Understanding our universe requires understanding how individual particles in the hearts of stars and in the first moments of existence have all been able to create the light that warms us and the particles that make up our planets, our technology, and ourselves. 

It’s hard to build massive telescopes, and the decade-long delay in the launch of the JWST is a case study of how hard it is to build scopes that allow us to see back to the earliest epochs of star and galaxy formation. On the other end of the scale, we build massive accelerators like FermiLab in Illinois and CERN in Europe, and we smash together particles after accelerating them to huge velocities, and out of the energy of the collisions, new particles can pop into existence. From these collisions, we’ve detected quarks, the Higgs boson, and myriad other particles. Unfortunately, there is a limit to what we can do, and some theorized reactions are beyond the capacity of our current engineering and our current power grid. 

Back in the 1960s, Sheldon Glashow predicted that at the right energy, an antineutrino and electron could interact and produce a W-minus boson. This unstable particle plays a role in nuclear decay, and the W-, in particular, lets quarks change flavors, literally make down to become up, or at least the down quark to become the up quark. Every nuclear decay, for instance in cancer treatment or in the glowing radium hands of an antique watch, every nuclear decay is thanks to these particles. Now, we’ve been able to detect W- bosons with CERN and other accelerators since 1983. Electrons — Benjamin Franklin studied those. But anti-neutrinos are harder.

We do have various detectors that can broadly detect neutrinos and antineutrinos, but we can’t always figure out if we’re seeing a neutrino or antineutrino. Sheldon Glashow’s prediction of an antineutrino interacting with an electron and a W- boson offers a specific reaction that, if observed, will make it clear that an antineutrino is being seen.

It isn’t enough to mix these three particles together, however. The antineutrino in the reaction has to have just the right energy – an energy 1000 times greater than what CERN can produce. This means, to see this reaction, the universe must accelerate an antineutrino to just the right energy, fire that antineutrino in Earth’s direction and that particle must intersect with an electron inside a detector capable of seeing the interaction.

Basically, a lot of things in the universe have to precisely align. And on December 6, 2016, this is exactly what happened.

Down in Antarctica, at the Amundsen-Scott South Pole Station, there are thousands of sensors distributed over a cubic kilometer of ice watching for high-energy cosmic neutrinos and antineutrinos to interact and create cascades of particles and bits of energy. On that summer day, the specific cascade of particles distinctive of Sheldon Glashow’s resonance was spotted, signifying that a 6.3 petaelectronvolt antineutrino, likely accelerated by a massive black hole somewhere out in space, had precisely hit an electron.

This result is now published in the journal Nature by the IceCube collaboration.

According to researcher Lu Lu: Previous measurements have not been sensitive to the difference between neutrinos and antineutrinos, so this result is the first direct measurement of an antineutrino.

To me, this is amazing because so many things literally had to line up in time and space to allow this detection to happen, but because the universe is vast, the fact this has happened once means it should happen again and again and again. With each detection, we’ll be able to better understand the ratio of how many neutrinos and antineutrinos are out there being accelerated our way.

Just think: some of them could be passing through you right now, and you would never know. Science is weird. And it is wonderful. 

More Information

IceCube press release

University of Wisconsin-Madison press release

Michigan State University press release

“Detection of a particle shower at the Glashow resonance with IceCube,” The IceCube Collaboration, 2021 March 10, Nature