Astroparticle Colloquia

The present status of our knowledge about the highest-energy particles in Nature

by Prof. Alan Watson (University of Leeds)

Online (GSSI)




After a brief introduction to the subject of cosmic rays I will describe how we detect those particles that are too rare to be observed directly from satellites or by using instruments such as AMS located on the space station. The method is to study the cascades of particles that are produced in the atmosphere, many of which reach ground level, the so-called ‘extensive air-showers’. The existence of cosmic rays with energies above 10^{19} eV has been known for over 60 years but limited data have allowed speculations about their origins to range freely. For example, while it has long been thought that the sources of these particles must lie outside our galaxy, it is only recently, through the work of the Pierre Auger Collaboration, that this hypothesis has been confirmed experimentally with the demonstration of a significant modulation in the arrival directions in right ascension above 8 \times 10^{18} eV, the amplitude of which increases with energy. Our understanding of the birthplaces of these particles has recently been further enhancement through measurements of the energy spectrum of ultra-high energy cosmic rays recently derived from an unrivalled sample of over 215,000 events with energies over 2.5 \times 10^{18} eV.

While I will primarily discuss data taken at the Pierre Auger Observatory, I will also comment on points of agreement and disagreement of these data with those measured using the Telescope Array that is located in the Northern Hemisphere.