Astrophysicists from Germany and North America have revealed plans for the world’s largest neutrino telescope to be built on the seabed off the coast of Canada. The Pacific Ocean Neutrino Experiment (P-ONE) aims to capture extremely high-energy neutrinos produced by severe events outside our galaxy.

Neutrino telescopes study the Cerenkov radiation released when neutrinos travelling through the Earth connect with atomic nuclei on rare occasions, producing fast-moving secondary particles. Neutrinos may pierce gas and dust as they travel through the universe because they are uncharged and extremely inert, allowing astronomers to pinpoint the extremely energetic processes that generate them in theory. Photons from such occurrences, on the other hand, are absorbed as they travel.

We’re about to open up neutrino astronomy, says Elisa Resconi

IceCube, the world’s largest neutrino telescope, is made up of dozens of photomultiplier tubes dangling in holes bored deep into the ice near the South Pole. IceCube, which has a volume of 1 km3, made history in 2013 when it announced that it had intercepted the first extragalactic neutrinos. It then recorded an event four years later that, according to contemporaneous gamma-ray observations, could be linked to a very distant, brilliant galactic core known as a blazar.

IceCube’s 2017 result, according to P-ONE director Elisa Resconi of the University of Munich, is strictly speaking simply “evidence” for the blazar source. She claims that to truly claim a finding and determine the origin of other cosmic neutrinos, further neutrino observatories as well as the extension of IceCube are required. “We are on the edge of opening up neutrino astronomy,” she continues, “but if we rely on just one telescope, this process might take years, if not decades.”

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We’re going underwater

P-ONE will be made up of seven groups of ten detector strings, with a total volume of around 3 km3. Because it is larger than IceCube, it will detect fewer, higher-energy neutrinos and will be most sensitive at a few tens of teraelectronvolts rather than a few thousand. It will also observe a different portion of the sky, catching neutrinos primarily from the southern rather than the northern hemisphere. According to Resconi, there will be some overlap between the two, allowing for independent observations of the same occurrence.

The new facility will lie 200 kilometers off the coast of British Columbia, at a depth of around 2.6 kilometers in the Cascadia Basin. As a result, it will rely on pre-existing infrastructure, such as an 800-kilometer-long fiber-optic cable maintained by the University of Victoria’s Ocean Net­works Canada, which provides power and transports data to and from existing sea-floor devices.

The P-ONE collaboration is planning to deploy a steel cable with additional detectors to investigate the site, including spectrometers, lidars, and a muon detector, after confirming that the site has the necessary optical properties by sending down two initial strings of light emitters and sensors in 2018. According to Resconi, the first element of the observatory, a ring including seven 1 km-long strings, will be installed around the end of 2023. If that goes well, the researchers will apply for the majority of the $50–100 million needed to finish the project, with personnel costs adding another $100 million.

By the end of the decade, Resconi hopes to have the entire observatory up and running and collecting data. This timeline, on the other hand, she defines as “extremely ambitious.” She argues that, in addition to the delays created by the current COVID-19 outbreak, it will be difficult to ensure that the detectors perform as intended, given the extreme pressures and presence of salt and sea critters that make the seabed such a harsh environment.

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Indeed, scientists had intended to launch KM3NeT, a cubic-kilometer-scale neutrino observatory, on the Mediterranean Sea floor in 2014, but it was postponed until 2020. Only two of the 230 strings due to be installed off the coast of southern Italy have been installed so far, according to collaboration member Feifei Huang, while another site in French waters currently has six out of a planned 115 strings running – with completion not expected until 2026 and 2024, respectively.

Part of the problem with that project, according to Resconi, is a lack of professional people, with the physicists essentially doing everything themselves – their self-built junction boxes, which join cables on the sea floor, have failed, for example. She hopes that Ocean Networks Canada’s experience will prevent P-ONE from suffering a similar fate. She claims that her crew can “concentrate on the physics” with 30 or 40 individuals committed to laying cables in the ocean.

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