mass was a big discovery, it was the first crack in the Standard Model," Halzen adds. "Everybody thinks that the neutrino is like the canary in the coal mine that will lead us from the Standard Model to the next model of physics."
It's important to note that scientists don't yet know the mass of any of the neutrino flavors, only that their masses are very, very tiny. Studying neutrino oscillations may help scientists better understand neutrino masses, including which flavor is heaviest and which is lightest.
Neutrinos are also important because they allow scientists to do astronomy using particles other than photons. The beginning of neutrino astronomy can be traced back to SN 1987A — the famous 1987 supernova in the Large Magellanic Cloud. Hours before visible light from the supernova reached Earth, several neutrino observatories detected the event (because matter doesn't typically absorb or scatter neutrinos as it does light). "What told us [the supernova] happened was not the light, it was the neutrinos," Halzen says. "It put stellar theory on observational footing."
Moreover, the neutrinos — all 24 of them — gave astronomers significant insight into supernovas. "The model is that a neutron star forms from a core collapse," Whitehorn says. "The neutrinos gave the only direct evidence of that." Halzen adds: "Neutrinos are the ideal way to study the violent universe."
The IceCube project is looking to discover more extra-solar neutrinos. Bert and Ernie (the neutrinos, not the muppets) had energies of more than a petaelectronvolt — that is, they were 100 million times more energetic than the supernova neutrinos and a thousand times more energetic than the neutrinos produced in accelerators. The 26 additional neutrinos the project scientists found were also very high energy, at 30 teraelectronvolts (a teraelectronvolt is one-thousandth of a petaelectronvolt).
Ultimately, the scientists hope the neutrinos will help them better understand the origins of high-energy cosmic rays, which are a mystery to scientists. Essentially, anything that could produce cosmic rays would also produce these high-energy neutrinos, Whitehorn says. So they are now trying to trace the neutrinos back to their source(s) — a task that should be easier than tracing back cosmic rays because neutrinos travel in a straight line, unperturbed by strong magnetic fields.
"These neutrinos could be clues to the origins of cosmic rays," Whitehorn says. "We could be opening a new window into the universe."