New infrastructure for astroparticle physics research in South America

In addition to benefiting from an upgrade to the Pierre Auger Observatory, the continent may host the world’s largest gamma-ray observatory

By Elton Alisson

Agência FAPESP – In the next few years, South America’s astroparticle physics research community expects to receive substantial reinforcements to the infrastructure available for experiments in this interdisciplinary area, which studies ultrahigh energy cosmic rays – the most energetic subatomic particles known today and of as yet uncertain origin.

The world’s largest facility for observing cosmic rays, the Pierre Auger Observatory, installed in Mendoza Province, Argentina, and with Brazil’s participation supported by FAPESP and other funding agencies, is being upgraded, with a completion date of 2018.

Meanwhile, the host country for the Cherenkov Telescope Array (CTA) in the Southern Hemisphere will be chosen in 2015. The CTA will be the world’s largest observatory dedicated to the study of celestial bodies that emit gamma rays, the highest-energy form of radiation known in the universe.

In addition, discussions are under way to prepare for construction of the Agua Negra Deep Experiment Site (ANDES), Latin America’s first underground laboratory. The lab will be located off a tunnel to be built under the Andes Mountains between Argentina and Chile and will be used for experiments in several fields, including astroparticle physics (also called particle astrophysics).

“The astroparticle physics community in South America is currently enjoying a very important and exciting phase thanks to the expected materialization of these projects,” said Luiz Vitor de Souza Filho, professor at the University of São Paulo’s São Carlos Institute of Physics (IFSC-USP), in an interview with Agência FAPESP.

To discuss the prospects for the community offered by these infrastructure projects in South America, Souza Filho and a group of 100 researchers from several other countries met in November at the University of São Paulo’s Institute of Physics (IF-USP) for the 3rd Astroparticle Physics Workshop: The future in South America. “The purpose of the event was to bring together the international community of researchers in astroparticle physics in order to begin planning the future of experiments in the area in a more organized manner,” said Souza Filho, one of the organizers of the meeting.

“This is the right time for us to try to create a research and investment plan, taking into account important scientific questions that we can answer through the development of these projects,” he added.

Upgrade program

Among these questions, according to Souza Filho, is the origin of ultrahigh-energy cosmic rays and the type of subatomic particles that reach Earth with macroscopic energies in the range of 10 to the power of 18 electron-volts, i.e., a billion billion eV.

In the past ten years, the Pierre Auger Observatory has enabled cosmic rays above 10 to the power of 20 eV to be observed. The researchers at the meeting concluded that the facility has been highly successful in this regard.

The results obtained by the Auger Collaboration, however, do not yet allow the sources of these ultrahigh energy cosmic rays to be totally identified.

“We’ve measured several properties of cosmic rays, but we haven’t yet managed to locate their source or sources. And we don’t know exactly whether the particles that come to Earth are pure protons, heavier atomic particles or even a mixture of both,” said Souza Filho.

According to researchers in the field, one of the challenges to identifying the source and composition of these particles from space is that they can only be measured indirectly.

When an ultrahigh-energy cosmic particle enters Earth’s atmosphere, it collides with the nucleus of an atom of atmospheric material, producing new particles that in turn collide and interact. The resulting particle cascade, called an extensive air shower, contains a billion particles or more.

The Auger Observatory studies the ultraenergetic cosmic rays that reach Earth by measuring the extensive air showers they produce in the atmosphere.

The upgrade program being implemented at the Auger Observatory is expected to help researchers answer these questions by considerably enhancing the resolution of its particle detectors.

“The upgrade will allow us to measure different types of particles and do so more precisely,” said Souza Filho.

Several proposals to upgrade the Auger Observatory are currently being evaluated internally by a committee of physicists. They all focus on enhancing the resolution of cosmic ray composition measurements.

Each of the various proposals involves a different technique for the identification of muons – ultraenergetic subatomic particles present in air showers – and requires different combinations of new electronic devices, new detectors and internal modifications to the observatory’s 1,600 existing detectors.

Spread over an area of 3,000 km2 on a vast plain overlooked by the Andes, the detectors are polyethylene tanks filled with 12,000 liters of purified water and instrumented with photomultiplier tubes.

When the particles in an air shower pass through the water in the tank, they emit light that can be measured by the photomultipliers.

Antennas mounted on top of each tank transmit the data by radio to the main campus of the observatory in Malargüe, in western Argentina. From there, the data are sent for analysis to some 450 researchers in other parts of the world.

The proposals to upgrade the observatory call for the addition of new devices to detect muons in the showers that are identified. This will require modification of all the existing detectors and will cost an estimated US$15 million.

Prototypes of the proposed muon detection systems are being tested to make sure they really work.

“One of the proposals will be chosen in early 2015. We hope the new detectors can be installed soon and commissioned, at the latest, by 2023,” said Carola Dobrigkeit Chinellato in an interview with Agência FAPESP. Chinellato is a professor at the State University of Campinas’s Gleb Wataghin Institute of Physics (IFGW-Unicamp) and chairs the Auger Publications Committee.

Chinellato also represented FAPESP at a meeting of the Auger Finance Board, where funding agencies from the Collaboration’s 18 member countries reviewed the project’s finances and approved its annual operating budget. The meeting took place on November 15 at FAPESP’s headquarters in São Paulo.

CTA

Also in early 2015, the Southern Hemisphere host country will be chosen for CTA, an international consortium of 28 countries, including Brazil, that plans to build by 2020 the world’s largest astronomical observatory dedicated to the study of astrophysical objects that emit gamma rays.

The observatory will have approximately 100 telescopes that will be installed at two different sites, one in the Southern and the other in the Northern Hemisphere. In the Southern Hemisphere, the candidate countries are Chile and Argentina in South America and Namibia in southern Africa, according to Souza Filho, one of the Brazilian researchers who are participating in the project (for more information, see http://agencia.fapesp.br/16755).

The initial idea, he explained, is to build an array of seven telescopes, which will form an embryo of the observatory called the CTA Mini-Array, and build the rest around it at a later stage.

Three of the first seven telescopes will be built by Brazil as part of a Thematic Project supported by FAPESP.

Testing of the first of these telescopes began in Catania, Italy, at the end of September. “The goal is to start by testing this mini-array, obtain the first scientific data, and move on from there until we have approximately 100 telescopes,” said Souza Filho.

During the workshop at USP, Werner Hofmann, CTA spokesman and a researcher at the Max Planck Institute for Nuclear Physics in Germany, said the CTA is unlikely not to come to South America.

ANDES project

Another astroparticle physics research initiative in South America that is under discussion concerns the construction of the ANDES deep underground lab. The idea being developed by the research community is to build the lab adjoining a 14 km tunnel that Argentina and Chile plan to excavate under the Andes Mountains to facilitate access by the countries of South America to the Pacific Ocean so that exports can be shipped more easily to Asia.

The underground lab project involves the installation of several types of equipment for research in different fields, including a large detector capable of identifying low-energy neutrinos and geoneutrinos. Geoneutrinos are produced by the decay of radioactive elements in Earth’s crust and mantle, such as potassium, uranium and thorium, and are believed to play a very important role in the planet’s heat balance.

The tunnel would be a suitable place to measure these particles, according to researchers in the field.

“The ANDES facility would enable us to perform experiments in different areas that require low levels of radiation, such as measuring dark matter and neutrinos,” said Souza Filho.

So far, only Argentina, Brazil, Mexico and Chile have invested in the project, which is seeking participation by other countries.