Abstract
The detection of ultra-high energy (UHE, in excess of ~10^20 eV) cosmic rays is one of the most challenging problem of modern physics due to their very low flux. Some of the novel detection methods exploit radio emission by cosmic-ray induced cascades. We consider two mechanisms: 1) Askaryan mechanism, which involves Cherenkov radiation by a charge excess in a cascade, and 2) incoherent bremsstrahlung emission by electrons in the cascade on the neutral air molecules. The first of these mechanisms uses the FORTE (Fast On-orbit Recording of Transient Events) satellite. This satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We look in FORTE database for an RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in Greenland ice sheet. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the fluxes of UHE cosmic ray neutrinos and hypothetical supersymmetric particles neutralinos and compare them to existing theoretical models. To investigate the second mechanism, a group of UH researchers performed an experiment at the Argonne Wakefield Accelerator in the summer of 2003. We measured radio emission from an electromagnetic cascade in an isolated chamber filled with air at C, Ka and Ku bands. The preliminary analysis uncovered a strong coherent and an insignificant incoherent component. The results are inconclusive with respect to incoherent bremsstrahlung emission and the mechanism of the coherent emission. Complete understanding may involve more experiments, but is very important since this mechanism gives a possibility of a viable radio detector addition to large area cosmic ray detectors such as AUGER