A tiny electron anti-neutrino has traveled incredible cosmic distances at almost the speed of light, colliding with an electron in the ice of Antarctica. This interaction produced a W- boson, one of the carrier particles for the weak nuclear force (like the photon is the carrier particle for the electromagnetic force). This interaction was first proposed theoretically 60 years ago, and now it has been observed for the first time. The discovery is reported in the journal Nature.
Now, a bit of background for this extraordinary discovery. Let’s start with neutrinos. Neutrinos are the known fundamental particles with the smallest mass, and they do not have an electric charge. This makes them very fast, and they really don’t like to interact with other particles. They come in three types or “flavors”: electron neutrino, muon neutrino, and tau neutrino.
The neutrinos have their antimatter counterparts – the antineutrinos. It is difficult to study these particles given how they stubbornly refuse to interact. Every second, 100 trillion neutrinos pass through our bodies.
Physicists use huge detectors to try and catch the rare moments when a neutrino hits another particle – and this is what happened inside IceCube. Deep within the ice of Antarctica, physicists have placed array after array of detectors. If a neutrino slams into a particle within that large volume, the observatory can catch it.
Back in 1960, Sheldon Glashow, who was working at Niels Bohr Institute in Copenhagen, suggested in a paper that an electron anti-neutrino interacting with an electron could produce a new particle if the anti-neutrino had the right energy. This process became known as the Glashow resonance.
The particle in question, the W- boson, was discovered in 1983 – but the energy required for it to actually be produced by the Glashow resonance is beyond what humans can produce. The anti-neutrino should have almost 1,000 times the energy we can give particles in the Large Hadron Collider.
But natural particle accelerators can do much better than us. A supermassive black hole in another galaxy most likely accelerated a large number of particles to incredible energies, among them an anti-neutrino. This one traveled intergalactic space until it hit an electron in the ice.
"When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal for producing the W- boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice," principal investigator of IceCube professor Francis Halzen, from the University of Wisconsin-Madison, said in a statement.
IceCube is just over a decade old, and since 2013 it has consistently reported detections of neutrinos from beyond the solar system – including from a black hole ripping a star apart. This newly reported detection – which actually happened on December 6, 2016 – is the first time that researchers could confirm that the event was caused by an anti-neutrino.
"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," explained lead author Dr Lu Lu, a professor at the University of Wisconsin–Madison.
Correction: An earlier version of this article had the wrong affiliation for professor Lu