Brazilian physics drives progress in international research on neutrinos

International cooperation is helping to increase our knowledge of particles that may account for the asymmetry between matter and antimatter in the universe.

By Diego Freire

Agência FAPESP – Brazil will produce some of the enhancements that are set to make international research on neutrinos advance considerably in the years ahead. The statement was made by Robert Svoboda, Professor of Physics at the University of California, Davis, during FAPESP Week UC Davis in Brazil, hosted by FAPESP and UC Davis in São Paulo on May 12-13, 2015.

Svoboda is one of the spokespersons for the Deep Underground Neutrino Experiment (DUNE), the world’s largest experiment devoted to detecting and studying neutrino interactions.

“Neutrinos are the smallest known particles. It would take ten million neutrinos to make an electron, which means that for every atom there are at least 1 billion neutrinos. In other words, we’re visitors in the neutrino universe, which in itself is a very good reason to try to understand them. Brazil has a major share in the knowledge obtained to date about these particles, and Brazilians are working on important new contributions,” Svoboda told Agência FAPESP.

He was referring to the participation of five Brazilian institutions in the international collaboration responsible for DUNE. Researchers affiliated with the University of Campinas (UNICAMP), the Federal University of ABC (UFABC), the Federal University of Goiás (UFG), the Federal University of Alagoas (UFAL) and the University of Southwest Bahia (UESB) are working to upgrade the sensors for the experiment, located in the United States.

The world’s most powerful instrument for studying the elusive neutrino spans more than 1,200 km between the Fermi National Accelerator Laboratory (Fermilab) in Illinois and the Sanford Underground Research Facility in South Dakota.

The distance between the neutrino detectors installed at either extremity will enable the international physics community to study the changes undergone by neutrinos as they pass through the earth’s mantle.

“We’re currently working on development of the experiment’s photon detection system, using acrylic fiber doped with a chemical compound that shifts the light produced by neutrino interaction to the visible spectrum so that the experiment’s sensors can see the interaction with greater precision,” explained Ernesto Kemp from UNICAMP’s Cosmic Rays & Chronology Department. Kemp was another speaker at FAPESP Week.

“New techniques are also being researched and developed to improve light reflection and collection in the experiment, so that the light travels to the fiber and from there to the light sensors more efficiently, as well as numerical simulations to validate these enhancements and new sensitivity calculations,” Kemp said.

For Kemp, the capabilities of its community of experimental physicists have positioned Brazil as an important collaborator in international research on neutrinos.

“The precision we pursue in measuring neutrino oscillation parameters, which depend on the energy generated and the matter they pass through, among other factors, will enhance our ability to understand how these particles can have been responsible for the predominance of matter in the universe instead of antimatter, for example,” he said.

Matter versus antimatter

Until the 1990s physicists believed neutrinos had no mass. According to Svoboda, the discovery of evidence to the contrary and a better understanding of the neutrino’s behavior may explain why the universe is made up predominantly of matter, whereas antimatter was just as likely to emerge from the Big Bang but is now practically nonexistent.

“When the universe was formed, matter and antimatter existed symmetrically in equal amounts. The increase in the amount of matter that took place after the Big Bang and accounts for things as we know them may have been caused by neutrinos,” Kemp said.

The answer appears to lie in the strange behavior of the neutrino’s mass and its relationship with its antiparticle. Scientists know the neutrino’s mass can change spontaneously and neutrinos can oscillate among the three available species or “flavors”.

“These properties aren’t well understood, but they may have distorted the balance of matter and antimatter produced at the beginning of the universe,” Svoboda said.
 

Cooperation

Besides DUNE, Brazil is participating in other important international experiments designed to investigate the behavior of neutrinos. Svoboda and Kemp are collaborating in Double Chooz, an experiment that aims to measure neutrino oscillation by observing antineutrinos produced in a nuclear reactor at Chooz, France. Brazil’s participation, which was supported by FAPESP under the aegis of the project “Measurements of neutrinos at nuclear reactors”, coordinated by UFABC’s Pietro Chimenti, led to the development of electronic sensors used to measure the energy of cosmic muons as they pass through the detector.

Based on the results of this and other experiments around the world, scientists now know the difference in mass between neutrino flavors but not how much mass each one has. This question remains unanswered, along with many others, but in Svoboda’s view international cooperation has contributed to significant progress.

“These particles went from theory to experimental proof in a very short time,” he said. “The first natural occurrence of neutrinos was observed in the 1960s in a South Africa mine. Neutrino-muon oscillations indicating that they have mass were detected in the 1990s at the Super-Kamiokande neutrino observatory in Japan. We must step up our efforts still further to keep the field advancing and find the answers mankind is seeking to the question of why things are as they are,” he said.