The Royal Swedish Academy of Sciences has awarded the 2026 Nobel Prize in Physics to Francis Halzen, University of Wisconsin–Madison, USA, “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.” [S1] Halzen was born in 1944 in Tienen, Belgium, and received his PhD in 1969 from KU Leuven. [S2] He is the first sole winner of the physics prize since Georges Charpak in 1992. [S6]

Here is the story in five facts. Each number matches a section below.

[1] The particle. Neutrinos are everywhere, but they pass through the Earth and through our bodies without us noticing. [S1] The Nobel Committee calls the neutrino “the shyest particle in the universe”: it has no electric charge and almost no mass. [S2]

[2] The source. The cosmos contains natural particle accelerators that fire out particles with energies up to a million times higher than laboratories on Earth can reach. [S1] Neutrinos are created in the same environments and reach us without changing direction or losing energy. [S1]

[3] The instrument. Halzen first presented his vision of capturing neutrinos at the South Pole in 1988. [S1] The result is IceCube: a cubic kilometre of ice fitted with 5,160 light sensors on 86 cables, which reached its full size in 2011. [S2]

[4] The challenge. Every day, IceCube registers more than a hundred million particles from the atmosphere above Antarctica. [S2] Finding the rare cosmic neutrinos means separating them from this background. [S2]

[5] The discovery. In 2013, the IceCube team reported the first evidence of cosmic neutrinos, and a couple of years later they were certain of the discovery. [S2] Since then, IceCube has seen high-energy neutrinos from our own Milky Way and found possible sources such as the active galaxy NGC 1068. [S2] In the words of the Nobel Committee, this paved the way for “a new kind of astronomy.” [S1]

[1] Meet the neutrino: an ID card for the universe’s shyest particle

  • CHARGE: Zero [S2]
  • MASS: Almost zero. It is less than 1 eV, and the exact value is still unknown. [S2][S3]
  • FAMILY: Three types, called “flavours”: electron, muon and tau neutrino [S3]
  • SPECIAL SKILL: Shapeshifting. A neutrino created as one type can arrive on Earth as another. This discovery won the 2015 Nobel Prize for Takaaki Kajita and Arthur B. McDonald. [S2]
  • FAVOURITE HOBBY: Ignoring everything. It passes through matter almost unnoticed. [S2]

How shy is it? Every second, 65 billion neutrinos from the Sun pass through your little fingernail. [S2]

Here is a simple way to picture it. The chance of a particle hitting something depends on how “big a target” it sees. Physicists call that the cross-section, σ. Even for the most energetic neutrinos (1 PeV), it is only:

σ ≈ 10⁻³³ cm² [S3]

That means the average distance a neutrino travels in ice before hitting something is:

λ = 1 / (n × σ)

Here n is the number of protons and neutrons in each cubic centimetre of ice (about 5.5 × 10²³). [calc]

λ ≈ 1 / (5.5 × 10²³ × 10⁻³³) ≈ 18,000 km of solid ice [calc]

That is longer than the diameter of the Earth, which is about 12,742 km. [ref]

A neutrino crossing 1 km of ice therefore interacts with a probability of roughly:

P ≈ L / λ ≈ 1 / 18,000 [calc]

This tiny chance is why the detector has to be a cubic kilometre. [S3] See post [3].

A short history of neutrino astronomy: five Nobel Prizes Neutrino research has won the Nobel Prize in Physics five times, and each prize adds a new chapter:

  • 1988: Leon Lederman, Melvin Schwartz and Jack Steinberger discover the second type of neutrino, the muon neutrino, using a neutrino beam. [S10]
  • 1995: Frederick Reines receives half the prize for the first detection of the neutrino. The other half goes to Martin Perl for discovering the tau lepton. [S11]
  • 2002: Raymond Davis Jr detected neutrinos from the Sun from the 1960s onward. Masatoshi Koshiba’s Kamiokande detector in Japan caught a burst of neutrinos from a 1987 supernova in the Large Magellanic Cloud, our neighbouring galaxy. Together they founded neutrino astronomy. [S2]
  • 2015: Takaaki Kajita and Arthur B. McDonald show that neutrinos change type as they travel, which means they have mass. [S2]
  • 2026: Francis Halzen takes neutrino astronomy beyond our galactic neighbourhood, to high-energy neutrinos from the distant universe. [S1]

The story runs from detecting the particle, to finding its family, to catching it from space, to discovering that it shapeshifts, and finally to using it to study the most violent events in the universe.

Why use such a shy messenger? Protons from space carry an electric charge, so magnetic fields bend their paths, and we can’t trace where they came from. Neutrinos have no charge, so they point straight back to their source. [S2] Gamma rays get weaker as they interact with light and matter on their journey, but neutrinos are unaffected. [S2]

[2] Recipe: how the universe cooks a cosmic neutrino

  • Possible kitchens: regions where stars have exploded, or distant active galaxies [S2]
  • Serves: one very patient detector

INGREDIENTS

  • 1 cosmic particle accelerator [S1]
  • Protons (the bare nuclei of hydrogen atoms), which make up most cosmic rays [S2]
  • A “target”: gas, or a dense cloud of light (photons) [S3]

STEP 1: Heat the protons The cosmic accelerator gives protons energies up to a million times higher than laboratories on Earth can reach. [S1]

STEP 2: Smash them A fast proton hits its target and breaks into short-lived particles called pions (π): [S3]

p + p → pions (π⁰, π⁺, π⁻) + other particles
p + γ → Δ⁺ → p + π⁰ or n + π⁺

STEP 3: Let them decay. Two dishes come out of the same oven. [S3] Neutral pions become gamma rays (high-energy light): π⁰ → γ + γ

Charged pions become neutrinos: π⁺ → μ⁺ + ν_μ
μ⁺ → e⁺ + ν_e + ν_μ

So one charged pion ends up producing three neutrinos. [calc]

STEP 4: Read the receipt Each neutrino carries only about one-twentieth of the original proton’s energy: [S3]

E_ν ≈ E_p / 20

So a 1 PeV neutrino (10¹⁵ electron-volts) means that a proton somewhere was pushed to about 20 PeV. [calc] Neutrinos from tens of TeV to tens of PeV can therefore reveal protons with energies from 10¹⁵ to 10¹⁸ eV. [S3]

CHEF’S TIP Gamma rays can also be made by electrons, so light alone can’t prove that protons were in the kitchen. [S3] High-energy neutrinos come from accelerated protons, which makes them clues to the origin of the most energetic cosmic rays. That origin has been “one of the universe’s best-kept secrets.” [S2]

[3] The 30-year bet: how one theorist convinced the world to build a telescope inside the ice

In 2026, Francis Halzen became the first person in 34 years to win the Nobel Prize in Physics alone. The last sole winner was Georges Charpak in 1992. [S6] The road to that prize was anything but smooth. Here it is, phase by phase.

PHASE 1: A theorist with a strange idea (1980s) Halzen was a theorist, not an experimenter. [S7] Others had been trying for years to catch cosmic neutrinos in the deep ocean off Hawaii, in a project called DUMAND, and it was struggling. [S5] When Halzen heard that Soviet researchers planned to listen for neutrino radio signals in Antarctic ice, he had a different thought: put light sensors in the ice. [S2] Why ice? Partly, he joked, because “it doesn’t slosh around.” [S5] In 1988, he and John G. Learned presented the idea at a conference in Poland. Learned went back to his own project, and the story could have ended there. [S2]

PHASE 2: Proving it isn’t crazy (1990–1991) Instead, researchers gathered around Halzen. [S2] The first test was not at the South Pole but on top of the Greenland ice sheet. [S2] A 1991 paper in Nature by Halzen, Bob Morse, Buford Price and colleagues concluded that “a full-scale Antarctic ice detector is technically quite feasible.” [S4] Halzen later admitted he had to switch from theory to experiment, and said: “I finally found what I wanted to do with life.” [S7]

PHASE 3: The first pitch (1992) Halzen, Steve Barwick, Buford Price and others asked the US National Science Foundation (NSF) to fund a pilot project called AMANDA. [S5] It had one job: to prove that ice could work. [S5] A big advantage was that NSF already ran a station at the South Pole. “If we had to build a station to do the experiment, we couldn’t have afforded it,” Halzen said. [S7]

PHASE 4: The first failure (Christmas 1993) On Christmas Eve 1993, Halzen sat at the dinner table with his computer on his lap, waiting for news that the first cable was in the ice. [S2] The first array had 4 strings at 800 to 1,000 m. [S4] It was a disappointment: the ice at that depth was full of air bubbles, which scattered the light and blurred the signals. [S2][S4] Halzen later joked about the team’s early confidence: “Showing we were stupid came later.” [S5]

PHASE 5: Going deeper, and getting lucky (1995–2000) The team drilled deeper, to depths where the ice turned out to be crystal clear. Below 1,400 m, flashes of light could travel 300 m, which was much further than anyone had expected. [S2] “We were incredibly lucky,” Halzen said. [S5] “Nature was kind to us.” [S7] By 2000, AMANDA-II had 677 sensors on 19 strings. [S4]

PHASE 6: Asking for the big one (1999–2001) In November 1999, the young IceCube team sent a 67-page proposal for a cubic-kilometre detector to NSF and to partners in Belgium, Germany and Sweden, with Halzen as principal investigator. [S4] To show the new design would work, they put 41 new digital sensors on AMANDA’s “String 18,” and it worked better than promised. [S5] In 2001, AMANDA published its first neutrino sky map in Nature: 263 neutrinos in 138 days. [S4] “This is a proof of concept,” Halzen said. [S4]

PHASE 7: The money (2002–2003) NSF’s governing board approved $15 million in March 2002 and up to $24.54 million more in May 2003 for the first phase. [S4] In the end, the project cost $279 million, of which NSF paid $242 million. [S4] Halzen later said NSF “took a chance” and added: “I am not sure this could happen today.” [S8]

PHASE 8: “A failure on our hands” (2004–2005) The plan assumed the team could drill a 2.5 km hole in two days. In the first season, they drilled only one hole. Halzen said he was “sure we had a failure on our hands.” [S5] At that pace, building IceCube would have taken more than a quarter of a century. [S5] There was no room for error either: once a sensor freezes in, nobody can reach it again. “It’s like launching a satellite, except you launch it down,” Halzen said. [S7]

PHASE 9: The ballet on the ice (2005–2010) The drillers learned fast. A 4.8-megawatt hot-water drill reached more than 2 km in under two days, and lowering each string took about 11 hours. [S4] By the end, the team was installing three strings a week. Halzen said it was “really like a ballet.” [S5] All the work had to fit into the short Antarctic summer, from November to February. [S2] The 86th and last string went into the ice on 18 December 2010. [S4]

PHASE 10: The payoff (2011–2026) The complete detector started its first full physics run on 13 May 2011. [S4] In 2013 came the first evidence of cosmic neutrinos. [S2] Looking back, Halzen said: “very few thought it would work, including myself.” [S9]

Why a solo prize is so rare The Nobel Prize can be shared by up to three people. [ref] Modern physics is mostly team science, and IceCube itself has about 450 people from 58 institutions in 14 countries. [S3] Even so, the Committee named only Halzen and pointed to his “vision and scientific leadership.” [S1] The Committee’s discussions stay secret for 50 years, so the exact reasoning isn’t public. [ref]

PHYSICS BOX: a “sonic boom” made of light When a neutrino hits a nucleus, it makes a charged particle that travels in the neutrino’s direction and gives off blue light. [S2] This happens because the particle moves faster than light travels in ice:

v > c / n, with n ≈ 1.31 [S3]
Speed of light in ice ≈ 300,000 km/s ÷ 1.31 ≈ 229,000 km/s [calc] The light forms a cone at an angle where cos θ ≈ 1/n, so θ ≈ 40°. [ref][calc]

IceCube in numbers: 1 km³ of ice, about 1 billion tonnes [S2][S3]. It has 5,160 sensors on 86 cables, buried between 1,450 and 2,450 m deep. [S2] Halzen calls each sensor “a lightbulb in reverse.” [S2]

[4] Case file: finding 1 cosmic neutrino among a million impostors

THE SUSPECTS

  • Suspect A: muons from the atmosphere. They can reach the sensors deep in the ice, and IceCube registers more than 100 million of these particles every day. [S2] That is about 3,000 per second. [S3]
  • Suspect B: atmospheric neutrinos, which are made when cosmic rays hit the air. A few hundred of them reach IceCube every day after passing right through the Earth from the northern hemisphere. [S2] That adds up to about 100,000 neutrinos per year above 0.1 TeV. [S3]
  • The target: cosmic neutrinos. About 100 per year. [S3]

That is about 1 cosmic neutrino for every 1,000 atmospheric ones. [calc]

CLUE 1: Use the planet as a filter Muons can’t pass through the Earth. If investigators keep only the tracks that come up from below the horizon, the muon rate falls from about 3,000 per second (3 kHz) to the microhertz level, which means about one every few days. [S3] The Earth itself becomes part of the detector.

CLUE 2: Follow the energy The number of neutrinos N at energy E falls off as: [S3]

Atmospheric: N ∝ E⁻³·⁷ (drops very fast)
Cosmic: N ∝ E⁻²·⁵ (drops more slowly)

N_cosmic / N_atmospheric ∝ E^(3.7 − 2.5) = E¹·² [calc]

Every time the energy goes up 10 times, the cosmic neutrinos gain about 16 times (10¹·²) on the impostors. [calc] Above a few tens of TeV, the cosmic signal finally stands out. [S3]

CLUE 3: Read the fingerprint [S3]

  • Muon neutrino: a long track, which is best for finding the direction.
  • Cascade (from an electron neutrino or a neutral-current collision): a compact ball of light less than 10 m across. It measures energy to about 8% at 100 TeV.
  • Tau neutrino: a “double bang,” two cascades separated by about 50 m for each PeV of energy.

THE VERDICT Investigators judge the evidence as a whole and ask whether the selected events, taken together, look like cosmic neutrinos rather than atmospheric ones. [S2]

  • The 2010–2013 data rejected a purely atmospheric explanation at 5.7σ. [S3]
  • In 2024, seven high-energy tau neutrinos were reported, with energies from 20 TeV to 1 PeV. [S3] Above 100 GeV, a tau neutrino must have come from space. [S3]

How sure is “sure”? 5σ is the usual threshold for a discovery in physics. It means the chance of a random fluke is roughly 1 in 3.5 million. [ref]

[5] Postcards from the neutrino sky

POSTCARD 1: “Somewhere out there,” 2013 “Dear Earth, we arrived with 1.04 PeV and 1.14 PeV.” [S3] This was the first evidence of cosmic neutrinos [S2]. The same data later revealed 28 candidates between 30 TeV and 1.14 PeV. [S3]

POSTCARD 2: Blazar TXS 0506+056, 22 September 2017 “One neutrino, about 290 TeV, from within 0.06° of me.” [S3] IceCube sent an alert to observatories around the world within a minute. In the months before, the source’s gamma-ray brightness had risen 7-fold. [S3] This was multi-messenger astronomy in action: one source seen through neutrinos and light at the same time. [S3]

POSTCARD 3: Active galaxy NGC 1068 (M77), 46 million light-years away “79 of us seem to come from my direction.” [S2] The signal stands out at 4.2σ, in the 1–10 TeV range. [S3] The galaxy is much fainter in gamma rays than in neutrinos. [S3] This fits the idea that neutrinos can escape from dense places that trap light. [S3] But be careful: the evidence is not yet strong enough to call NGC 1068 a confirmed neutrino source. [S2] At close to the speed of light, these neutrinos have been travelling for about 46 million years. [calc]

POSTCARD 4: Our own Milky Way, 2023 and 2026 “Greetings from home.” The neutrinos probably come from cosmic rays hitting atoms in the thin gas between the stars. [S2] The signal reached 4.5σ in 2023 and 5.7σ in 2026, using 12 years of data. That makes the Milky Way the first source of high-energy neutrinos to pass the 5σ discovery threshold. [S3]

POSTCARD 5: The record holder, 31 March 2019 (published March 2026) “About 11.4 PeV. Beat that.” This is IceCube’s highest-energy neutrino published so far. [S3] For comparison, a proton at CERN’s Large Hadron Collider has about 6.8 TeV, roughly 1,700 times less. [ref][calc]

WHAT’S NEXT

  • IceCube-Gen2 will be eight cubic kilometres of ice at the South Pole. [S2]
  • New telescopes in water are being built in Lake Baikal, the Mediterranean, the South China Sea and off the west coast of Canada. [S2]
  • IceCube is operated by about 450 people from 58 institutions in 14 countries. [S3]

The aim is knowledge “that cannot be obtained in any other way.” [S2]

Sources