Auger and IceCube: Cosmic Ray and Neutrino Probes of the High Energy Universe

Nearly a century after they were first discovered, the origin of cosmic rays, the sub-atomic particles that constantly rain down on us from outer space, remains a profound mystery. Their energies extend far beyond what can be achieved even in the giant particle accelerators at the international laboratory CERN in Geneva. The most energetic cosmic ray recorded has an energy equivalent to that of a Pete Sampras serve -but carried on a particle ten million million times smaller than a tennis ball. We are fortunate that such particles cannot hit us directly! The Earth's atmosphere acts as a giant blanket which absorbs them. In the process the incoming particle generates a huge 'air shower' - a disc of plasma travelling at nearly the speed of light - which can spread to several tens of kilometres by the time it hits the Earth's surface. By this time its force is spent but sensitive electronic detectors can pick up the signals and tell us how energetic the 'primary' particle was and which direction in the sky it came from. Such events are however rather rare, on average only one per square kilometre, per century. How does nature create the conditions to accelerate a tiny particle to such an energy? Are they coming from the giant black holes that lurk at the centres of galaxies, gobbling up stars and as and spewing out huge jets of plasma, or are they from gamma-ray bursts, the biggest explosions in the Universe, which release an energy equivalent to the mass of an entire star (E = mc^2!) within a few seconds to minutes. Perhaps these energetic cosmic rays are in fact created when massive particles left over from the Big Bang, which make up the dark matter of the Universe, undergo a sort of slow radioactive decay. Tracking these ultrahigh-energy particles back to their sources will enable us to answer such questions. To this end, two hundred scientists from 15 countries around the world are building the $55 million 'Pierre Auger Observatory' in the shadow of the Andes mountains in Malargue, Argentina. When completed in 2005, this array of electronic detectors will cover an area the size of Lancashire, allowing scientists to catch many of these events. As Nobel Prize winning physicists Jim Cronin has noted: 'The existence of these high energy rays is a puzzle, the solution of which will be the discovery of new fundamental physics or astrophysics'. In other words, we are sure to learn something profound and interesting about the Universe by undertaking this challenging task. Another, complementary, way to investigate the cosmic engines that accelerate sub-atomic particles to such stupendous energies is to study ghostly particles called neutrinos which are created in the process. Unlike other forms of radiation these neutrinos interact very weakly with matter and usually travel right through the Earth without even noticing it is there. Even so scientists have learnt how to catch them - by building giant underground caverns filled with water, watched by sensitive electronic eyes. Nearly all the neutrinos hitting the Earth, for example from the Sun where they are created by nuclear fusion in its core, pass straight through the detector, but one in a trillion may interact thus emitting a faint flash of light which can be detected. For the very high energy neutrinos expected from cosmic accelerators it is necessary to scale up the size of the detector considerably. In fact scientists are planning to use a kilometre size block of ice deep under the South Polar cap to look for these neutrinos. This $272 million project, called 'IceCube', will sink 'strings' carrying detectors (using a five MW jet of hot water!) into the crystal clear ice compressed to enormous pressures over two kilometres underground. This will allow us to detect neutrinos coming form very far away and peer straight into the heart of the most violent and awesome phenomena in the heavens. We will open a new 'window' on the Universe and new