International experiment drives forward experimental neutrino physics in Brazil

Brazilian researchers participate in the Double Chooz experiment in France, where neutrino oscillation measurements of unparalleled precision are made in flows produced by a nuclear reactor.

By Fábio de Castro

Agência FAPESP – The international Double Chooz experiment published its first results, including an important discovery related to the phenomenon known as neutrino oscillation, in Physical Review Letters. The discovery was considered an important step toward understanding phenomena that could help explain the asymmetry between matter and antimatter in the universe.

According to the Brazilian scientists that contributed to the article, the Brazilian participation in Double Chooz will allow the country to develop experimental neutrino physics, which is currently considered an important area of study, in Brazil.

With the success of the first measurements, Double Chooz, which began collecting data in 2011, will continue to refine its research. A new article was recently submitted to Physical Review Letters, and this work was accomplished with the participation of a Brazilian team. The researchinvolves scientists from the Brazilian Center for Physics Research (CBPF), the Universidade Federal do ABC (UFABC) and the Universidade Estadual de Campinas (UNICAMP).

One of the coauthors that described the first Double Chooz results was Ernesto Kemp, a professor in the Department of Cosmic Rays and Chronology at UNICAMP. He explained that the purpose of the experiment is to measure neutrino oscillations with unparalleled precision by observing the antineutrinos produced in a nuclear reactor in Chooz, located in the Ardenas region in France.

“The experiment obtained an indication that the antineutrons of the electron disappeared during their propagation between the Chooz nuclear reactor and a detector located one kilometer away. This result allowed us to establish a first measurement of what is called the mixing angle theta13,” Kemp told Agência FAPESP.

Under its Regular Research Support Program, FAPESP funded the Brazilian team’s participation in the “Measurements of neutrinos at nuclear reactors” project coordinated by Pietro Chimenti from UFABC. João dos Anjos coordinated the CBPF team.

Chimenti and Anjos are also co-authors of the article along with scientists from France, Germany, the United States, the United Kingdom, Japan and Russia. Kemp is one of the principal researchers on the Thematic Project titled “Study of cosmic rays of highest energies with the Pierre Auger Observatory,” which is financed by FAPESP and coordinated by Carola Chinellato, who is also a professor at the UNICAMP Institute of Physics.

Kemp stated that a measurement of the mixing angle theta13 is crucial for future experiments that seek to measure the difference between neutrino and antineutrino oscillations, as well as the phenomena that could someday explain the origin of the asymmetry between matter and antimatter in the Universe.

“Our great victory was to prove that the angle theta13 has a value different than zero. There was much speculation on the measurement of this angle, and if the value had been zero—meaning the absence of neutrino mixing—it would have had innumerous implications in physics, especially in cosmology and particle astrophysics. The next stages of the mission will be to refine this value more and more, increasing the precision of the angle measurement,” affirmed Kemp.

The Brazilians developed and built electronics capable of measuring the energy of the cosmic muons that cross the detector. Muons, such as electrons and taus, are particles from the lepton family.

“This technology will make it possible to identify and label highly energetic muons that are candidates to produce neutrons by spallation, one of the most important sources of noise for neutrino events,” asserted Kemp.

The elimination of this noise will reduce the systematic errors in measurements of theta13. The electronics designed at CBPF and the farthest detector modules are being built in cooperation with Brazilian industries, and according to Kemp, will be added to the main detector in 2012 during a maintenance shutdown.

Beyond the standard model

Neutrino physics has undergone many advances over the last decade; the experimental proof that neutrinos can oscillate between their different physical states and the implication that neutrinos have mass is one of the most important results in particle physics today. This result unites strong evidence for the existence of physics beyond what we call standard model physics.

“The experiment had been carried out at the Chooz nuclear plant at the end of the 1990s, seeking out the phenomenon we call neutrino oscillation. However, it was discovered that the instrument wasn’t sensitive enough to measure the value. At the time, it was only possible to establish the maximum value that the angle would have if, in fact, oscillation did exist. What we did in 2011 was prove that this value is different from zero. We are getting closer to a more precise measurement,” said Kemp.

The mixing angle is fundamental to understanding the phenomenon of neutrino oscillation. According to Kemp, the interactions of elementary particles that produce neutrinos can occur in three different “flavors,” which are determined by the leptons related to each neutrino: electrons, muons and taus.

“In a given interaction, neutrinos with these three flavors can be produced. However, when the neutrino is propagating itself, what determines this dislocation from one point in space to another isn’t the flavor, but mass. Every mass is made of a combination of different proportions of the three flavors. This proportion, in turn, is determined by the mixing angle,” he explained.

When an antineutrino is produced in the nuclear reactor and is transported a certain distance, the different mass states propagate at different velocities.

The antineutrinos with the same energy but smaller mass states propagate more quickly and vice-versa. “During propagation, a phenomenon of quantic interference between different masses increases or diminishes the chance of detecting a neutrino of a certain flavor after it has moved a specific distance,” said the researcher.

The experiment has two detectors located at previously chosen distances and measures the flow of neutrinos where the interference presents a minimum and maximum, making it possible to measure the intensity of the effect—in other words, the proportion of the mixing between the different masses.

“This is why we built detectors at different distances. One is built very close to the reactor. There, we know we can measure a previously known flow of neutrons through theoretical calculations. As the distance is small, the interference doesn’t change the flavor of the neutrinos. We built another detector farther away where we know that interference will cause a change in flavor, meaning oscillation,” said Kemp.

By building two identical instruments at different distances, the scientists overcame the problem called “systematic error.” “We built two identical instruments, one to measure an expected flow and another to show the suppression of the antineutrino flow, so that we could observe the oscillation. We will never build a perfect instrument because there are technological limitations. But, with the two similar instruments, we can cancel out the systematic effects that would distort the results,” he said.

Open pathway to new discoveries

The next step for the Double Chooz researchers will be to further refine the mixing angle measurement to arrive at a defined value.

According to Kemp, the main result of the discovery that the angle different from zero is associated with the property called the CP phase. According to Kemp, C is a transformation of the charge of the particles, and P is a change in parity.

“A CP transformation inverts the spatial behavior of the particles, like in a mirror. C changes the electrical charge of the particles involved in the interaction. The CP phase determines how much asymmetry exists in the fundamental interactions that lead to the creation of matter and antimatter,” explained Kemp.

The measurement of the mixing angle theta13 is crucial for future experiments to verify the existence of CP violation in the lepton sector. “Understanding this phenomenon will open the doors to explain why, when fundamental interactions generating leptons happen, asymmetry occurs that causes more matter to appear than antimatter,” he said.

Thus, scientific observations have shown that there is much more matter than antimatter in the universe, but until now, researchers have been unable to explain why. New discoveries may lead to a more effective theory concerning this problem.

“If the value of the angle theta13 were zero, we would never have access to a measure for the CP phase. It would be definitively impossible to someday uncover the mystery of the disproportion between matter and antimatter that we observe in nature,” said Kemp.

“With our results, we can establish criteria and experiments for elaborating measures and have experimental access to the number of CP phase violations. It won’t be easy, but we now know that we aren’t at a scientific dead-end,” he said.

The article “Indication of Reactor v e Disappearance in the Double Chooz Experiment” by Ernesto Kemp and others may be read by subscribers of Physical Review Letters at: http://prl.aps.org/abstract/PRL/v108/i13/e131801.