Exploring Outer Space
Frozen Lab – detecting neutrinos in ice
October 31, 2023
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Dr. Nahee Park (Physics, Engineering Physics, and Astronomy)
At SNOLAB, 2 km underground in Sudbury, Ontario, Queen’s researchers are leading experiments to advance our knowledge of dark matter and neutrinos – the tiny particles abundant in the universe but hard to spot because they rarely interact with matter. Thousands of kilometers to the south, in the middle of Antarctica, Queen’s is part of another international collaboration devoted to exploring the mysteries of outer space from right here on Earth.
The IceCube Neutrino Observatory is a one-cubic kilometer detector buried deep in Antarctica’s ice, at a depth of 1.45 km to 2.45 km. Operated by the University of Wisconsin-Madison, IceCube is the largest neutrino detector in the world and brings together 58 institutions in 14 countries, with Queen’s officially joining the collaboration in 2020. Professor Nahee Park (Physics, Engineering Physics, and Astronomy) is currently the Queen’s lead on IceCube. She recently spoke to the Gazette about the importance of this facility and why this is an exciting time to be part of this research.
Front view of the IceCube Lab at twilight, with a starry sky showing a glimpse of the Milky Way overhead and sunlight lingering on the horizon. [Credit: Martin Wolf, IceCube/NSF]
What are the main differences between the neutrino research being done at SNOLAB and IceCube?
Both facilities are looking at neutrinos, but SNOLAB focuses on low-energy neutrinos, while IceCube investigates high-energy neutrinos. We work with particles that have 10 to 15 times more energy than our eyes can see – this opens, literally, the universe to us. Higher-energy particles are rarer, which is why using a larger detector like IceCube is an advantage, and we can investigate where these high-energy particles are created.
How does it work? Why is it important that IceCube be buried in ice?
The detector was built by drilling holes in the ice to vertically place 86 long cables, covering an area of approximately one cubic kilometer. Each of these cables holds 60 digital optical modules with very sensitive light detectors and minicomputers that record and transmit data. On the surface, an additional 324 digital optical modules complete the detector. The construction took seven years of work and was completed in 2010.
We needed a medium that was transparent and in a large volume. We can’t really see or feel the neutrinos without equipment, but, when the neutrinos interact with a medium like ice, sometimes they create electrical charge and a secondary particle that can move faster than the speed of light through ice. When that happens, they create “Cherenkov radiation,” a blueish light that is captured by the optical devices in IceCube.
When a neutrino interacts with molecules in the clear Antarctic ice, it produces secondary particles that leave a trace of blue light as they travel through the IceCube detector. [Credit: Nicolle r. Fuller/NSF/IceCube]
What questions are you trying to answer with your current research?
Over the last decade or so, I’ve been studying cosmic rays, easy to detect high-energy particles that come to Earth from outer space. They hold more energy than the particles created in the Sun. My main question is, where are these particles created? It sounds like a simple question, but it’s one not easy to answer. When these high-energy particles come to Earth, their trajectory is twisted by magnetic fields they find on the way, and so it is very hard to understand their origins. Scientists have been baffled by this question for over 100 years.
Neutrinos, on the other hand, are hard to detect, but can provide the most direct evidence of where these rays come from: unlike cosmic rays, that bend inside the magnetic field, neutrinos don’t bend, because they are charge-neutral particles.
That’s the part of neutrino physics that is most exciting to me. High-energy neutrinos can escape very opaque environments in the universe – for example, around a black hole – and they “survive” because they hardly interact with anything. In other words, neutrinos can give us information about the Universe that no other observational method can find.
Education and outreach – In 2023, for the first time, Queen’s, with the support of the Arthur B. McDonald Canadian Astroparticle Physics Research Institute, participated in the IceCube Masterclass program. The initiative is aimed at high school students and Queen’s received six students from Kingston and Toronto for an introduction to high-energy neutrino physics and the work being done at the IceCube. Queen’s plans to host this program annually to inspire the next generation of physicists.
How does IceCube support this research?
IceCube is currently the only facility that can detect high-energy neutrinos coming from far away in the universe, outside our galaxy. But even so, these particles are hard to catch.
IceCube detects around 275 million cosmic rays every day. Numbers for neutrinos are much lower: daily, it detects 275 atmospheric neutrinos (the ones created when cosmic rays interact with the Earth’s atmosphere) – about 100,000 per year. But for these outer space neutrinos, we have even fewer events, about 120 detections over a 10-year period. To really find out where these high-energy neutrinos come from, we will need to observe many events.
Late in 2022, we published a paper in Science describing how IceCube detected evidence of neutrinos coming from a nearby galaxy, “only” 47 million light-years away. This was the first evidence of neutrinos from an obscure environment.
The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers collecting raw data. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real time in this lab. Only events selected as interesting for physics studies are sent to UW-Madison, where they are prepared for use by any member of the IceCube Collaboration. [Credit: Felipe Pedreros, IceCube/NSF]
How will high-energy neutrino science help us better understand the Universe?
A huge share of our astronomy knowledge relies on the information gained by low-energy photon observations. Neutrino observations bring in new information that complements that knowledge and opens a new window to look at and further understand galaxies and other astronomical objects.
Right now is a very exciting moment to work on high-energy particles physics. We are making amazing progress in answering some big questions, and I’m happy to be part of it all, both through research and through the development of new particle detectors.
For decades, Queen’s has been a global leader in the study of neutrinos. The excellence of this research has been recognized in numerous awards, including the 2015 Nobel Prize in Physics, awarded to Arthur B. McDonald for showing that neutrinos have mass. Queen’s researchers continue to advance neutrino physics and help shape the next generation of neutrino experiments.
