Teresa Montaruli, University of Geneva
When we think about astronomers, we picture them hidden in a dome on top of a mountain during sleepless nights. So how comes that in the news we have a telescope buried between 1.5 and 2.5 kilometers in the Antarctic ice that restlessly decodes information from the sky? What brought scientists to exploit their fantasy to the point of trying to see the far universe from the most remote location of the Earth, the South Pole?
After more than 20 years of pioneering the drilling technique in the ice and after the AMANDA detector was successfully operated at a smaller scale in the 90ies, its successor IceCube has found first signs of the most powerful messengers of the Universe: ultra-high energy neutrinos.
IceCube is huge, a cubic kilometre ice volume instrumented with ‘electronics eyes’ along 86 cables transporting power and data. They detect the weak blue light produced in a cone around the direction of charged particles when they travel faster than the speed of light in the medium. Rarely such light-emitting charged particles are induced by neutrinos that penetrate to the depths of IceCube. Such neutrinos are hidden between a huge amount of other particles and backgrounds and require quite an effort to be filtered out. Despite their more challenging detection, these elusive particles are potentially the most powerful messengers of the universe. They are the only possible tools we know to have the capability of traveling from the interior of sources up to the farthest regions of the observable universe.
In 1960 Greisen realized the power that these messengers could have in astronomy: they can open a new window on the non-thermal emissions of the Universe. These are not directly related to the temperature of matter in the universe but to acceleration processes in shocks caused by the death of stars, by the formation of jets coming out of black holes or by collisions of galaxies. Neutrinos provide a picture of such portions of the universe that are not accessible by messengers like the photons since they are lost on the way to us from their sources or they cannot exit the sources themselves.
Contrary to gamma-rays, which are produced also in electromagnetic phenomena associated to the presence of electrons and magnetic fields in the universe, neutrinos are signs of the presence of matter, such as protons and nuclei, accelerated in the universe. As we exploit high energy machines at CERN to learn about the fundamental properties of matter and the origin of the universe, similar goals can be achieved intercepting the powerful beams produced by the powerful primordial accelerators in the universe.
Once IceCube achieved its completion in 2011, the Collaboration made a huge effort to exploit the large dimensions of the detector and the powerful information recorded by such embedded photosensors with associated computers in the ice. By using background rejection techniques to eliminate particles produced in the atmosphere, 28 high-energy neutrino events of more than 50 Tera-electronvolts have been detected. The events, including two in the highest PeV energy region, cannot be explained by other neutrino fluxes, such as those from atmospheric neutrinos, nor by muons produced by the interaction of cosmic rays in the atmosphere.
The results above could be achieved only with the full detector. As a matter of fact only with a large enough detector the veto rejection technique of the atmospheric background can be successfully applied. The technique tags downgoing tracks that are typically produced by downgoing muons that are more than 5 orders of magnitude more numerous than atmospheric neutrinos. It is also capable of tagging atmospheric neutrinos, which are a dangerous background for astrophysical neutrinos, since muons and neutrinos are produced in the same meson decays in the atmospheric showers. Hence, when a muon crosses the detector, also an inner track, most probably due by a neutrino, can be tagged and rejected. This leaves more room to detect signal neutrinos directly produced in sources in the upper hemisphere of IceCube in the region above few tens of TeV. Previously we mostly used the lower hemisphere (so only one half of the sky) since we used the earth itself as a filter to select upgoing neutrinos against atmospheric muons. Now, the full sky can be covered with better sensitivity.
Once these high energy neutrino events have been collected, the analysis continued and we have achieved now a larger sample. A part of this has been unblended and is being published and another part is waiting for approval for unblinding. Unblinding is a procedure that prohibits analyzers to look into the signal region before they have selected their search criteria in order to not be biased by the desire for discovery.
By accumulating the statistics we will be able to understand better the energy spectrum of these events and to understand if they are compatible with shock acceleration in sources or with neutrinos produced in interactions of ultra-high energy cosmic rays on the microwave background left by the Big Bang. Identifying the sources is our next challenge. The ultra-high energy events produce huge light showers in the detector and it is not trivial to identify the direction with sub-degree accuracy. Nonetheless, the data acquisition system in IceCube is sophisticated and provides the full information on the time of each of the photons produced by particles. The associated charge released tells us how the energy lost by the particle distributes in time and space in the instrumented region. We have learned in time how to exploit this information better.
Only after enough statistics will be accumulated, we will fully understand how the signal is reaching us. For the moment we know that the sunrise of neutrino astronomy has finally begun.
The IceCube Neutrino Observatory was built under a NSF Major Research Equipment and Facilities Construction grant, with assistance from partner funding agencies around the world. The NSF's Division of Polar Programs and Physics Division continue to support the project with a Maintenance and Operations grant, along with international support from participating institutes and their funding agencies. These include the University of Geneva funded by the Swiss National Science Foundation. The international collaboration includes 250 scientists.
The paper reporting the details of this scientific result is: "Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector," IceCube Collaboration: M. G. Aartsen et al. Science 342, 1242856 (2013). DOI: 10.1126/science.1242856
[Released: January 2014]