All potential sources produce 's as well as 's. One could detect interactions by observing electromagnetic and hadronic showers of contained events. However, at this stage, we will only concentrate on detection.
High energy muon neutrinos can be detected by observing long-range muons produced in charged current neutrino-nucleon interactions with matter surrounding the detector (see figure ). To reduce the background from direct muons produced in the atmosphere, the neutrino telescope should be located at a depth of several kilometers water equivalent and one should only consider muons with a zenith angle greater than 80, i.e. 10 above the horizon, the value for which the flux of direct atmospheric muons becomes smaller than the flux of atmospheric neutrino induced muons under 3000 meters of water. (The zenith angle of an incoming particle is defned as that of the source it originates from, so that zenith 0 corresponds to a downwards coming ray). At high energy, the outgoing muon is emitted in the same direction as the incident neutrino () allowing to point back to the source of the neutrino emission.
When passing through sea water, the muon emits Cherenkov light which is detected by a three-dimensional matrix of photo-multiplier tubes. The measurement of the arrival time of the Cherenkov light on the photo-multiplier tubes allows the reconstruction of the muon direction. The amount of light allows to estimate the muon energy giving a lower limit on the energy of the parent neutrino.
In order to calculate the flux of muons going through such a detection set-up, we need to know, besides the and fluxes, the neutrino-nucleus interaction cross-section, the attenuation of the neutrino flux in the Earth and the range of the induced muon. Details of the calculation can be found in .
Physical backgrounds to cosmic neutrinos essentially come from neutrinos and muons produced in atmospheric showers resulting from the interaction of primary cosmic rays with the Earth atmosphere.
The flux of atmospheric neutrinos has a 3.7 . However for cosmic sources we expect to be around 2. The greater slope of the atmospheric neutrino spectrum makes the signal/background ratio improve with energy.