The Morning the IceCube Team Woke Up to a Nobel Prize, and Their Ongoing Hunt for Cosmic "Ghost Particles"
“For no pre-planned reason at all, I jolted awake at 5:30 a.m., and the very first thing I did was pull up the New York Times website to scan the top headlines,” Juan Carlos Díaz Vélez recalls of the surreal morning the 2024 Nobel Prize in Physics was announced. “I could barely believe what I saw: a top breaking alert said Francis Halzen had just been awarded the prize. I immediately fired off messages to every team group chat I was in, only to find all my colleagues based in Europe had gotten the news hours earlier.”
Díaz Vélez works at the University of Wisconsin–Madison’s Wisconsin IceCube Particle Astrophysics Center (WIPAC) — and the newly named Nobel laureate he was rushing to message? His direct supervisor.
“When I walked into the WIPAC offices that day, every single person was grinning, shaking hands, and congratulating each other nonstop,” he says. “We all piled into the main conference room to stream Halzen’s official Nobel press conference, since he was stuck at an unrelated academic summit in Italy at the time. We’re holding the big full-team celebration for when he flies back, but we haven’t paused our neutrino research for a second in the meantime. This work never stops.”
That decades-long line of research centers on catching one of the universe’s most elusive particles, using a custom-built detector buried deep beneath a cubic kilometer of solid Antarctic ice.
“Neutrinos are nearly massless subatomic particles that only engage in reactions inside atomic nuclei,” Díaz Vélez explains. “They are produced by natural radioactivity on Earth — you’ll find trace amounts even in watermelons and bananas, thanks to their naturally occurring potassium-40 content, as well as inside our own bodies.”
The average person never notices these tiny particles, for good reason: they barely interact with ordinary matter at all. True to their name, neutrinos carry no electric charge, meaning they can travel across the universe in perfectly straight lines, completely undisturbed by cosmic magnetic fields, and pass straight through the entire planet Earth without colliding with a single atom.
“Neutrinos pouring off the sun are so abundant that 1 billion of them pass through the nail on your thumb every single second,” Díaz Vélez notes.
The neutrinos Halzen’s team tracks are not the low-energy, unnoticeable ones we all pass through every day. Their work targets ultra-high-energy neutrinos that originate from the far reaches of deep space. These cosmic particles likely form in the violent outflows of supernova remnants, or at the edges of supermassive black holes: the extreme magnetic fields generated by these phenomena can accelerate protons to near-light speeds, and when those high-energy particles collide with interstellar gas and plasma, they spit out both gamma rays and the high-energy neutrinos the team hunts.
“That’s what makes neutrinos such perfect unobscured messengers from the universe’s most violent high-energy events,” Díaz Vélez says. “The highest-energy neutrino sources we’ve ever traced back are active galactic nuclei, anchored by supermassive black holes millions of times the mass of our sun.”
The team detects these rare particles at the IceCube Neutrino Observatory, a one-of-a-kind research facility at the geographic South Pole (and no relation to the famous hip-hop artist of the same name). The observatory hosts a grid of 5,160 ultra-sensitive optical sensors buried deep underground, spread across a full cubic kilometer of dense, ancient Antarctic ice.
“The South Pole ice sheet becomes nearly perfectly transparent at a depth of 1.5 kilometers,” Díaz Vélez, who helped build the IceCube array during its 2010 construction phase, explains. “That clear ice lets our sensors pick up the faint flash of light generated when rare, chance collisions between a passing neutrino and an ice atom create a secondary heavy subatomic particle called a muon.” The observatory went fully online in 2011, and delivered its first landmark breakthrough just a few years later: the first confirmed detection of high-energy neutrinos coming from astrophysical sources outside our solar system.
One of their most famous confirmed sources is TXS 0506+056, a blazar — an extremely bright, active galaxy anchored by a ravenous supermassive black hole — that the team identified in partnership with other observatories tracking matching gamma-ray, optical, and radio wave signals. The team has since confirmed this entire class of blazar objects is capable of spitting out the ultra-high-energy neutrinos they study, and most recently, they have mapped the Milky Way itself as a diffuse, wide-area source of these particles.
Beyond astronomy, IceCube has also driven major advances in fundamental particle physics. There are three confirmed known “flavors” of neutrinos, and prior Nobel Prize-winning research found these particles can spontaneously transform from one flavor to another as they travel across space. IceCube’s large, high-precision array has let researchers make increasingly exact measurements of this poorly understood oscillation phenomenon.
“Another key research area, and the one I focus on personally, is cosmic rays,” Díaz Vélez says. “We’ve proven these high-energy proton and heavy nucleus particles don’t rain down on Earth uniformly from every direction — they cluster in preferred arrival directions, which tells us how their source objects are distributed across the Milky Way.”
“Catching a single neutrino is exactly like looking for a needle in a haystack,” he adds. “For every one neutrino IceCube successfully detects, we pick up more than a million background muons generated by regular cosmic rays.”
Díaz Vélez serves as product coordinator for the global IceCube collaboration, overseeing real-time processing of experimental data and large-scale physics simulations. The system calculates the exact arrival direction, energy, and type of every detected particle every two milliseconds, and runs parallel cosmic ray and neutrino simulations at matching volumes. All of this work runs on a distributed global network of high-performance computing clusters, totaling roughly 10,000 CPU cores and 1,000 GPUs.
While IceCube’s hardware is stationed in Antarctica and its administrative headquarters are in Wisconsin, it is a fully international collaboration of 450 researchers across 58 institutions in 14 countries, with a huge portion of the hands-on work led by master’s and doctoral students. The Nobel Prize may only carry Halzen’s name, but the breakthrough it honors rests on the cumulative work of hundreds of team members across decades.
“Francis is a towering figure in physics, an incredible leader, but also an incredibly friendly, generous person,” Díaz Vélez says. “Most of the time we see him more as a close colleague than a boss. To be honest, if it weren’t for his decades of relentless passion and refusal to give up when the project hit repeated roadblocks, this experiment never would have succeeded at all.”
This story was originally published in Spanish in WIRED en Español, and has been adapted from the original translation.
