The elusive Glashow resonance was observed deep within Antarctic ice

More than 60 years ago, theoretical physicist Sheldon Glashow predicted that the collision of an electron with an electron antineutrino would form a negatively charged W boson. For decades the event, now called a Glashow resonance, wasn’t observed, in part because it occurs only when an electron antineutrino’s energy reaches 6.3 PeV. Current particle accelerators cannot produce such energetic particles. Protons in the Large Hadron Collider, for example, have energies three orders of magnitude lower.

The standard model incorporates Glashow’s prediction, and its validity would be further cemented if such an event was detected from a nonterrestrial, astronomical source. A detection would also give researchers an opportunity to study the astrophysical processes that produce neutrinos. In 2016 the first step toward that goal was made when a promising high-energy particle shower was first detected using the IceCube Neutrino Observatory , shown in the picture above, near the Amundsen–Scott South Pole Station in Antarctica.

Now the researchers of the IceCube collaboration have completed painstaking analyses of the event and have reported , with a confidence level better than 2σ, that the results are consistent with a Glashow resonance. The University of Paris’s Antoine Kouchner, who was not involved with the research, said, “It is beautiful to see that the demanding efforts to construct a large detector in a hostile environment can lead to an additional confirmation of the standard model of particle physics.”

The first neutrinos to be detected from outside the solar system came from Supernova 1987a in the Large Magellanic Cloud (see Physics Today online, 23 February 2017 ). Some of the universe’s other highest-known energy sources—including active galactic nuclei, cosmic-ray and gamma-ray bursts, and the merger of two black holes or neutron stars—may also conceivably produce neutrinos (see Physics Today online, 12 July 2018 , and the article by Francis Halzen and Spencer Klein, Physics Today, May 2008, page 29 ). Because neutrinos interact so infrequently with other matter, they’re capable of escaping from those potential production sources and avoiding deflection from magnetic fields during their journey to Earth.

When neutrinos do interact with matter, via the weak nuclear force mediated by W and Z bosons, charged particles are produced that emit Cherenkov radiation when traveling through ice or other transparent mediums. In 2016 that radiation was spotted by the IceCube detector, composed of some 5000 digital optical modules (DOM) packed into a cubic kilometer of ice. The visualization below shows the DOMs at the observatory that detected the event. The larger red and orange sensors made the detection before the smaller blue and green ones.

During the five years between the first observation of the PeV-energy event and the recent publication of the discovery, the researchers conducted several quality-control checks to ensure the Cherenkov radiation did indeed come from a high-energy astrophysical neutrino interaction. A machine-learning algorithm analyzed events from 2012 to 2017 in the PeV energy range and compared those data with the 2016 particle shower. The analysis showed an event with visible light energy of about 6 PeV, consistent with the 6.3 PeV W⁻ found in a Glashow resonance event. The theory predicts that some 5% of the energy is associated with decay products that aren’t detectable as Cherenkov radiation.

Another confirming clue came when the researchers reconstructed the timing of the event’s energy and from what direction the Cherenkov radiation arrived. They detected a few pulses of energy earlier than would be possible for photons traveling in ice. The researchers suspect that the likely culprit for those early pulses is near-relativistic muons, a known decay product from the hadronic shower of a high-energy event.

Muons are also generated when cosmic rays collide with Earth’s atmosphere. The researchers ruled out, with an estimated certainty greater than 5σ, the possibility of atmospheric muons generating a PeV-level event over the same area as that of the 2016 particle shower. That degree of improbability allowed the researchers to reject the possibility that the Cherenkov radiation was an artifact of atmospheric muons.

In a final analysis the researchers tested whether the particle shower could have arisen from one of two relatively more common processes: charged-current or neutral-current interactions. The first occurs when electron neutrinos or electron antineutrinos interact with nucleons through charged W bosons; the second interaction arises when any of the three neutrino flavors exchange a neutral Z boson. Using a best-fit flux model and a likelihood-ratio test, the researchers concluded that the event is most consistent with a Glashow resonance, at a statistical significance of 2.3σ.

The results aren’t iron-clad, though Carla Distefano of the National Institute for Nuclear Physics in Catania, Italy, wrote in a Nature news and views piece that “the IceCube Collaboration’s observations are cause for celebration, because they are the first to be consistent with a Glashow resonance.” Although based on one event, the finding suggests that astrophysical fluxes contain electron antineutrinos. That information may help physicists uncover the mechanisms that make neutrinos: proton–proton or proton–photon interactions. (IceCube collaboration, Nature, 2021, doi:10.1038/s41586-021-03256-1 .)