Physicists studying the data of the IceCube Neutrino Observatory in Antarctica noticed an outstanding physical event. With a statistical accuracy of 2.3σ, a super-energetic cosmic particle (electron antineutrino) interacted with an electron deep within the ice of the South Pole, resulting in a rare phenomenon predicted 60 years ago in a theory – the Glashow resonance event.
This phenomenon consists of a resonant increase in the W-boson production cross-section in the collision of an electron antineutrino with an electron when the antineutrino energy in the electron rest frame approaches 6.3 petaelectronvolts (PeV).
Scientists have determined the apparent energy of particles born in the event, which turned out to be close to the desired one and also determined the region of origin of neutrinos and the role of background processes in the registration of such a phenomenon.
In the future, the developed methods will make it possible not only to study the Glashow resonance phenomenon itself but also to learn more about the sources of astrophysical neutrinos.
What is a neutrino?
Neutrinos are extremely light elementary particles that interact very weakly with matter. The interaction itself occurs through the exchange of W- and Z-bosons with matter – carriers of weak interaction.
Neutrinos can be born in seemingly very different processes: solar neutrinos, for example, are formed during the thermonuclear reaction of hydrogen burning with the formation of helium, and atmospheric neutrinos – during the decay of pions and kaons, which appear when cosmic rays interact with nuclei in the air.
Astrophysical neutrinos, which arise in various “cosmic accelerators” – objects in the universe capable of accelerating particles to extremely high energies, stand apart. Potential sources of astrophysical neutrinos can be, for example, active galactic nuclei, supernova explosions, and other sources of gamma-ray bursts.
Each type of neutrino is characterized by the dependence of their flux on energy – a spectrum. In addition to the source itself, it is the spectrum that distinguishes astrophysical neutrinos from all others – very few of them are born and arrive on Earth, and their energy can be very large: up to 10 20 electron volts.
In addition, astrophysical neutrinos have a particularly small cross-section for interaction with matter (on the order of 10 -20 barn), which makes them very convenient observables for studying processes in their potential sources.
The fact is that the environment surrounding potential “space accelerators” is very dense, and light and neutral neutrinos are able to overcome it and reach the Earth without even deflecting under the influence of a magnetic field. That is, astrophysical neutrinos can allow physicists to indirectly study the physics of space objects involved in their birth.
What is the Glashow Resonance?
Another interesting effect is associated with high-energy astrophysical neutrinos – the Glashow resonance event. It was theoretically predicted back in 1959 and consists of a resonant increase in the W-boson production cross-section in the collision of an electron antineutrino with an electron when the antineutrino energy in the electron rest frame approaches 6.3 petaelectronvolts.
This energy is unattainable for existing “terrestrial” accelerators, but it is quite accessible for their cosmic counterparts, which means that the resonant production of the W-boson is possible on Earth, but with the participation of an astrophysical neutrino.
Observation of such a process is interesting not only as another potential confirmation of the Standard Model: only antineutrinos can participate in it, which means that an experimental study of the Glashow resonance would make it possible to directly compare the fractions of astrophysical neutrinos and antineutrinos.
In many ways, the IceCube neutrino observatory was created just for the registration of astrophysical neutrinos and related processes, in which a cubic kilometer of Antarctic ice acts as a working body of the detector. It contains photomultipliers that register Cherenkov radiation from charged particles and their decay products generated by the interaction of neutrinos with ice and the earth.
Physicists can determine the direction of movement of the neutrino itself by the direction of propagation of radiation in the thickness of the ice, and by the intensity of this radiation, they judge its energy. The construction of the observatory was completed back in 2010, and the first neutrino event was registered even earlier – on January 29, 2006.
However, as mentioned above, the higher the neutrino energy, the less likely it is to register it, and until recently IceCube could not distinguish in the accumulated data events with the participation of cosmic particles with sufficiently high energy close to the 6.3 petaelectronvolt Glashow resonance characteristic.
Now physicists have found traces of the desired neutrino in the detector data for 4.6 years of operation between 2012 and 2017:
Detecting the cosmic particle and the phenomenon it caused
To detect this event, an algorithm based on machine learning was used, which, unlike the algorithms in previous analyzes, looked for events at the edge of the detector, thereby indirectly increasing its useful volume. To find the exact energy and direction of movement of neutrinos, physicists performed Monte Carlo simulations of the recorded event, varying its possible parameters.
After such a simulation, it was found that on the photodetectors closest to the event, the signal appeared even before the photons from the initial flux of particles from the high-energy W-boson born in the ice mass could reach the detector.
This feature of the event is explained by the fact that the light in the ice moves at a speed of 2.19 × 10 8 meters per second, while muons, born in the decays of mesons in the hadron shower of the original event, move through the ice at almost the speed of light in a vacuum of 3 × 10 8 meters per second.
Thus, the first registered photons were Cherenkov radiation from these very muons, and then photodetectors recorded radiation from the initial cascade of particles.
Separation of the signal from the muons that reached the detector and from the initial cascade of particles made it possible to verify the correctness of the determination of the direction of motion of the neutrinos: from kinematic considerations, they should have flown in the same direction.
The same considerations narrowed the possible area in the starry sky, from where the cosmic particle flew to Earth. To make sure that the detected neutrino was astrophysical, physicists modeled the background of cosmic muons and found that they could only generate 1.1 × 10 -7 events in 4.6 years with the same detector response.
Similar calculations showed that atmospheric neutrinos over the same period of time could only lead to 2 × 10 -7 events, which, in combination with data on muons, indicates the registration of astrophysical neutrinos with a statistical accuracy of 5σ.
In addition, the scientists needed to make sure that the recorded event was a manifestation of the Glashow resonance and not some other interaction of the astrophysical neutrino with matter.
The main background process, in this case, is the interaction of neutrinos with nucleons through interaction through charged currents (that is, through the exchange of virtual W ± -bosons). The calculations also took into account the interaction through neutral currents (by exchanging virtual Z 0-bosons).
As a result, Monte Carlo simulations showed that the probability of such an occurrence of a registered event is 100 times less than the same probability for the Glashow resonance, for which the modeling predicted the registration of 1.55 events over 4.6 years of observation. That is, the observation of the Glashow resonance can be said with a certainty of 99 percent, or 2.3σ.
Scientists note that although the work describes the processing of only one event, the developed methods can be used for future data and experiments, as well as for existing results and the search for neutrinos of lower energies in them.
In addition, accurate registration of the antineutrino flux, including in large-scale experiments such as IceCube-Gen2, will be able to limit the existing models of astrophysical neutrino production, in which the ratio of neutrino and antineutrino fluxes strongly depends on such parameters of sources as the photon density, mass cosmic ray spectrum, and magnetic field strength.
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