Neutrinos are nearly massless, electrically neutral particles that constantly pass through Earth, human bodies and even dense cosmic environments with little interference. Their rarity of interaction makes them exceptionally difficult to detect, but observatories such as the IceCube Neutrino Observatory are capturing evidence of these particles and using it to study some of the universe’s most extreme and distant environments.
Why neutrinos are so mysterious
Neutrino research has played a prominent role in modern physics. Discoveries involving these particles have contributed to five Nobel Prizes in physics. This year, physicist Francis Halzen received a prestigious award for his “definitive contribution to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.”
George Fuller, Distinguished Professor of Physics at the University of California, San Diego, has spent decades studying nuclear physics, particle astrophysics and cosmology. He also worked with Halzen on early scientific papers on IceCube. His expertise helps explain what makes neutrinos unusual and why scientists have built enormous observatories deep beneath the Antarctic ice to study them.
Neutrinos are elementary particles, meaning scientists do not believe they are made of smaller components. There are three known types: electron neutrinos, muon neutrinos and tau neutrinos. Unlike charged particles, neutrinos have no electric charge. They do have mass, but it is extremely small—possibly about 10 million times less than the mass of an electron. Their exact mass remains unknown, as does the mechanism that produces it.
Neutrinos can also move through matter with little resistance. Because they rarely interact with other particles, they can travel vast distances at speeds close to the speed of light.
These particles come from many sources, including the Sun, distant regions of space and sources that scientists have not yet identified. They are also extraordinarily abundant. Estimates suggest that the universe contains about 1089 neutrinos, or approximately 1 billion neutrinos for every atom.
“Neutrinos are truly ghostly particles,” Fuller said. “They’re hard to stop, they’re hard to catch. How do we know they exist and where they are?”
Ghost particles that pass through almost everything
Although neutrinos are elusive, they can interact with matter through the fundamental force known as the weak interaction. This force is about 20 orders of magnitude weaker than electromagnetism. When weak interactions occur, neutrinos can be scattered or absorbed, giving scientists a way to detect them. Such events are extremely rare, however, and require highly sensitive equipment.
Neutrinos are valuable messengers from otherwise inaccessible regions of the universe because they can pass through dense matter. They can escape environments surrounding collapsing stars and black holes, carrying information that other forms of radiation may not reveal.
The scale of this invisible activity is immense. Approximately 100 billion neutrinos pass through every square centimeter of the human body each second, almost imperceptibly.
“Because the interaction is so weak, neutrinos can pass through almost anything,” Fuller said. “Light years of lead—depending on their energy, they’ll get through it.”
How scientists learned to detect neutrinos
The search for neutrinos began long before scientists had the technology to observe them. In the 1930s, physicist Wolfgang Pauli proposed that an unknown particle could explain why energy appeared to be lost during certain forms of radioactive decay. He initially used a different name for the hypothetical particles, calling them “neutrons” at the time.
Pauli understood how difficult his proposal would be to test. He famously acknowledged the problem by saying, “I did something terrible. I assumed an undetectable particle.”
For decades, neutrinos remained beyond the reach of experiments. That changed in 1956, when physicists Clyde Cowan and Frederick Lines succeeded in detecting them in an experiment known as “Project Poltergeist.”
The breakthrough established that neutrinos were real, but detecting them remained a major challenge. Scientists needed increasingly sophisticated instruments to study the rarest and most energetic neutrinos arriving from space.
Why Antarctica is ideal for cosmic neutrino hunting
In the 1990s, Halzen recognized that Antarctica offered an excellent natural setting for neutrino research. Far below the Antarctic surface, enormous quantities of ice are highly transparent and relatively free of the bubbles and contaminants that can obscure faint flashes of light.
His vision eventually led to the IceCube Neutrino Observatory, a giant scientific instrument embedded in about 1 cubic kilometer of Antarctic ice. To build it, researchers drilled deep holes and lowered long strings of sensitive light detectors called photomultiplier tubes.
The instruments search for brief flashes of blue light produced by a phenomenon called Cerenkov radiation. The light appears when neutrinos interact with the ice and produce charged particles that travel faster than light can travel through that material.
By recording these flashes, researchers can identify evidence of neutrinos that would otherwise pass through Earth without leaving a detectable trace.
Unusual particles from deep space
IceCube is particularly important for studying neutrinos that arrive from distant cosmic sources at extremely high energies. The Sun produces vast numbers of relatively low-energy neutrinos, but these powerful visitors from space are exceptionally rare. Observatories detect only a few dozen of the most energetic examples each year.
When one of these neutrinos interacts with ice, it can produce a fast-moving electron or muon. Muons, for example, produce a cone of Cerenkov light as they move through the ice. Scientists analyze the resulting light pattern to estimate the neutrino’s energy and reconstruct the direction from which it arrived.
For Fuller, these highly energetic neutrinos are among IceCube’s most exciting discoveries because their origins and the physical processes that accelerate them remain poorly understood.
“Some of the very high-energy neutrinos detected in IceCube have about 10 billion times more energy than typical neutrinos from the sun. Neutrinos detected in similar experiments have 200 times more energy than that. We don’t actually know where that neutrino came from,” he said. “The fact that it got here may be telling us something profound about the universe. In that sense, Halzen is right at the forefront of experimental particle physics.”
Neutrinos could reveal secrets of the Big Bang
Efforts to understand neutrinos extend beyond Antarctica. Researchers at UC San Diego are also working on another major observatory that could provide new insights into these elusive particles and the universe’s earliest history.
The Simmons Observatory, located high in Chile’s Atacama Desert, collects information about the cosmic microwave background radiation—ancient radiation left over from the early universe. Often described as the oldest light in the universe, the cosmic microwave background provides a record of conditions that existed shortly after the Big Bang.
The University of California, San Diego helped establish the observatory, and Brian Keating, the university’s Distinguished Professor of Physics, is its principal investigator.
Although the observatory primarily studies ancient light, its measurements could also reveal information about neutrinos from the earliest stages of cosmic history. The project could provide large amounts of new data about these ancient particles and offer insights that are unavailable through other experiments.
Fuller believes the anticipated observations could improve scientists’ understanding of the Big Bang, the early evolution of the universe and other fundamental questions about how the cosmos developed.
“Halzen’s work, and that of those whose vision he inspired, was the basis for giving humanity a new window on the universe and the mysterious and notoriously slippery neutrinos,” he said.
Source: www.sciencedaily.com