Particle Astrophysics with the AMANDA neutrino telescope
Authors: Spiering C. 
Published: May 2022

Whereas MeV neutrin astronomy has been established by the observation of solar neutrinos and neutrinos from supernova SN1987, neutrinos with energies of GeV to PeV, which must accompany the production of high energy cosmic rays still await discovery. Detectors underground have turned ut to be too small to detect the feeble fluxes of energetic neutrinos from cosmic accelerators. The high‐energy frontier is being tackled by much larger, expandable arrays constructed in open water or ice. These telescopes detect the Cherenkov light generated by secondary particles – typically muons – produced in neutrino interactions. The Cherenkov radiation is detected by an array of photomultipliers which measures the arrival times of the photons to a precision of a few nanoseconds. Timing as well as amplitude information is used to reconstruct the track of the muon. To ensure that the muon is produced by a neutrino, the Earth is used as a filter: up‐going muon tracks can be generated only by neutrinos since this is the only particle which can pass through the Earth. Upgoing muons must be identified in an intense flux of downgoing muons. At 1 kilometer depth, the flux of downgoing (background) muons exceeds the upgoing signal by nearly six orders of magnitude.

The Amanda neutrino detector consists of photomultipliers embedded at a depth of 1.5‐2.0 km in the ice sheet covering the geographic South Pole. The photomultipliers are housed in pressure glass spheres which are attached to vertical cable strings. With 677 photomultipliers at 19 strings, the present AMANDA‐II array reaches an effective detection area of a few 104 m2 for 1 TeV muons. Although still far below the square kilometer size suggested by most theoretical models, AMANDA‐II may be the first detector with a realistic discovery potential for extraterrestrial high‐energy neutrinos.

First physically relevant limits have been obtained from the analysis of data taken with the three times smaller AMANDA‐B10 in 1997. The limit on the diffuse flux from unresolved sources with an assumed E—2 spectrum is of the order of 10—6 Eν—2 cm—2 s—1 sr—1 GeV—1, close to recent theoretical bounds. The flux limit on point sources with an E—2 spectrum at the northern sky (declination larger than 30 degrees) is of the order of 10—7 cm—2 s—1 for Eν > 10 GeV. The overall sensitivity of AMANDA‐B10 has been verified by a sample of more than 200 events which is dominated by atmospheric neutrinos. These are neutrinos which have been generated by interactions of primary cosmic rays in the atmosphere at the other side of the Earth. Since their flux is known with an accuracy of about 20%, they serve as a standard candle to calibrate the telescope.

The main purpose of Amanda is the search for neutrinos from cosmic acceleration processes in galactic sources like binary systems or supernova remnants (SNR), or extragalactic sources like active galactic nuclei (AGN) or gamma ray bursters (GRB). On top of that, questions closely related to particle physics are studied. They include the search for neutrinos from the annihilation of Weakly Interacting Massive Particles (WIMPs), the search for magnetic monopoles, or the investigation of neutrino oscillations, using neutrinos from accelerators, the atmosphere or extraterrestrial neutrinos.

A magnetic monopole with unit magnetic Dirac charge g = 137/2 · e and velocities above the Cherenkov threshold in water (β > 0.75) would emit Cherenkov radiation smoothly along its path, exceeding that of a bare relativistic muon by a factor of 8300. No event with this rather unique signature has been observed by Amanda, yielding a limit of 0.5 · 10—16 cm—2 s—1 sr—1 on the flux of relativistic magnetic monopoles. This value is 20 times smaller than the bound which is derived from the observed stability of galactic magnetic fields and nearly five times below earlier limits obtained by other direct search experiments.

The annihilation of WIMPcandidates like neutralinos (the lightest supersymmetric particle) trapped within the core of the Earth or in the Sun would generate high‐energy neutrinos from the nearly vertical direction or from the Sun, respectively. Present searches by experiments underground and underwater for an excess of muons from the center of the Earth exclude a fraction of supersymmetric models with neutralin masses above 50‐100 GeV. Amanda performed a search for upgoing muons in a narrow cone around the opposite zenith. The number of events detected is consistent with the expectation from neutrinos produced by cosmic‐ray interactions in the atmosphere at the opposite hemisphere of the Earth. This measurements places a limit on the flux of neutrinos from neutralin annihilation. Indeed already the data from only 135 days taken in 1997 yields limits competitive to those derived from underground experiments over many years data collection.

The ultimate goal of the Amanda collaboration is a detector of the scale of one cubic kilometer, named IceCube. This is the order of magnitude required by many models of neutrino production in active galactic nuclei, in gamma ray bursts, in the center of our galaxy or in young supernovae. This scale is als suggested by the pretention to open a really new window – i.e., to increase the sensitivity compared to existing devices by 2‐3 orders of magnitude. The proposed IceCube detector consists of 4800 photomultipliers at 80 strings. The deployment of the detector is scheduled to start in the austral summer 2002/03 and to extend over six years.