Built in one of the most extreme environments on Earth, IceCube is the largest neutrino telescope in the world.

300 scientists at 53 institutions in 12 different countries. This is what it takes to design and operate of the world largest scientific facility dedicated to the advancement of our understanding of nature’s ghost particles: neutrinos. Neutrinos are chargeless, near-massless elementary particles which only interact through the weak nuclear force and gravity. Usually, particles are detected and identified by observing the effects of their charge, mass, or interactions. As neutrinos have no charge, barely any mass, and do not interact using the strong nuclear force, they are notoriously difficult to detect and are often referred to as the ghost particle. As a result, we are only able to “detect” neutrinos by inferring their presence from the detection of other particles and energy that were produced in interactions involving a neutrino. Even this is a difficult task, as the weak interactions of a neutrino have a very short range, meaning it rarely interacts with matter at all.

While neutrinos are one of the smallest sub-atomic particles in the standard model, they are created in the most energetic events in the universe. For instance, neutrinos are known to be produced in a large number in nuclear reactions in the core of the Sun. The first detection of solar neutrinos provided direct evidence that the sun produced its energy through nuclear fusion, as only fusion events could create neutrinos with the materials in the Sun. Neutrinos are also incredibly useful tools for examining supernova and neutron stars. It is theorised that, when a star’s core collapses and a supernova occurs, 99% of the expelled energy is emitted via neutrinos.

Benefits of the South Pole location

But why on Earth did scientists choose such an extreme environment as the South Pole to build IceCube? The IceCube detector is an array of detector modules buried at depths between 1450m and 2450m within the Antarctic ice sheet. The principal advantage of having an in-ice detector is that this offers researchers the possibility to look at the detection of burst neutrinos from supernovae. This is only possible in ice, due to low dark counting rate of the photomultipliers. The dark count rate is essentially a rate of counts that do not come from a photon event, but rather are intrinsic and come from the detector itself. This phenomenon is temperature-dependent: dark count-rate decreases with temperature, making the extremely cold Antarctic ice a perfect medium to increase sensitivity. The low dark count rate allows detection of even very small signals produced by MeV-range interactions of neutrinos from a supernova burst. This means certain supernovae can be detected with 5σ significance, and would thus be enough to trigger the SuperNova Early Warning System. The use of an in-ice detector is also advantageous in eliminating background events. As with deep-sea detectors, its location deep under the ground limits background from atmospheric muons, but IceCube has the additional benefit of not being subject to background photons produced by bio-luminescent bacteria which are present in the ocean. Seawater is also slightly radioactive: in particular, decay of Potassium-40 is a source of background radiation that needs to be accounted for in deep-sea neutrino telescopes.

Image of the ice cube diagram. Credits: IceCube Science Team – Francis Halzen, Department of Physics, University of Wisconsin

IceCube has undoubtedly contributed massively to the field of particle physics. IceCube detected a high energy neutrino in 2017 with a clear signal showing its direction, allowing the first identification of the source of a high energy neutrino. The source was found to be a blazar, which is a supermassive black hole at the centre of a galaxy which produces relativistic jets of matter. This has provided the first direct evidence of a likely source of cosmic rays. Additionally, though we are aware of neutrino oscillations, certain key parameters of the phenomenon are yet to be determined. In 2013, the first measurements of the neutrino oscillation parameters were analysed by IceCube. It was clear that IceCube could provide key insight to the nature of neutrino oscillations, so further measurements and analysis were made a year later, using three years of data. The precision on the parameters was improved by a factor of ten. Another recent update of measurements was published in 2018 using three years worth of data, providing some of the most competitive results to date. IceCube was the first experiment to use atmospheric neutrinos in this research, and a high precision was achieved due to an improved event selection of neutrino events.

A bright future

Additional key research topics are pushed forward in parallel, spanning from galactic supernovae to distant gamma-ray bursts coincident with neutrinos and indirect dark matter searches. With continuous upgrades of this already majestic facility, we can only expect more exiting results from all these active research areas. The future is certainly bright and promising for neutrino research!

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