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Tuesday, November 28, 2023

IceCube, 2013 breakthrough of the year, brings neutrinos to the fore

Buried deep within one and a half miles of dark, clear Antarctic ice lies IceCube. After seven years of construction, the IceCube detector was completed in 2010: one cubic-kilometer of ice instrumented with over 5,000 optical sensors.

Unlike an optical telescope, which looks at photons, IceCube tries to view the sky using high-energy neutrinos that originate from objects and processes such as the sun, dark matter, radioactive decay, cosmic rays and violent galactic events such as exploding stars.

IceCube is therefore a neutrino telescope in that it “[takes] a picture of the sky using neutrinos,” according to Francis Halzen, a University of Wisconsin-Madison professor and principal investigator of IceCube.

Through detecting neutrinos, which are nearly massless and electrically neutral particles, researchers can begin to understand the objects and phenomena by which neutrinos are produced. They also can understand properties of neutrinos, such as how neutrino oscillations may relate to the changing flavors (types) of neutrinos.

IceCube instruments, buried in the ice to shield them from radiation at the Earth’s surface, detect the light emitted by particles that are produced when a neutrino interacts with ice. The resulting nuclear reaction produces secondary particles that give off a blue light called Cherenkov radiation.

Neutrinos travel at nearly the speed of light and typically do not interact with matter or magnetic fields. Because neutrino interactions with matter are rare and produce light of long wavelengths, the South Pole was chosen as the ideal location for the IceCube detector. While most ice contains air bubbles that would distort IceCube’s measurements, the compressed South Pole ice sheet contains few of these bubbles, making the ice there ideal for neutrino detections.

Projects competing with IceCube to detect high-energy neutrinos are located in the deep ocean water of the Mediterranean Sea. Halzen notes that this medium holds several disadvantages over ice, the main one being movement of the water itself.

The advantages of IceCube are clear. “History has shown the importance of our choice—we got there first,” Halzen said.

IceCube was awarded the 2013 Breakthrough of the Year award by Physics World for their discovery of 28 extremely high-energy neutrinos.

UW-Madison also received the award in 2012 for the discovery of the Higgs boson.

Receiving the award “was a total surprise,” Halzen said. “You do not do this type of highly risky research if fame or a better career is on your mind…The biggest surprise [was that we ended up] doing the science described in the proposals written 15 years ago.”

The IceCube collaboration published data and statistical methods on their discovery of 28 high-energy neutrinos in the November 2013 issue of Science. These data show the first observations of cosmic neutrinos rather than simply neutrinos of atmospheric origin.

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In the analysis published in Science, Halzen said, “we specialized [our analysis] to neutrinos of such high energy that they no longer make it through the Earth. As a consequence, most of the cosmic neutrinos collected so far originate in the southern sky above Antarctica.” In routine observation, IceCube looks at the sky above Madison from the South Pole.

While IceCube detected the first high-energy neutrinos to a high statistical significance, Halzen says that, unlike social sciences research, “significance is not the central issue” in the statistical analysis published in Science.

“The key issue is whether some systematic effects have been overlooked,” Halzen said. “We have ways to mitigate those by finding the same signal in totally different ways; further results will become public soon.”

There is still much to be done. Halzen hopes to compile more data to help determine the origins of these high-energy cosmic neutrinos. He suspects that the neutrinos originate in the cosmic particle accelerators that produce cosmic rays, “which are particles with ten million times the energy of those produced in Geneva with the Large Hadron Collider.”

While looking for the sources of the newly discovered cosmic neutrinos is IceCube’s main priority right now, Halzen said, “I am still looking for a total surprise, an observation made possible with this totally novel equipment.”

IceCube might just be the detector that is up to the task.

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