Ice Cube Neutrino Observatory - Experimental Mechanism

Experimental Mechanism

Neutrinos are electrically neutral leptons, and interact very rarely with matter. When they do react with the molecules of water in the ice, they can create charged leptons (electrons, muons, or taus). These charged leptons can, if they are energetic enough, emit Cherenkov radiation. This happens when the charged particle travels through the ice faster than the speed of light in the ice, similar to the bow shock of a boat traveling faster than the waves it crosses. This light can then be detected by photomultiplier tubes within the digital optical modules making up IceCube.

The signals from the PMTs are digitized and then sent to the surface of the glacier on a cable. These signals are collected in a surface counting house, and some of them are sent north via satellite for further analysis. More of the data is kept on tape and sent north once a year via ship. Once the data reach experimenters, they can reconstruct kinematical parameters of the incoming neutrino. High-energy neutrinos may leave a large signal in the detector, pointing back to their origin. Clusters of such neutrino directions indicate point sources of neutrinos.

Each of the above steps requires a certain minimum energy, and thus IceCube is sensitive mostly to high energy neutrinos, in the range of 1011 to about 1021 eV. Estimates predict a neutrino event about every 20 minutes in the fully constructed IceCube detector.

IceCube is more sensitive to muons than other charged leptons, because they are the most penetrating and thus have the longest tracks in the detector. Thus, of the neutrino flavors, IceCube is most sensitive to muon neutrinos. Electron neutrinos typically scatter several times before losing enough energy to fall below the Cherenkov threshold; this means that they cannot typically be used to point back to sources, but they are more likely to be fully contained in the detector, and thus they can be useful for energy studies. These events are more spherical, or "cascade"-like, than "track"-like; muon neutrinos are more track-like.

Taus can also create cascade events; but are short-lived and cannot travel very far before decaying, and are thus usually indistinguishable from electron cascades. A tau could be distinguished from an electron with a "double bang" event, where a cascade is seen both at the tau creation and decay. This is only possible with very high energy taus. Hypothetically, to resolve a tau track, the tau would need to travel at least from one DOM to an adjacent DOM (17 m) before decaying. As the average lifetime of a tau is 2.9×10−13 s, a tau traveling at near the speed of light would require 20 TeV of energy for every meter traveled. Realistically, an experimenter would need more space than just one DOM to the next to distinguish two cascades, so double bang searches are centered at PeV scale energies. Such searches are under way but have not so far isolated a double bang event from background events.

However, there is a large background of muons created not by neutrinos from astrophysical sources but by cosmic rays impacting the atmosphere above the detector. There are about 106 times more cosmic ray muons than neutrino-induced muons observed in IceCube. Most of these can be rejected using the fact that they are traveling downwards. Most of the remaining (up-going) events are from neutrinos, but most of these neutrinos are from cosmic rays hitting the far side of the Earth; some unknown fraction may come from astronomical sources, and these neutrinos are the key to IceCube point source searches. Estimates predict the detection of about 75 upgoing neutrinos per day in the fully constructed IceCube detector. The arrival directions of these astrophysical neutrinos are the points with which the IceCube telescope maps the sky. To distinguish these two types of neutrinos statistically, the direction and energy of the incoming neutrino is estimated from its collision by-products. Unexpected excesses in energy or excesses from a given spatial direction indicate an extraterrestrial source.