2011 Faraday Medal of the Institute of Physics

Professor Alan Andrew Watson

University of Leeds

For his outstanding leadership within the Pierre Auger Observatory, and the insights he has provided to the origin and nature of ultra high energy cosmic rays.

Cosmic rays are high-energy particles impacting the Earth’s atmosphere. Originating from violent astronomical phenomena, they are used both as an astronomer’s tool for exploring the most exotic cosmic sites, and by particle physicists as a laboratory accessing energies far higher than achievable in terrestrial accelerators. Alan Watson has been a pivotal figure in cosmic ray experiments for four decades, and is one of the world’s most distinguished and respected experimental physicists.

Following PhD work on condensation nuclei, he has worked almost exclusively on the extensive air shower technique for the detection of cosmic rays, with particular interest in those of the highest energies.

Famously, the very existence of particles with these energies challenges modern physics, it being difficult to conceive of sufficiently powerful production mechanisms, and once produced, interaction with background photons should prohibit propagation over large distances (the GZK effect). Alan Watson played a major role in the pioneering Haverah Park (UK) experiment leading the science from the front, personally developing the analysis techniques and establishing the energy spectrum that served as a key reference for over 15 years. In 1987 he led a US-UK project at the South Pole searching for gamma-rays from supernova 1987A. In 1995 he helped to discover flared emission from Markarian 421 at ~ 1TeV (Whipple Observatory).

In 1991, together with J W Cronin, he established the largest ever cosmic-ray project, the Pierre Auger Observatory, covering an area 30 times the size of Paris. The ambitious project has delivered, demonstrating an anisotropy in the arrival direction of the highest-energy cosmic rays, possibly indicating active galactic nuclei as the source. This may well be the most significant experimental discovery made to date in particle astrophysics.

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