The Pierre Auger Observatory is a "hybrid detector," employing two independent methods to detect and study the cosmic rays with the highest energies. One technique detects the high-energy particles of the cosmic ray showers using water tanks spread over a surface of 6000 km2. The other technique tracks the development of air showers by observing with fluorescence telescopes the ultraviolet light emitted high in the Earth"s atmosphere by the shower. This Observatory is providing the most accurate data ever reached of cosmic rays with energies up to 1020 eV.
The High Energy Physics group joined the Pierre Auger collaboration in 2004. Fernando Arqueros has been the representative of UCM in the collaboration board of the Auger Observatory. The group has had relevant contributions in the determination of the atmospheric fluorescence yield that is a key parameter for the energy calibration of the Observatory. In addition it has participated in data analysis, in particular by developing a novel technique for the determination of the mass composition of cosmic rays at the highest energies.