Science

 

 

Neutrino Telescopes in water and ice are multipurpose detectors. They address physics questions ranging from astrophysics over particle physics to environmental science. 

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  • Search for the sources of high-energy  cosmic rays                                                                                                                                                                                                   

  • Cosmic rays consist essentially of protons and nuclei of heavier elements; electrons contribute only at the percent level. They reach breath-taking energies: up to 10 million times the beam energy of the Large Hadron Collider in Geneva! Moreover, all galactic cosmic rays together carry a similar amount of energy as star light or cosmic magnetic fields.  All that makes the question about their origin one of the prime questions of astrophysics.  Since cosmic rays are electrically charged, they are deflected by cosmic magnetic fields on their way to Earth. Precise pointing (i.e. astronomy ) is only possible with electrically neutral, stable particles: electromagnetic waves (i.e. gamma rays at the energies under consideration) and neutrinos. High energy neutrinos, with energies much beyond a GeV, must be emitted as a by-product of collisions of charged cosmic rays with matter. Actually, only neutrinos provide incontrovertible evidence for acceleration of nuclear particles since gamma rays may also stem from scattering processes of accelerated electrons and other electromagnetic processes. Moreover, neutrinos can escape much denser celestial environments than light. Therefore they can be tracers of processes which stay hidden to traditional and gamma ray astronomy. At the same time, however, their extremely low reaction cross section makes their detection a challenge and requires huge detectors.

  • The window of neutrino astronomy has been opened recently by the discovery of first extraterrestrial high-energy neutrinos with the iceCube detector.

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    • Indirect Search for WIMP Dark Matter                                                                                                                                                                                                      Weakly Interactive Massive Particles (WIMPs) belong to the prime candidates for Dark Matter. Over billions of years they may have accumulated in the Sun. Eventually, after their density has become sufficiently high, they would hit each other and transform into normal matter particles, among them neutrinos. All other particles are absorbed in the dense core of the Sun, but neutrinos can escape and detected in neutrino telescopes on Earth.  No sign of an excess of high-energy neutrinos from the direction of the Sun has been observed so far. This challenges models on the character and properties of WIMPs.
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    • Magnetic Monopoles and other super-heavy particles                                                                        
    • Magnetic Monopoles have been predicted more than 80 years ago. Interest in these particles has been renewed in the context of Grand Unified Theories (GUTs) of particles physics. They appear as extremely bright tracks in neutrino telescopes.  GUTs predict very heavy magnetic monopoles, so heavy that they do not fly with a speed close to that of light (like most of the lighter elementary particles) but rather with the speed of meteorites.
    • Other hypothetical super-heavy particles are quark-nuggets (or “nuclearites”).
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    • Violation of Lorentz invariance 
    •  "Lorentz invariance" is one of the central principles of Einstein's Theory of Special Relativity. Violation of  Lorenz Invariance is possible e.g. in quantum field theories and could manifest itself in a depletion of neutrinos at very high energies.

 

  • Neutrino Physics
  • The three different neutrino types can transform into each other, a phenomenon known as "neutrino oscillations".  These transformations can only happen if neutrinos have a mass, be it even very tiny. Over the last 20 years, neutrino oscillations have been firmly established. Yet, some of the relevant parameters governing oscillations are only known with a moderate accuracy, and some are not known at all. Neutrino telescopes can measure oscillations via a depletion of the flux of neutrinos generated in the atmosphere of the Earth, at certain angles and energies. ANTARES and IceCube have delivered first results on oscillations in 2012 and 2013.  Accuracies of these measurements will be considerably improved over the next years. The big challenge, however, is to fix a parameter which presently is not known: the so-called neutrino mass hierarchy. We actually know the mass differences between the "mass eigenstates" of neutrinos but neither their absolute masses nor which of the 3 is the lightest, i.e. how the exact hierarchy of the masses is. Detectors to determine the mass hierarchy are considered both for the South Pole (project PINGU) and the Mediterranean Sea (project ORCA).
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  • Other particle physics questions being studied include the neutrino cross section at highest energies and  the production of particles containing a charm quark which are generated in very high energy interactions of cosmic rays in the atmosphere.
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  • Supernova Collapse Physics         
  • Stars much heavier than the Sun finish their life in a gigantic collapse called "Supernova". During the first seconds after the collapse, only neutrinos can escape the extremely compact object in the center of the collapse (a "neutron star"). They carry information about the initial temperature in the neutron star as well as other important messages. Neutrinos from a supernova have been measured in 1987. At present, about ten neutrino detectors worldwide are waiting for the next galactic Supernova. These neutrinos would be a bonanza for astrophysics as well as particle physics. IceCube, for instance, would detect a huge number of neutrinos from such an event and would measure the risetime of the Supernova neutrino signal with unprecedented precision.
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  • Cosmic Ray Physics                                          
  • Cosmic rays interacting in the atmosphere generate muons. The most energetic of them punch through down to the depths of neutrino telescopes and carry information about the primary particles - their energy, their composition and their direction. Underwater and deepice detectors can measure these muons with high precision.  
  • Another way to investigate cosmic rays is to install an air shower detector at the surface - if a solid surface exists. This is the case at the South Pole (IceTop array).
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  • Environmental physics
  • Water neutrino telescopes can be used to study oceanographic (Sea) or limnological (Lake) phenomena, ice neutrino telescopes provide data on Glaciology. Such data have been obtained and published by the Baikal experiment, the ANTARES experiment and AMANDA/IceCube at the South Pole.