The planned IceCube Upgrade will have two new types of sensor modules, which will be tested for a ten-times-larger future extension of IceCube known as IceCube-Gen2. The modules to be deployed in this first extension will be two to three times more sensitive than the sensors that make up the current detector. This is an important benefit for neutrino studies, but it becomes even more relevant for planning the larger IceCube-Gen2.
The $37 million IceCube Upgrade, to be deployed during the 2022-23 polar field season, has now secured $23 million through an NSF mid-scale research award. Last fall, activities got underway, including setting up the IceCube Upgrade office, thanks to initial funding from NSF and additional support from international partners in Japan and Germany as well as from Michigan State University and the University of Wisconsin–Madison.
“Neutrinos are the least understood particles in the Standard Model of particle physics,” explains Kael Hanson, director of the Wisconsin IceCube Particle Astrophysics Center at UW–Madison and principal investigator of the IceCube Upgrade, referencing the scientific model that with unprecedented success explains the behavior of most subatomic particles. “Neutrinos have properties the Standard Model can’t account for.”
The principal goal of this IceCube extension is to enhance the cubic-kilometer detector to gain precision in studies of the oscillation properties of neutrinos, which can transform—or oscillate—from one type of neutrino to another as they interact with other particles and travel through space.
Another goal is to better characterize the ice around the IceCube sensors and thereby obtain better performance with the existing detector, thus yielding more precise reconstructions of neutrinos at all accessible energies. Most notably, this will give high-energy neutrino astronomy a boost, as IceCube will be able to resolve the neutrino sky more sharply. Furthermore, understanding the ice better will enable the collaboration to improve the reconstruction of archived data collected over the past nine years.
The new strings will be deployed below the center of the existing detector, a mile deep in the Antarctic ice. The ice in and around the detector is extremely transparent, which makes it an ideal medium in which to study the properties of relativistic particles.
Neutrinos are sometimes called “ghost particles” for their ability to travel through matter and space over galactic distances without hitting a thing. This ghostly behavior is due to their weak interaction with matter. Yet when neutrinos interact with other particles in or near the detector, they create secondary particles such as muons, which cross the detector at such high speeds in the ice that they give off a bluish light—called Cherenkov light—that can be detected to reconstruct the trajectory and energy of the parent neutrino. Depending on the light pattern, IceCube scientists can also recognize different types of neutrinos.
“The Upgrade is designed to realize significant improvements in our measurements of neutrinos for astronomy and our understanding of the nature of the particle itself,” says Darren Grant, spokesperson for the IceCube Collaboration. “It comes at a crucial time for the project as we look towards an exciting future of advancements in the field that the IceCube detector will provide.”
A very high energy neutrino that was detected by IceCube in September 2017 was critical for pointing to the first source of cosmic neutrinos—those created in accelerators like massive black holes in distant galaxies. With the improved ice characterization obtained with devices deployed on the new strings, scientists will be able to estimate with greater precision the direction that very high energy neutrinos come from, thus providing a clearer view of the extreme universe and most likely allowing the identification of new sources.
Lower energy neutrinos, such as those created when cosmic-ray particles collide in Earth’s atmosphere, can reveal how neutrinos morph from one kind into another. In fact, the IceCube Upgrade is designed to make precision measurements of neutrino physics.
We know that neutrinos come in at least three types, or “flavors”: tau, muon, and electron. They can oscillate from one flavor into another following patterns that change with the energy of the neutrinos, their masses, and how far they have traveled in space and through matter.
A “sweet spot” for observing neutrino oscillations, according to Hanson, is when muon neutrinos are created in the atmosphere by a cosmic-ray interaction with the nucleus of an atom. As the muon neutrino travels through the Earth, it can oscillate into a tau neutrino.
Atmospheric neutrinos are almost perfect for understanding oscillations because they span a wide range of energies, reaching energies higher than those of devoted long-baseline neutrino detectors. And that’s important, because even though we know that neutrinos change from one flavor to another, we don’t know enough about this morphing.
Neutrino oscillations—a quantum effect that earned its discoverers the 2015 Nobel Prize in Physics—proved neutrinos have small but well-defined masses. Surprisingly, the three neutrino mass states are not exactly the same as the electron, muon, or tau flavors, but rather mixtures of the three. The mixing phenomena are not fully understood, but they are related to what physicists call the neutrino mass ordering, i.e., which of the neutrinos is the heaviest and which is the lightest.
Understanding how neutrinos change flavor will help refine the Standard Model. The IceCube Upgrade, for example, will provide world-leading measurements of the tau neutrino appearance, which if found to be different from standard oscillations would point to new physics, such as the existence of a fourth type of neutrino––the so-called sterile neutrino.
In general, testing the model at higher energies with IceCube will look at different scenarios of new physics than those using lower energy neutrinos. The quest is on, and many think the higher the energy, the greater the chance of encountering new physics.
The IceCube Neutrino Observatory is located at NSF’s Amundsen-Scott South Pole Station. Management and operation of the observatory is through the Wisconsin IceCube Particle Astrophysics Center at UW–Madison. The scientific program is run by the international IceCube Collaboration, with more than 300 scientists from 52 institutions spanning 12 countries.
Nick van Eijndhoven, 0032 (0)488.074166, Nick.van.Eijndhoven@vub.ac.be