e-Science : April 2012 Issue 1
The arrival durections of Muon and neutrino events in IceCube. What does IceCube see? So far, IceCube has seen many tens of thou- sands of neutrinos – but none from a source in the distant Universe. The neutrinos that we do see are produced at the Earth, in the atmo- sphere, when high-energy cosmic rays crash into air molecules. In these collisions, the cosmic ray protons and nuclei are destroyed and large showers of other particles are pro- duced. In these showers, some of the particles have very short lifetimes and when they de- cay, neutrinos are made. These are the “atmo- spheric neutrinos” that IceCube sees for now. Our hope is that if we let IceCube run for many more years, that we will eventually see a hot spot in the sky – many events coming from one direction, and in statistical terms – too many to be explained as a random chance fluctuation of the atmospheric neutrino back- ground. Even though IceCube doesn’t see neutrinos from distant sources, we can still make useful contributions to scientific research by testing to see if predictions of what might be seen are actually seen. Simply, if a scientist is study- ing the possible properties of a distant object, and predicts that neutrinos should be seen, then not seeing them tells us something about the prediction – it must be wrong! This has already had some important consequences for our understanding of the Universe. As we’ve discussed, one of the mysteries is figuring out where the highest energy cosmic rays are pro- duced. We see these particles at the Earth, but can’t tell where they come from. One of the leading ideas on where they do come from is that they are made in gamma-ray bursts – huge explosions off in deep space. To put it simply – if gamma-ray bursts are accelerating cosmic rays in these explosions, then there should be lots of neutrinos, some of which IceCube should see. Importantly, IceCube has looked at the specific directions in the sky and closely around the exact times when these bursts are observed, and didn’t see the neu- trinos. This likely means that the cosmic rays are not being accelerated in these bursts, or, if they really are, then something about the way this happens is very different to what we currently believe. Either way, there is a lot to learn about these objects, and all the other bizarre high energy objects in the Universe!
July 2012 Issue 2