Sources of Neutrinos

Neutrinos actually aren't incredibly exotic particles. As a matter of fact there are 100,000 billion passing through your body every second. Statistically speaking, your body will stop only one of the many neutrinos which pass through it during a lifetime. So neutrinos are produced all the time. One of the ways is in nuclear reactors. When particles decay they produce neutrinos, and in a nuclear reactor the flux is 10^13 neutrinos per square centimeter per second. They can also be produced in the Sun, getting solar neutrinos. This isn't surprising since the Sun is a big nucear reactor. They can also be produced by primary cosmic rays interacting in the atmosphere and causing a cascade, in which neutrinos along with other particles are produced.

But what IceCube is interested in are the neutrinos produced outside of the solar system, even outside of the Galaxy! But how are they formed? As mentioned, neutrinos were first suggested as a solution to the missing energy/missing mass problems of beta decay. Cosmic neutrinos are formed in a similar manner. Hadrons such as a protons, are accelerated to incredible energies and incredible speeds. These protons will then decay into particles known as pions. These pions can either be neutral or charged particles. The neutral pions will decay into two photons, which is useful for optical and radio astronomers. The charged pions will decay into a muon and a muon neutrino. The muon in turn will decay into a muon neutrino, an electron neutrino and an electron. This gives us a total of 1 electron neutrino and two muon neutrinos. So where does the tau neutrino come from? Neutrino oscillations. Neutrinos are know to spontaniously change from one flavor to another. Given enough time and space to get to Earth, the neutrino proportions even themselves out, and we recieve electron, muon and tau neutrinos in a ratio 1:1:1.

The neutrinos interesting to IceCube have to be accelerated to much greater energy than any accelerator on Earth can do. Even more so than what our own Sun can do. So what is so violent out there? There are a number of possible candidates. Some of them are active galactic nuclei, binary stars, accreting black holes, gamma ray bursts, supernovae, micro-quasars, galaxy clusters and blazars. WIMPS can also produce neutrinos when they are trapped in the core of celestial bodies such as the sun or the Earth and are annihilated. With neutrino astronomy so new, it's hard to know exactly where these sources are coming from. Most of the other types of astronomy know enough to get a fairly complete picture of the galaxy. See some of them here. Neutrino astronomy on the other hand, relies on information that comes very slowly, with one a handful of events a day. With that kind of stream, it is very hard to get an accurate picture of the galaxy... yet.