W boson spotted in Antarctica

On 6 December 2016, a high-energy particle hurtled from outer space and through an Antarctic ice sheet, where it slammed into an electron at nearly the speed of light. The enormously energetic collision created a completely different particle, which rapidly decayed into a cascade of others.

This event might have gone unnoticed – if a massive matrix of neutrino-detectors hadn’t been sunk into the ice, ready to capture such astrophysical phenomena.

In a paper published in Nature, high-energy astrophysicists at the IceCube Observatory  in Antarctica confirm that this 2016 collision provides observational evidence for a theory put forth in 1960, solidifying our understanding of the Standard Model of particle physics.

Called a Glashow resonance event, the phenomenon was first described by physicist Sheldon Glashow. He predicted that if an antineutrino with just the right amount of energy collided with an electron, it would create a then-undiscovered particle through a process called resonance.

This mysterious particle – the W boson – is an elementary particle with an electric charge. Along with the Z boson, it mediates the weak force, which is one of the four fundamental forces that govern how matter behaves.

When it was discovered 23 years later by CERN’s Large Hadron Collider via a different particle interaction, physicists realised that the W boson was much heavier than expected. This amended Glashow’s prediction: for a W^– boson to be created via an antineutrino-electron interaction, the antineutrino would need to have an astonishing energy of 6.3 petaelectronvolts (PeV). This is 1000 times more energy than the Large Hadron Collider can produce – and that’s currently the most powerful particle accelerator in the world.

Since physicists couldn’t simulate a Glashow collision in the lab, they instead had to wait for a cosmic coincidence. Supermassive black holes and other extremely energetic astrophysical processes can act like natural particle accelerators, but not only do they have to accelerate an antineutrino to the right energy, and not only must it be sent on the right trajectory to collide with the Earth, it needs to hit in the square kilometre of Antarctic ice where the IceCube Observatory’s detectors are watching.

Needless to say, scientists have been waiting a while. The IceCube Observatory was operational in 2011; this 2016 hit is its first W^– boson.

“When Glashow was a postdoc at Niels Bohr [Institute], he could never have imagined that his unconventional proposal for producing the W^– boson would be realised by an antineutrino from a faraway galaxy crashing into Antarctic ice,” says Francis Halzen, IceCube’s principal investigator from the University of Wisconsin-Madison in the US.

The results of the collision analysis show that it was one of the highest-energy events ever detected by the IceCube observatory; only two other events have had an energy greater than 5 PeV.

It’s also one of the first times the detector has been able to distinguish neutrinos and antineutrinos, their anti-matter equivalents.

“Previous measurements have not been sensitive to the difference between neutrinos and antineutrinos, so this result is the first direct measurement of an antineutrino component of the astrophysical neutrino flux,” explains co-author Lu Lu, from Chiba University in Japan.

This is an important distinction to make, according to another co-author Tianlu Yuan from the Wisconsin IceCube Particle Astrophysics Center: “There are a number of properties of the astrophysical neutrinos’ sources that we cannot measure, like the physical size of the accelerator and the magnetic field strength in the acceleration region.

“If we can determine the neutrino-to-antineutrino ratio, we can directly investigate these properties.”

Observing further Glashow resonances will help the team figure out this ratio – as well as double, triple and quadruple check that the 2016 event does indeed confirm the predictions.

Glashow himself – now an emeritus professor at Boston University – concludes: “To be absolutely sure, we should see another such event at the very same energy as the one that was seen. So far there’s one, and someday there will be more.”

The Royal Institution of Australia has an Education resource based on this article. You can access it here.