Seeing antineutrinos in a new way - the Glashow resonance

 Recently, IceCube made its first definitive observation of an antineutrino, as it interacts with an atomic electron.  The result was published in Nature, and is now available publicly on the arXiv preprint server, as arXiv:2110.15051.

The reaction is very different from the usual Deep Inelastic Scattering interactions, where a neutrino or antineutrino interacts with an atomic nucleus.   In this process, known as the Glashow Resonance, an antineutrino and an electron annihilate each other, producing a W boson, as is shown in the diagram to the right     The W boson is heavy (weighing about 85 times the mass of a proton), so it decays essentially immediately, usually into a quark and an antiquark which then fragment producing two jets of particles. In IceCube, this leads to a cascade of particles, which looks like (nearly) a point source of light.   For antineutrinos with the right energy (about 6.3 PeV), the interaction probability is very high - antineutrinos near the peak of the Glashow resonance only have a range in ice of about 100 km, only about 1% of the range for neutrinos of the same energy.

This reaction is of great interest because it only happens with antineutrinos.  Its not a big surprise that there are astrophysical antineutrinos, but it is nice to have clear confirmation.  Later, with enough statistics (this will take a while), we can measure the neutrino:antineutrino ratio, which will tell us something about how the neutrinos are produced.  

 

There were some interesting technical aspects of the event.  The event display (above) shows a large cascade near the edge of the instrumented volume.   In fact, the most likely location of the actual interaction is outside the detector, but close enough that we can reconstruct it well.   However, closer examination shows some interesting features.  

The bottom two parts of the graphic show the signals recorded in two of the optical modules, as a function of time.  The blue curves show the expected light profile from a pure cascade at the reconstructed interaction point.  The red curves show unexpected 'early' light.   We believe that this light came from muons produced in the cascade.  

The muons travelled at nearly the speed of light, while the light moves more slowly.  This may sound surprising but in dense materials like ice, the light interacts with the medium (one way to think about is as if the light bounces around as it moves), and so only moves at about 3/4 of the speed of light.  So, the muons will reach the vicinity of the optical sensors first, emitting early light which will reach the sensors before the light from the rest of the cascade.   This early light signals the presence of muons, which show that the cascade was a hadronic shower, rather than purely electromagnetic.  So, the cascade was not due to an electron-neutrino charged-current interaction.  By eliminating the electron-neutrino hypothesis, we strengthen the case that this is indeed the Glashow resonance.  Which, in turn, strengthens the case that we have observed an antineutrino.