  The Glashow resonance describes the resonant formation of a $W^-$ boson
during the interaction of a high-energy electron antineutrino with an electron,
peaking at an antineutrino energy of 6.3 petaelectronvolts (PeV) in the rest
frame of the electron. Whereas this energy scale is out of reach for currently
operating and future planned particle accelerators, natural astrophysical
phenomena are expected to produce antineutrinos with energies beyond the PeV
scale. Here we report the detection by the IceCube neutrino observatory of a
cascade of high-energy particles (a particle shower) consistent with being
created at the Glashow resonance. A shower with an energy of $6.05 \pm 0.72$
PeV (determined from Cherenkov radiation in the Antarctic Ice Sheet) was
measured. Features consistent with the production of secondary muons in the
particle shower indicate the hadronic decay of a resonant $W^-$ boson, confirm
that the source is astrophysical and provide improved directional localization.
The evidence of the Glashow resonance suggests the presence of electron
antineutrinos in the astrophysical flux, while also providing further
validation of the standard model of particle physics. Its unique signature
indicates a method of distinguishing neutrinos from antineutrinos, thus
providing a way to identify astronomical accelerators that produce neutrinos
via hadronuclear or photohadronic interactions, with or without strong magnetic
fields. As such, knowledge of both the flavour (that is, electron, muon or tau
neutrinos) and charge (neutrino or antineutrino) will facilitate the
advancement of neutrino astronomy.
