The neutrino is perhaps the most mysterious elementary particle. Theoretically postulated in 1930 as a “ghost” particle accompanying the electron in beta decay, it was not experimentally confirmed until 1956, using a nuclear reactor and detecting just a handful of the trillions of neutrinos passing through every square centimetre of the detector each second.
We are exposed to a similar shower of neutrinos coming from the Sun, constantly bombarding us and passing through our bodies—and through the entire Earth—with barely any absorption, because their interaction with matter is extremely weak. The neutrino carries no electric charge, and its mass is almost zero, by far the smallest of all the fundamental particles of matter. Its initial detection, the observation of cosmic neutrinos (originating from the Sun and from a supernova near our galaxy), and the discovery through neutrino oscillations that its tiny mass is not exactly zero were recognised with three Nobel Prizes, in 1995, 2002, and 2015, respectively.
In 2026, the detection of very-high-energy astrophysical neutrinos originating from distant cosmic sources has earned a new Nobel Prize, this time awarded to Francis Halzen, the driving force and leading figure behind the world’s largest cosmic-neutrino observatory: the IceCube experiment, located at the South Pole. Between 2 and 3 kilometres beneath the Antarctic ice lies a network of detectors covering an area of more than one square kilometre. In 2013, IceCube succeeded in identifying, among the rare flashes of light recorded by these detectors, those produced by neutrinos of enormous energy originating in the depths of the cosmos. After travelling across the Universe, these neutrinos reached Earth and underwent an interaction—highly improbable, but not impossible.
These astrophysical neutrinos are true messengers from the most extreme environments in the cosmos, complementing the incomplete information provided by other cosmic messengers: photons, whose usable energy range is limited because the highest-energy photons are absorbed before reaching Earth, and cosmic rays, protons and other atomic nuclei that are deflected by magnetic fields as they travel through the Universe, thereby losing directional information about their sources. These cosmic messengers have complementary properties, and multimessenger astronomy—that is, the observation of astrophysical events through more than one type of messenger—although still challenging, promises to revolutionise astrophysics just as astronomical observations at different wavelengths of light, such as radio waves and X-rays, did in the last century.
The neutrino’s unique properties also make it extremely sensitive to new-physics phenomena, which can therefore be investigated by studying these cosmic neutrinos, whose energies can be thousands of times higher than those we are able to produce on Earth. In particular, as explained in this reference, the use of very-high-energy astrophysical neutrinos as a window into physics beyond special relativity is one of the research areas pursued by the quantum-gravity phenomenology group (QuGraPheno) at the Center for Astroparticle and High Energy Physics (CAPA). One of its members, José Luis Cortés, worked with Francis Halzen at the University of Wisconsin, before Halzen had embarked on the journey that would ultimately lead to the creation of the IceCube Observatory.
Congratulations to Francis Halzen and to the field of very-high-energy astroparticle physics!
