During the lecture Prof Watson will outline why we are interested in such rare and energetic particles, describe some of the problems encountered in building this unique detector in Argentina, at a cost of $50M, and explain the significance of the new results on the energy spectrum, the arrival direction distribution and the mass of the primaries in the context of high-energy astrophysics. No knowledge of cosmic rays will be assumed.
The earth is continuously bombarded by the nuclei of atoms that have acquired high energies somewhere outside our solar system. These particles are called cosmic rays and, although discovered in 1912, we remain unsure about their origin. The major reason for this is that charged particles are strongly deflected by the magnetic fields that thread our galaxy, the system of stars in which the Sun sits, and which they must cross to reach us. Trying to do astronomy under such conditions is impossible unless the energies of the cosmic rays are very high. High-energy particles are very rare but, when the energies are comparable to that of a tennis ball moving at 60 mph, they can cut through the magnetic fields of the galaxy with little deflection. To study them in detail it has been necessary to build a detector that covers an area of 3000 km2, the size of Lancashire. A large international collaboration has achieved this feat, with early results giving strong hints that cosmic ray astronomy can finally be said to have started.