Shower of Particles

Pierre Auger Observatory captures high-energy cosmic rays

Located in the desert-like Malargüe, a town of 20 thousand inhabitants close to the Argentinean Andes, the Pierre Auger Observatory, the largest international project conceived to capture high-energy cosmic rays, a rare kind of subatomic particle that travels almost at the speed of light before falling onto the Earth, has just sent the first news. Encouraging, as well. In spite of being at work a little less than half its total capacity, the enterprise recorded, between January 2004 and July this year, 3,525 events in which the cosmic rays had colossal energies. On 20 occasions, the particles showed energy levels close to or higher than the so-called GZK cut-off, of 5 x 1,019 electron-volts (eV). The GZK should mark the maximum limit of energy that the rays could show when arriving at our planet. Theoretically, it would be impossible to catch particles with energy above this ceiling. Theoretically. But the Auger, in which Brazil is one of the partners, and other lesser experiments have already measured events with rays more energized than the GZK cut-off. This does not mean that the limit does not make sense any more. Far from it. Not least because these occasions have been so infrequent that they have not generated data with statistical relevance for formulating definitive judgments.

The information supplied by the Auger was presented in July to the community of physicists, at a congress on cosmic rays in India. “Two years from now, when the observatory is completely ready, we are going to have a quantity of data seven times greater than today”, comments Vitor de Souza, of the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of São Paulo (IAG-USP), one of the Brazilian researchers who are taking part in the international project. “Our margin of error will be far smaller, and we shall be able to answer questions that relate to the nature of these particles.?

One of the queries that most intrigues physicists is the polemic about the existence (or not) of particular sources of cosmic rays, sidereal objects, like a black hole, which could emanate enormous quantities of energized particles. When there is a systematic record in a given region of the sky of rays with much more energy than in other parts of the firmament, this measure may indicate that, in that direction of the Cosmos, millions of light-years away, there is a celestial body emitting the particles. For the time being, the Auger has not yet detected any particular source. Apparently, the cosmic rays captured by the experiment in Malargüe come from all the directions of the Universe, and not from specific points. This perception, though, may be an illusion. The original direction of the particles may have been altered in the course of the journey en route to the Earth.

A US$ 47 million project, which involves institutions from 18 countries, the observatory in the Argentinean Andes is the first one to join two methods for observing high-energy cosmic rays: surface detectors, called technically Cerenkov tanks, and fluorescence detectors, a special kind of telescope. As soon as it is concluded, the enterprise, in which Brazil invested about R$ 2.5 million, through FAPESP and the Ministry of Science and Technology, is going to have 1,600 tanks, spread over an area of 3 thousand square kilometers, and 24 telescopes, housed in four buildings. Today, 700 surface detectors and 118 fluorescence detectors are working. Everything to try to understand the nature of cosmic rays, a phenomenon whose comprehension may broaden knowledge about the constitution of matter and the formation of the Universe. “No future experiment with these particles can be smaller than Auger or equal to it”, comments physicist Carlos Escobar, from the State University of Campinas (Unicamp), the supervisor of the Brazilian team in the enterprise. “To be justified, it will have to be bigger.”