Proposed experiment seeks origin of cosmic neutrinos

Under deep water, far off the coast of British Columbia, Canada, the world is cold and dark. Rising from the sand below bob is a set of submerged buoys, securely fastened with mooring lines to the ocean floor. Attached at intervals along each line are large glass spheres housing sensitive light-detecting instruments.

The scientists who built these instruments on the high seas are neither biologists nor oceanographers. They are physicists and astronomers. It’s here, 2 kilometers below the freezing waves of the Pacific, that they hope to capture tricky, shape-shifting, nearly massless particles called neutrinos that could change our view of the universe.

Particle decays regularly produce mid- to low-energy neutrinos, the kind that physicists spend most of their time observing. But every once in a while, a different type of neutrino is detected: a high-energy cosmic neutrino.

Scientists know from the intense energy of these superpowered particles that they must have been accelerated by extreme objects outside our galaxy.

“These energies are really hard to imagine,” says Juan Pablo Yanez Garza, physicist and assistant professor at the University of Alberta. “When you consider how we accelerate particles in our labs, like the Large Hadron Collider, and plug in the magnetic fields typical of the universe, you realize you would need an ‘accelerator’ the size of an entire galaxy. to energize the neutrinos so much.”

Using gigantic, specialized detectors, scientists are just beginning to identify some of the extragalactic origins of these particles. Figuring out where high-energy neutrinos come from can help solve long-standing mysteries about the giant cosmic accelerators that produce them, answer unanswered questions about cosmic rays, and even provide clues to the origins of matter. black.

The instruments in the Pacific Ocean are some of the first steps toward a proposed experiment called the Pacific Ocean Neutrino Experiment, or P-ONE, which scientists hope will help them uncover the cosmic origins of neutrinos.

Cosmic messengers

Due to the vastness of space and the ubiquity of view-blocking dust clouds, most of the universe is hidden from photon-based telescopes. Instead, astronomers are looking for messenger particles, such as neutrinos, to learn more about these dark regions, which include some of the most powerful objects in the universe. Neutrinos are ideal cosmic messengers. Their limited interactions with other particles and their chargeless state mean they can travel through space unhindered by magnetic fields and clouds of dust. However, this introversion means they can be difficult to capture when they reach Earth.

Scientists have developed goliath detectors in hopes of increasing the odds. Even so, the chances of catching a high-energy cosmic neutrino are slim. During its 12 years of operation, the IceCube detector – one of the first cosmic neutrino observatories, located at the South Pole with a detection volume of one cubic kilometer – has captured only a few hundred.

“With more than 10 years of IceCube data, we still don’t know what the most cosmic sources of high-energy neutrinos are,” says Lisa Schumacher, a researcher at the Technical University of Munich, who is involved in both in IceCube and P-ONE.

But we know some of them. In 2018, the IceCube observatory was the first to identify a source of high-energy neutrinos: a blazar 3.7 billion light-years away. A blazar is the core of a galaxy fueled by a black hole that can accelerate particles in huge jets to near the speed of light. Then in 2022, IceCube announced a second source in another active galaxy just 47 million light-years away, where scientists believe neutrinos and other matter are being accelerated around a giant black hole.

While active galaxies are now a confirmed source, statistical analyzes show that they alone cannot account for all high-energy astrophysical neutrinos. “The problem is that we need more data,” says Elisa Resconi, a professor at the Technical University of Munich who has worked with IceCube. “Stats are what’s limiting us right now.”

A light in the dark

To collect more data, scientists like Resconi have devised new neutrino observatories. Resconi led an effort to launch the new P-ONE experience at scale. With the aim of building hundreds of neutrino detectors along several 1 kilometer long lines, the experiment is intended to complement IceCube. From its location in the northeast Pacific, P-ONE could pick up neutrinos from different parts of the universe that IceCube cannot see.

“The idea is to have a telescope that would be similar to IceCube, but with improvements given the advances in technology over the last decade,” Resconi says. “Our goal is to really complement the other detectors, because we want to be able to work together and pool our data.”

P-ONE will detect neutrinos the same way IceCube has for years – looking for the tiny streaks of light neutrinos created when they collide with other particles in a medium such as water or ice.

To block other atmospheric particles that can mimic these tiny contrails, scientists often install neutrino detectors underground. Some neutrino detectors, such as the Sudbury Neutrino Observatory in Canada and the Super-Kamiokande in Japan, are built in old or operating mines. Others, like the Baikal Deep Underwater Neutrino Telescope in Russia and the future Cubic Kilometer Neutrino Telescope off Italy and France, are being built at depth. P-ONE scientists hope to complement and expand the scope of the aquatic fleet.

P-ONE’s location in the water gives it an advantage over IceCube, which is embedded 2,000 meters deep in glacial ice. Although Antarctic ice is very transparent, its crystalline structure prevents a beam of light from traveling along a perfectly straight line. Since the angle and direction of the streaks are used to identify where the neutrino is coming from in the sky, this scattering makes it harder for IceCube to identify cosmic neutrino factories.

“The total amount of light that the detectors will measure with P-ONE will be a little less than with IceCube, but we can better reconstruct where the light is coming from,” Schumacher says.

Neutrinos, bioluminescence, and more

Scientists proposed to build P-ONE from the backbone of Ocean Networks Canada’s Oceanographic Observatory, the largest permanent oceanographic infrastructure in the world. If they do, P-ONE scientists will be able to tap into the network of hundreds of kilometers of optical cables and substations already installed on the ocean floor, saving the experiment time. and money.

In return, P-ONE could also open new doors in oceanography and biology. Additional detectors, like hydrophones or oxygen sensors, could be attached to the P-ONE lines to measure ocean vital signs and perform acoustic tomography, which uses low-frequency signals to measure currents and temperature. oceans over large areas. And since P-ONE’s detectors are light-sensitive, they could also be used to study long-term changes in bioluminescence activity at depth.

In 2018, project scientists deployed STRAW-a, a group of instruments designed to test the suitability of the site for the P-ONE experiment. With STRAW-b, which completed testing in 2021, the Pathfinder mission proved that the location’s clear waters would provide a good canvas for neutrino detection. Now, scientists are preparing for the next step: the installation of a prototype instrument, scheduled for spring 2024.

The prototype phase will see at least three lines, each with 20 detectors, installed on the ocean floor. This should allow scientists to capture around 30 atmospheric neutrinos, enough to calibrate and provide proof of concept. If all goes well, P-ONE will eventually consist of 70 lines 1,000 meters long spread over one square kilometer of ocean. And if interest in the project grows, the experience is extremely scalable.

“With P-ONE, IceCube and other detectors in the works, we can really do neutrino astronomy properly,” says Resconi. “We will be able to locate many objects and do population studies, which can tell us which objects produce the most neutrinos.”

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