Neutrinos are the most elusive of the fundamental particles and occur in three types: the electron neutrino, the muon neutrino and the tau neutrino. Almost massless and without electric charge, they communicate only with matter through a fundamental force, known as the weak interaction, which is mediated by force-bearing particles. W and Z bosone. In 1959, the theoretical physicist Sheldon Glashow used the standard model of particle physics to predict1 which is negatively charged W bosone (W– bosons) can be formed in the collisions between an electron and an electron antineutrino (the antimatter version of an electron neutrino). This process is now called the Glashow resonance, and occurs for electron neutrinos that have energy of about 6.3 petaelectron volts (1 PeV is 1015eV).
The Glashow resonance was not observed in laboratories because the required antineutrino energy is outside the range of available particle accelerators. Naturally occurring antineutrinos produced in cosmic processes can reach energy up to ten PeV2. In a paper in Nature, the IceCube Collaboration reports the detection of an event produced by an antineutrino from an astrophysical source, which may be the first observation of the Glashow resonance.3.
When a neutrino changes with matter, charged particles are produced. These emit light known as Cherenkov radiation when moving through a transparent medium (such as ice or water) at a velocity greater than the speed of light in that medium. High-energy astrophysic neutrinos can thus be observed by instruments that detect Cherenkov radiation in such a medium. Because the expected flood in the number of astrophysic neutrinos at the energy levels that are low is low, and it is suspected that this flood decreases rapidly as the energy of neutrinos increases2 , large volumes of the transparent medium are required.
The IceCube Neutrino Observatory is a neutrino detector buried in the deep ice near the Amundsen – Scott Antarctic Station in Antarctica (Fig. 1). Its main purpose is to observe neutrinos produced from the most powerful astrophysical sources in the Universe, such as active galactic nuclei and γ-ray bursts, or from catastrophic phenomena such as exploding stars and fusion of black holes or neutron stars. IceCube can detect all odors of astrophysic neutrino at energies outside the exaelectron volt range (1 EeV is 1018eV). It consists of 5,160 digital optical modules (DOMs; devices that can detect faint light signals), located over one cubic kilometer of Antarctic ice, buried at a depth of 1,450-2,450 meters. The DOMs are attached to 86 vertical strings that are 125 feet apart in the ice, on a hexagonal grid.
Figure 1 | The IceCube Neutrino Observatory, Antarctica. The IceCube Collaboration reports3evidence of an event known as a Glashow resonance, which occurs when a high-energy electron antineutrino (the antimatter counterpart of an electron neutrino) interacts with matter.Credit: Yuya Makino, IceCube / NSF
Cherenkov radiation detected by the DOMs is used to reconstruct the properties of the neutrino that caused it – such as the energy and the direction from which it came. The topology of the radiation burst can also be informative. For example, if a muzzle neutrino interacts with matter, it can create a muzzle particle moving a few kilometers4 . It produces an elongated, light orbit in the ice called an orbit. Other neutrino odors produce cascades of secondary particles within a spherical region of only 10 meters in diameter5. The spatial and temporal characteristics of detected light signals thus contain information about the neutrino odor and about the interaction channel (the process by which the neutrino deals with matter; the Glashow resonance is one kind of interaction channel).
Antineutrinos that interact with matter through the Glashow resonance are expected to produce characteristic events that result in W– boson decays into a cascade of secondary particles, including particles called hadrons. It is expected that about 5% of the neutrino energy in these events will be absorbed by secondary particles that are neutral or do not have enough energy to produce Cherenkov radiation.5which limits the amount of energy that can be observed to about 6.0 PeV. In addition, low-energy muons are expected to be produced in the cascade and that the wavefront of the Cherenkov radiation will exceed fast enough to cause early light pulses that can be detected by the DOMs.
On December 8, 2016, IceCube detected an event that had a visible energy of 6.05 ± 0.72 PeV. The IceCube Collaboration analyzed the parameters of the event using a machine learning algorithm optimized to recognize cascades with multi-PeV energies. The algorithm was trained to distinguish such signals from those associated with muons produced in the atmosphere, taking into account the differences between trace and cascade events.
Based on the characteristics of the detected event, the authors classified it as an astrophysic neutrino – the confidence of this classification was at the 5σ level, meaning that there is only about one in three million chances that it is incorrect. . In addition, early light pulses were detected by the DOMs, consistent with the production of low-energy secondary muons from the decay of a W–boson and hadrone. A statistical test of the data indicated that the neutrino interaction process that caused the event may be the Glashow resonance. However, the confidence of the result is only at the 2.3σ level, which falls too short to completely rule out the possibility of a non-Glashow resonance event.
Nevertheless, the observations of the IceCube Collaboration are cause for celebration because it is the first to match a Glashow resonance. An unambiguous detection will not only confirm the standard model of particle physics, but also prove that electron neutrinos occur in astrophysical flood. The fraction of astrophysical flood consisting of electron antineutrinos depends on the mechanisms by which neutrinos and antineutrinos are produced, of which there are two possibilities.6: Hadronuclear processes, which involve proton-proton interactions, and photohadronic processes, which involve proton-photon interactions. Future measurements of electron neutrinos will therefore open a window for the physics of neutrino sources. The current IceCube detector can only detect a low number of Glashow resonance events, but the next generation of the device – IceCube-Gen2 – was introduced last year.7. This detector has a sufficiently large amount of sensor-tested ice to detect higher numbers of Glashow resonance events, enabling a statistically meaningful analysis of astrophysic neutrino production mechanisms.