HIGH ENERGY COSMIC RAY EXPERIMENTS

Abstract

The so-called Cherenkov eect refers to the electromagnetic radiation that is emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. These charged particles polarize the molecules of that medium, which then turn back rapidly to their ground state, emitting radiation in the process. Cherenkov radiation is commonly used in experimental particle physics for particle identication, and in nuclear reactors to detect high-energy charged particles (i.e., to measure the intensity of the reaction), and also to characterize the remaining radioactivity of spent fuel rods.

The Cherenkov eect is also used in many dierent astrophysics experiments. When a high-energy photon or cosmic ray interacts with the Earth's atmosphere, it produces an extensive air shower of secondary particles with enormous velocities. The Cherenkov radiation (in air) from the secondary charged particles can be used to determine the incoming direction and energy of the primary cosmic ray or gamma ray. This is used by the Imaging Atmospheric Cherenkov Telescopes, such as VERITAS, HESS, MAGIC, and the future Cherenkov Telescope Array (CTA), to study very high energy gamma rays. Similar methods are used (in water and in ice) by very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory, and IceCube. Other experiments, like the Pierre Auger and the High Altitude Water Cherenkov (HAWC) Observatories, use tanks lled with puried water to observe the Cherenkov radiation caused in water by the secondary muons, electrons, and positrons of the extensive air showers.

In this lecture I will discuss the main characteristics of the Cherenkov radiation, and its use in particle physics, nuclear reactors, and astrophysics experiments. In particular, I will concentrate on the Pierre Auger and HAWC Observatories, CTA, and IceCube.

Figure 1. Prof. Mostafa and one of his babies, a.k.a. a full-size, fully- instrumented HAWC water Cherenkov detector. This prototype, lo- cated at Colorado State University, is the only detector outside the HAWC array in Mexico.

Short Bio

Miguel Mostafa is an Associate Professor of Physics at Colorado State University. After obtaining his Ph.D. in high energy particle physics (measuring W bosons with the D detector at Fermilab) from Instituto Balseiro in Argentina, he was a fellow of the Istituto Nazionale di Fisica Nucleare in Italy, and a postdoctoral research associate at the University of New Mexico. He was also Assistant Professor at the University of Utah before joining the CSU faculty in 2008. He has been working on ultra-high energy cosmic rays as a member of the Pierre Auger Collaboration for more than 10 years. In 2009 he also joined the High Altitude Water Cherenkov Col- laboration. His research interests are in high energy particle astrophysics, including the origin of cosmic rays, acceleration mechanisms, particle physics at energies above terrestrial accelerators, gamma-ray astronomy and the structure of the Galaxy, and the nature and properties of dark matter.