Francis Halzen receives the 2026 Nobel Prize in Physics for his contributions to IceCube

We congratulate Francis Halzen on receiving the 2026 Nobel Prize in Physics, which recognizes his contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. Our research group at TU Dortmund University has been involved in IceCube for many years, contributing to research on astrophysical and atmospheric neutrinos, the development of new simulation and analysis methods, and improvements to the detector.

Neutrinos are electrically neutral particles with very small masses that interact only weakly with matter. As a result, they can travel vast distances almost without interacting with matter, carrying information from distant astrophysical sources. However, these same properties also make them difficult to detect. Detecting neutrinos therefore requires a very large detection volume, which is why the IceCube Neutrino Observatory uses Antarctic ice as its detection medium. When a neutrino interacts with a particle in the ice, it can produce charged particles that emit Cherenkov light if they travel fast enough. This light is recorded by sensors distributed throughout the ice. The recorded signals can then be used to reconstruct properties of the incoming neutrino, including its energy and direction of arrival.

Francis Halzen is a physicist at the University of Wisconsin–Madison in the United States and played a central role in the development of the IceCube experiment. He pursued the idea of using Antarctic ice as a detection medium for high-energy neutrinos and, together with other researchers, demonstrated that this approach works. This work laid an important foundation for the IceCube experiment. Halzen played an important role in building the international collaboration and developing the observatory. He is still the Principal Investigator of the IceCube experiment. In 2013, IceCube achieved a major scientific milestone with the first detection of high-energy neutrinos of astrophysical origin. Another important discovery followed in 2023, when IceCube detected high-energy neutrinos from our own galaxy, the Milky Way, for the first time. Researchers at TU Dortmund University played a major role in the data analysis that led to this discovery.

Our research group has been part of the IceCube Collaboration for many years, contributing both to the development of the detector and to the analysis of its data. Even before IceCube was completed, researchers in Dortmund were involved in the search for astrophysical neutrino sources. Their work included analyses of data from AMANDA, IceCube’s predecessor experiment, which also operated in Antarctic ice, as well as early IceCube data. Since individual astrophysical sources often produce only a small number of detectable neutrinos, data from several potential sources were analyzed together to identify even weak signals. Using data collected during IceCube’s construction phase, researchers in Dortmund detected more high-energy neutrinos than could be explained by processes in Earth’s atmosphere alone. This suggested that some of the neutrinos must have come from astrophysical sources. Identifying and understanding such signals requires extensive simulations and sophisticated methods for analyzing the measured data. Developing simulation software and machine learning methods has therefore also been an important focus of our research group for many years. One example is PROPOSAL, a software package developed in Dortmund that simulates the propagation of high-energy particles through matter.

In addition to searching for astrophysical neutrinos, our research group also studies neutrinos and muons produced in Earth’s atmosphere. These particles are generated when high-energy cosmic rays interact with particles in the atmosphere. In searches for neutrinos from distant astrophysical sources, atmospheric neutrinos and muons form a background that overlaps with the signals of interest. Understanding these atmospheric contributions as accurately as possible is therefore essential for identifying astrophysical neutrinos in the data. At the same time, these particles provide valuable information about the interactions of high-energy cosmic rays. Researchers in Dortmund were the first to determine how seasonal variations in the atmospheric neutrino flux depend on energy. An analysis of atmospheric muons provided evidence for so-called prompt muons, which originate from the decay of particularly short-lived particles. In addition, the energy and angular distributions of the muon neutrino flux were reconstructed using eleven years of IceCube data. Beyond their work on atmospheric particles, researchers in Dortmund also develop analysis methods for neutrino astronomy. Their machine learning methods played a key role in the analysis that led to the discovery of high-energy neutrinos from the Milky Way in 2023.

Our research group also contributes to the further development of the IceCube detector. For the recent IceCube Upgrade, around 5000 photomultipliers were tested and calibrated at TU Dortmund University. Members of our research group participated in the installation at the South Pole. We also contribute to IceCube-Gen2, the planned next generation of the experiment. While IceCube detects neutrinos through the light produced by their interactions, an additional radio detector is designed to detect neutrinos at even higher energies. In Dortmund, we are developing a new, fully digital data acquisition system, neural networks to better identify radio signals, and methods to optimize the detector layout.

Today, the IceCube Collaboration brings together more than 450 researchers from 58 institutions in 14 countries. Francis Halzen’s Nobel Prize is therefore also a recognition of the scientific achievements made possible by this long-standing international collaboration.

Image credit: El País / Bernardo Pérez