Brazilian physicists join "space LHC"

A group of researchers in Brazil has joined the collaboration that operates a particle detector installed on the International Space Station.

By Elton Alisson

Agência FAPESP – The Alpha Magnetic Spectrometer (AMS-02), a particle detector that has been in service on the International Space Station for the past four years, is comparable in performance terms to CERN’s Large Hadron Collider (LHC) in Switzerland and can be considered a “space version” of the LHC.

The University of São Paulo’s São Carlos Physics Institute (IFSC-USP) in Brazil recently joined the international collaboration that designed, built and operates the AMS-02 and comprises more than 600 physicists affiliated with 56 research institutions in 16 countries.

The inclusion of IFSC-USP as the first South American institution to participate in the international project was made possible by the project “Indirect dark matter search with the AMS-02 detector”, supported by FAPESP under its Young Investigators Grants program.

The proposal for collaboration between researchers at IFSC-USP and colleagues at France’s Annecy-Le-Vieux Particle Physics Laboratory (LAPP-IN2P3-CNRS) to participate in the AMS-02 project was also one of those selected in the second call for proposals of FAPESP’s São Paulo Researchers in International Collaboration (SPRINT) program.

“Our research group will focus on the search for dark matter by measuring cosmic rays detected by the AMS-02,” said Manuela Vecchi, a professor at IFSC-USP and coordinator of the project.

The AMS-02 is designed to measure the properties of cosmic rays, made up primarily of high-energy particles traveling near the speed of light, such as protons, electrons, positrons (the antiparticles of electrons) and antiprotons (the antiparticles of protons), with the aim of contributing to a better understanding of how the universe was formed.

High-precision measurements of these cosmic rays’ compositions and fluxes could help determine whether there is anything left in our galaxy of the primordial antimatter that must have existed for the Big Bang to occur and the universe to be formed almost 14 billion years ago, for example.

“We know that at the very beginning the universe was probably made up of matter and antimatter in equal proportions, but the part of the universe that has been explored so far appears to consist mainly of matter,” Vecchi told Agência FAPESP.

The antimatter particles detected hitherto, such as positrons and antiprotons, are produced together with matter particles in certain astrophysical processes, according to Vecchi.

A detailed analysis of the compositions and fluxes of cosmic rays using the AMS-02 may enable researchers to identify potential exclusive sources of antimatter, which could be antistars or antigalaxies. “No exclusive source of antimatter has ever been found in the universe,” Vecchi said.

By measuring the compositions and fluxes of antimatter particles like positrons and antiprotons, the scientists will be able to study the presence of dark matter in the galaxy. Dark matter, which neither emits nor absorbs electromagnetic radiation, is thought to make up 25% of the universe, according to Vecchi.

“The measurements already made by the AMS-02 and other detectors brought on stream in recent years suggest there are more positrons in our galaxy than the number expected from conventional astrophysical processes,” she said.

“That means positrons are probably produced not just by conventional astrophysical processes but also by other sources in our galaxy. We don’t yet know what these are.”

One of the hypotheses is that there are zones in space with high densities of dark matter, which collide and mutually annihilate near the solar system.

Theoretical models predict the production of a significant flux of positrons and antiprotons during the process of dark matter annihilation. They also suggest that antimatter particle fluxes can be detected because their energy spectrum differs considerably from that of the particle fluxes from astrophysical sources, Vecchi explained.

“One of the AMS-02 collaboration’s goals is to understand the origin of positrons and antiprotons,” she said. “It’s important to try to ascertain whether these particles are really produced by astrophysical sources or result from the annihilation of dark matter.”

Installation in space

According to Vecchi, design and construction of the AMS-02 began over 15 years ago.

Before being sent into space it was calibrated on CERN’s test beam, used for the LHC as well as for other particle detectors.

It was then sent to the United States, where NASA’s space shuttle Endeavor took it to the International Space Station (ISS) in early 2011.

“The AMS-02 is the first and only particle detector in operation on the ISS,” Vecchi said.

“Most of the projects under way on the ISS have to do with biological research, such as evaluating different life forms under microgravity conditions. The astronauts living there also perform biological experiments on themselves.”

Data are acquired by the AMS-02 approximately 700 times per second and are processed by computers on the ISS and downloaded via NASA satellite to the experiment control center at CERN.

“The experiment control center, which controls the AMS-02 remotely, operates around the clock every day of the year because the detector captures and transfers data to Earth uninterruptedly,” Vecchi said.

The AMS-02 operates in space so that it can measure cosmic rays before the particles interact with Earth’s atmosphere. Scientists study the rays’ compositions in addition to looking for antimatter, given that positive and negative charges can be distinguished, owing to the magnetic field produced by the detector.

“The AMS-02 can identify protons, helium nuclei, electrons, positrons, antiprotons and heavier nuclei in cosmic rays,” she said.

There are terrestrial cosmic ray observatories, such as the Pierre Auger Observatory, the world’s largest facility for observing cosmic rays, installed near the town of Malargüe in Mendoza Province, Argentina, approximately 1,200 km from Buenos Aires (read more about the Pierre Auger Observatory at agencia.fapesp.br/20997).

However, the Auger’s detectors identify secondary particles produced by cosmic rays interacting with the atmosphere at energy levels above 100 billion billion electronvolts, higher than those detected by the AMS-02.

“The scientific objectives of the AMS-02 and Pierre Auger Observatory are different but complementary,” Vecchi said.

“We believe the cosmic rays detected by the AMS-02 are probably of galactic origin. To study higher-energy cosmic rays from sources outside our galaxy, we need larger detectors installed on Earth, like the Auger.”

The Italian-born physicist has been a member of the AMS-02 Collaborations since 2011, when she was a postdoctoral research associate at CERN in Switzerland.