How do astronomers detect neutrinos

Astronomers in Antarctica are kicking off a new way of studying the cosmos. Rather than studying light, researchers now plan to study neutrinos from outside of our solar system. These hard-to-detect particles can carry new information on distant galaxies. Unlike light, neutrinos can pass through most all matter in their path, allowing researchers to see exactly where they came from and to learn more about black holes, supernovas, and other cosmic occurrences.

Though neutrinos have been known about for some time, and were detected in one isolated experiment in , researchers have since only able to detect ones from within our own solar system. In an article published today in Science , researchers from the IceCube Neutrino Observatory in the South Pole detail the detection of 28 of them, which they say must have originated farther out in the cosmos.

We have finally discovered those. Because neutrinos pass through most matter, detecting them is a tricky task. To do so, researchers at IceCube burrowed through a mile of ice to place their detectors deep underground where neutrinos would be forced to interact with a large amount of matter in front of them.

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The "keystone" proton-proton reaction, which created the detected neutrinos, is the first step of a reaction sequence responsible for about 99 percent of the Sun's power. Because they only interact through the nuclear weak force, they pass through matter virtually unaffected, which makes them difficult to detect and distinguish from trace nuclear decays of ordinary materials, he said.

These pp neutrinos, emitted when two protons fuse forming a deuteron, are particularly hard to study. This is because they are low energy, in the range where natural radioactivity is very abundant and masks the signal from their interaction.

Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet. Indeed, it is the only detector on Earth capable of observing the entire spectrum of solar neutrinos simultaneously. Although neutrinos have mass, it is negligible relative to the TeV to EeV energies targeted by neutrino telescopes. They do however give rise to neutrino oscillations. Unfortunately, their weak interactions make neutrinos very difficult to detect.

High-energy neutrinos interact with matter via deep inelastic scattering off nucleons. Whereas the former interaction leaves the neutrino state intact, the latter creates a charged lepton associated with the initial neutrino flavor. The struck nucleus does not remain intact and its high-energy fragments typically initiate hadronic showers in the target medium. Immense particle detectors are required to collect cosmic neutrinos in statistically significant numbers.

Already by the s, it had been understood that a kilometer-scale detector was needed to observe the cosmogenic neutrinos produced in the interactions of CRs with background microwave photons.

There exist a variety of methods to detect the high-energy secondary particles created in CC and NC interactions. This requires the use of transparent detector media like water or ice.

A sketch of the signal is shown in Figure 2. Photomultipliers placed in the medium transform the Cherenkov light of muons generated in neutrino interactions into electrical signals using the photoelectric effect. This information allows scientists to reconstruct the various Cherenkov light patterns produced in neutrino events and infer their arrival directions, their energies and—in most cases only on a statistical basis—their flavor. Because of the large background of muons produced by CR interactions in the atmosphere, the signal is limited to upgoing muon tracks that are produced in interactions inside or close to the detector by neutrinos that have passed through the Earth see Figure 3.

Even in a cubic kilometer detector, the production of atmospheric neutrinos is suppressed once their energy exceeds hundreds of TeV.

Thus, energy alone allows separating atmospheric and high-energy cosmic neutrinos. The hadronic particle showers that develop after the neutrino strikes a nucleus in the ice or water are also visible by optical Cherenkov emission. The direction of the initial neutrino can only be reconstructed from the Cherenkov emission of secondary particles close to the neutrino interaction point, and the angular resolution is much worse than for track events.

On the other hand, cascade events allow for a better energy resolution since the Cherenkov light is proportional to the energy transferred to the cascade, which is fully contained in the instrumented volume.

It has demonstrated the feasibility of neutrino detection in the deep sea and has provided a wealth of technical experience and design solutions for deep-sea components. However, the recent breakthrough of neutrino astronomy by the first observation of high-energy astrophysical neutrinos was achieved by the IceCube observatory.

The IceCube detector transforms deep natural Antarctic ice 1, m below the geographic South Pole into a Cherenkov detector. The instrument consists of 5, digital optical modules that instrument a cubic kilometer of ice; see Figure 4.

Each digital optical module consists of a glass sphere that contains a inch photomultiplier and the electronics board that digitizes the signals locally using an onboard computer. The digitized signals are given a global time stamp with an accuracy of two nanoseconds and are subsequently transmitted to the surface. Processors at the surface continuously collect the time-stamped signals from the optical modules, each of which functions independently. These signals are sorted into telltale patterns of light that reveal the direction, energy, and flavor of the incident neutrino.

Even at a depth of 1, m, IceCube detects a background of atmospheric cosmic-ray muons originating in the Southern Hemisphere at a rate of 3, per second Figure 3. Two methods are used to identify neutrinos.

Traditionally, neutrino searches have focused on the observation of muon neutrinos that interact primarily outside the detector to produce kilometer-long muon tracks passing through the instrumented volume.

Although this allows the identification of neutrinos that interact outside the detector, it is necessary to use the Earth as a filter in order to remove the huge background of cosmic-ray muons.

This limits the neutrino view to a single flavor and half the sky because of the presence of an energetic muon secondary in the debris of the neutrino interaction. An alternative method exclusively identifies high-energy neutrinos interacting inside the detector, so-called high-energy starting events HESE. Furthermore, with this method, neutrinos from all directions in the sky can be identified, including both muon tracks as well as secondary showers, produced by charged-current interactions of electron and tau neutrinos, and neutral current interactions of neutrinos of all flavors.

The Cherenkov patterns initiated by an electron or tau neutrino of 1 PeV energy and a secondary muon depositing 2. The two methods, upgoing muon tracks and HESE, of separating neutrinos from the cosmic-ray muon background have complementary advantages.

In contrast, the reconstruction of the direction of cascades in the HESE analysis, in principle possible to a few degrees, is still in the development stage in IceCube. For high-energy neutrino astronomy, the first challenge is to select a pure sample of neutrinos, roughly , per year above a threshold of 0.

Above this energy, however, the atmospheric neutrino flux reduces to a few events per year, even in a kilometer-scale detector, and thus events in that energy range are cosmic in origin.

Using the Earth as a filter, a flux of neutrinos has been identified that is predominantly of atmospheric origin. Looking for galaxies in all the wrong places.

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