A major international event in the Argentine pampas celebrated, from November 14 to 16, 2019, the 20th anniversary of the Pierre Auger Observatory Located in Malargüe, about 100 kilometers (km) from the Andes Mountains and approximately 370 km south of the city of Mendoza, Auger, as it is sometimes called, is the largest cosmic ray observatory in the world, operated by an international collaboration of more than 400 scientists from 17 countries, involving physicists, engineers, technicians and graduate students.

Dozens of Brazilian researchers have actively participated in the research conducted there, from the design of the observatory in the 1990s, through construction, development of detectors, operation and data analysis. In total, FAPESP has already provided 32 grants and scholarships for projects developed at Auger, and provided funds for the manufacture of part of the 1,660 water tanksused in the detection system, for the purchase of batteries for the surface detectors and for the manufacture of correcting lenses for the telescopes.

Several parts of the detectors and other equipment were manufactured by Brazilian industries, such as Alpina Termoplásticos, Rotoplastyc Indústria de Rotomoldados, Equatorial Sistemas, Schwantz Ferramentas Diamantadas and Acumuladores Moura.

A first-time participant on the observatory team is physicist Carola Dobrigkeit Chinellato born in Germany and living in Brazil, where she has worked for more than four decades as a professor at the Physics Institute of the State University of Campinas (Unicamp).

Researcher responsible for the Thematic Project " Studying ultra-high energy cosmic rays with AugerPrime ", initiated in 2010 with support from FAPESP, Dobrigkeit also chairs, since 2013, the Publications Committee of the Pierre Auger Observatory.

"The Pierre Auger was one of the first projects in which FAPESP partnered with foreign agencies to support experiments of great magnitude and impact. FAPESP's initial support for the project dates back to 1996, when Professor Carlos Escobar, from the Institute of Physics at Unicamp, created the opportunity. The participation, under the leadership of Professor Carola, has been great for the developmentscientific and technological development of the State of São Paulo and to open opportunities for international connections and, mainly, for the creation of good science," said Carlos Henrique de Brito Cruz, scientific director of FAPESP.

"Cosmic rays are high-energy particles that are continuously reaching the Earth from outer space. They are mostly made up of protons and other atomic nuclei, but also electrons, neutrinos, etc. The Pierre Auger Observatory is especially interested in studying the particles with the highest energy, which are the most interesting and also the rarest. ByThat's why we built an observatory with such a large area, twice the size of São Paulo. Our detectors are spread over an area of 3,000 km2," Dobrigkeit told FAPESP Agency .

The energy of cosmic rays is distributed in a very wide range, ranging from 109 to 1021 electron-volts. The lower energy ones originate in the Sun, while the very high energy ones come from extragalactic sources. These constitute, in fact, the most energetic particles ever observed by mankind, reaching energy levels millions of times higher than those obtained in a beamat the LHC (Large Hadron Collider), the largest particle accelerator on the planet, located on the French-Swiss border.

"The cosmic rays collide with nuclei present in the atmosphere. New particles result from the interactions, which multiply in a cascade up to the detectors located on the ground. When they reach the ground, the particles already have much lower energies and pass through our bodies without us realizing it," said the researcher.

Extragalactic origin

The study of cosmic rays, especially in this very high energy range, aims to know where these particles come from, by which astrophysical objects they are produced and what physical processes are involved in their production. The highest energy ones certainly come from outside the Milky Way and take millions of years or more to reach the Earth.

"You'd expect the most energetic ones to come from the center of our galaxy, because that's where there's a supermassive black hole and a higher density of objects. But no. They come mostly from a direction that's about 120 degrees from the center of the Milky Way. And that's a strong indication of their extragalactic origin. We have some evidence that these very high-energy cosmic raysThey may have originated in galaxies with active cores, or in brane galaxies, which generate a lot of stars. But these indications are not yet conclusive. We are just improving our detection systems to confirm or not such a hypothesis," said Dobrigkeit.

It is worth remembering that, for being electrically charged particles, the cosmic rays are deviated by magnetic fields during their propagation. So, to know exactly where they come from is not something easy. It requires a real gymnastics of calculations and interpretations.existing magnets along the route," explained the researcher.

Cosmic rays are like messengers from deep space and the remote past. Studying them is a way to look far away and back in the history of the Universe. For this, the Pierre Auger Observatory uses basically two detection systems. One is composed of fluorescence telescopes, which capture, in the ultraviolet range, the radiation emitted by the nitrogen of the air when excited by theparticles produced by the cosmic ray. The more light the telescopes capture, the greater the amount of particles in the cascade, and, the more particles in the cascade, the greater the energy of the progenitor cosmic ray. Thus, by measuring the light, it is possible to estimate the energy of the original particle.

The other detection system is made up of hermetically sealed tanks, each containing 12,000 litres of very pure water. When the particles of the cascade pass through the water, they generate a luminosity that is captured by photomultipliers existing inside the tanks. The reasoning is exactly the same: by measuring the amount of light, we arrive at the energy of the parent particle.

"Those were the two original techniques of the observatory, since its foundation 20 years ago. Over time, the detection system has been improved, with the inclusion of antennas, to capture the shower of particles in the radio range; of underground detectors, to capture muons, which are one of the types of particle generated in the cascade; and, now, of scintillators, which are being installed on top of thewater tanks. All this to improve the measures," said Dobrigkeit.

The researcher was a student, advisor and assistant of the great pioneer in the study of cosmic rays, the physicist César Lattes (1924-2005), who, in 1947, discovered the píon, or meson pi as it was called at the time. Formed by a quark and an antiquark, the píon helps to explain the interactions that hold the atomic nucleus together. Lattes discovered the píon by collecting traces of cosmic rays in plates filled withborax at an altitude of more than 5,000 metres (m), on Mount Chacaltaya in the Bolivian Andes.

"The detection we do now at the Pierre Auger Observatory occurs at a much lower altitude, about 1,400 m above sea level. Lattes' goal was to study the cosmic ray interaction. So, for him, the altitude was an advantage, because the interaction produced fewer particles, which made detection easier. Our goal is to catch the whole particle shower. So, we seekdetect near the maximum development of the propagation cascade," explained Dobrigkeit.

The 20th anniversary event of the Pierre Auger Observatory comprises a scientific meeting, a science fair and an official ceremony, and includes a guided tour of the observatory site with a visit to the detectors. Parallel activities will take place in the small town of Malargüe with which the observatory staff maintains close collaboration.

The agenda includes a symposium covering the following topics: very high energy cosmic rays; cosmic ray sources and their propagation; neutrinos; high energy gamma rays; cosmic ray interactions; multiseminar astronomy; and a review on the current state and perspectives of astroparticle physics. In parallel, the Science Fair will take place, involving students from thelocations.

The complete program of the event can be checked at www.auger.org/index.php/observatory/20th-anniversary .

This text was originally published by Agência FAPESP under the Creative Commons license CC-BY-NC-ND. Read the original here.