Nobel of physics for Francis Halzen, the hunter of the “ghost particles” of the Universe

BarcelonaThe Royal Swedish Academy of Sciences announced this Tuesday that the winner of this year's Nobel Prize in Physics is University of Wisconsin-Madison researcher Francis Halzen. Born in 1944 in Belgium, Halzen has been awarded for having laid the foundations for a radically new type of astronomy: neutrino astronomy. His contributions have been decisive in creating the IceCube Neutrino Observatory in Antarctica and in discovering high-energy neutrinos of astrophysical origin, which allows researchers from all over the world to learn more about the cosmos and its characteristics, some of which are still unknown.

For four decades, the laureate has dedicated his career to pursuing precisely neutrinos, particles that are almost undetectable, yet pass through the entire Earth and also our body without us noticing. They have no electric charge and almost no mass, and they rarely make themselves felt by colliding with an atomic nucleus. Even so, every second 65 billion neutrinos from the Sun flow through the nail of our little finger. But neutrinos also come from other regions of space, and studying them provides clues about some of the mysteries of the cosmos.

In the eighties, Halzen realized that Antarctic ice could be turned into an immense neutrino detector. "His scientific vision and leadership have been fundamental to the IceCube Neutrino Observatory, a cubic kilometer of ice equipped with light sensors. With it, researchers can detect neutrinos coming from extremely energetic processes in the distant Universe," highlights the Royal Swedish Academy of Sciences in a press release.

Just a few days ago, at a science festival, some colleagues said that Halzen could be awarded the Nobel Prize. This Tuesday, the Belgian researcher attributed his success to a mixture of the talent of the researchers who participated in the project and a bit of "luck." "This was a kind of adventure in which success was not guaranteed and we were lucky to overcome the various challenges," he said in a digital press conference. Before starting the project "everyone thought it might be a good idea, but very few believed it would work, including myself," he admitted.

The neutrino interactions that Halzen's strategy continuously records –including those originating in the distant cosmos– provide "new insights into the violent environments in which high-energy neutrinos can be generated, and could even reveal cosmic phenomena hitherto unknown," state the award committee. The universe contains natural accelerators that eject particles with energies up to a million times higher than those that can be achieved in terrestrial laboratories. But there are still many questions about these sources: what they are, where they are found, and what the main processes taking place within them are.

IceCube: beyond the Solar System

Extremely high-energy neutrinos are created in the same environments as other types of particles. But, unlike these, neutrinos reach us without changing direction or losing energy. This means they can provide information that cannot be obtained in any other way. In 1988, Francis Halzen conceived and promoted a way to turn Antarctic ice into a giant neutrino detector.

When a neutrino collides with the nucleus of an atom, it produces a flash of light that can be detected by sensors installed in the transparent glacial ice. South Pole ice offers many advantages, as it is free from various types of interference and the area is geologically stable, without earthquakes. Halzen and his idea soon gained the support of other researchers, and a few years later the first tests were carried out with sensors installed in the ice.

Cosmic neutrinos of extremely high energies are very rare, so an immense volume of ice is needed to observe a sufficient number of collisions. IceCube occupies an entire cubic kilometer and was completed in 2011. Researchers soon discovered the first high-energy neutrinos there and, a few years later, were able to publish the discovery of neutrinos that necessarily had to come from far beyond our Solar System.

"Francis Halzen has led an international team of researchers and engineers who have provided us with a fantastic instrument. His tenacity and scientific vision have paved the way for a new kind of astronomy," says the chair of the Nobel Committee for Physics, Mark Pearce. Halzen is also director of the Institute for Elementary Particle Physics Research, and has been an associate scientist at the European Organization for Nuclear Research (CERN) in Geneva. Throughout his career, he has written countless publications about IceCube in the prestigious journals Science, Nature and The Astrophysical Journal.

"It is a well-deserved prize"

Researchers from all over have celebrated the recognition of Halzen, which they describe as the "culmination" of a "brilliant" career, already backed by numerous awards. The CSIC research professor at the Institute for Corpuscular Physics (IFIC) in Valencia and co-principal investigator of the KM3NeT research team at the IFIC, Juan José Hernández Rey, considers it "a well-deserved award". "Professor Halzen, with whom we were just two weeks ago, not only proposed the idea of using Antarctic ice as a natural medium to detect neutrinos, but he pursued this idea with extraordinary determination," he states in statements to SMC Spain.

This is also highlighted by the professor at the University of Valencia and former director of the Institute for Corpuscular Physics (IFIC), Nuria Rius, who describes Halzen as an "excellent and tenacious" researcher, and the researcher at the Department of Theoretical Physics and the Cosmos at the University of Granada, Patricia Sánchez-Lucas: "Today is a day of celebration for the entire neutrino physics community, as this award is also a recognition of the importance of continuing to study these particles. Everything that we still have left to discover about them will allow us to continue advancing in our quest to unravel the great mysteries of the universe."