The Highest Energy Particles in Nature – the Past, the Present and the Future
University of Leeds
Since the earliest days cosmic-ray physicists have been studying the highest-energy particles in Nature. A basic understanding of the development of electromagnetic cascades led to the first targeted searches for air showers and, soon after the discovery of charged and neutral pions, the concept of using the muon content to find the mass of the primary particles was proposed. Progress in the field has relied on the conception and mastery of new techniques, including the development of Monte Carlo simulations, and by the levels of funding available. The challenge of measuring the direction of high-energy cosmic rays was solved through the development of scintillator arrays, while the ability to detect Cherenkov light and fluorescence radiation has aided model-free estimates of the primary energy. The radio technique, demonstrated in 1965 but abandoned 10 years later, is again showing promise. I will describe something of the history of the development of the experimental methods exposing the long lead-times between their conception and successful implementation.
However the challenge of determining the mass of the primary particles requires knowledge of hadronic physics at centre-of-mass energies well-beyond those reached at the LHC, while details of key pion interactions are seriously lacking. It may be that we need to exploit anisotropies and magnetic fields in some smart manner to solve this problem. I will comment on the present state of knowledge of the key parameters to set the stage for the Working Group reports that are an important feature of this meeting. I will also discuss briefly discuss prospects for future developments in space and on the ground.