An experiment designed to measure neutrino oscillation has produced knowledge that is of crucial importance to an understanding of the phenomenon that led to the emergence of the material universe. Referred to by particle physicists as charge-parity symmetry violation (a difference between the behaviors of particles and their antiparticles), the phenomenon produced a small surplus of matter over antimatter shortly after the Big Bang. Without this matter-antimatter imbalance, the known universe could not exist.
The experiment, called Double Chooz, is still under way in France as an international collaboration in which Brazil is participating. Pietro Chimenti, an Italian physicist who lives in Brazil, contributed to the collaboration with the project “Bayesian analysis of Theta_13 in the Double-Chooz experiment”, supported by FAPESP.
“Some 40 million euros have been invested in Double Chooz,” Chimenti told Agência FAPESP. “If you spend that amount to obtain a measurement, you’ll want to be sure the measurement is performed with great care, which means double- and triple-checking all the calculations using different methods in order to rule out any possible sources of error. My analysis using the Bayesian method confirmed the data obtained through more conventional techniques. That’s highly positive.”
There are three types, or “flavors,” of neutrino: the electron neutrino, the muon neutrino and the tau neutrino. Scientists have discovered that neutrinos can switch flavor through a process called neutrino oscillation. “This is a probabilistic phenomenon that occurs during the propagation of neutrinos through space,” Chimenti explained.
The Double Chooz experiment measures neutrino oscillation by comparing the neutrino fluxes and spectra in two identical detectors at different distances (400 m and 1,050 m) from the reactor cores of the Chooz nuclear power plant in the Ardennes, near the French border with Belgium.
The difference in the quantities of neutrinos detected is used to calculate the oscillation, that is, the transformation of one flavor of neutrino into another. Above all, Double Chooz aims to achieve an even more precise measurement of theta one-three (θ13), one of the mixing angles that describe this oscillation.
Precise measurement of θ13 was chosen as the goal for the experiment because of the vital information it provides about the intrinsic nature of neutrinos and because of its connection with charge-parity violation, the phenomenon believed to have produced the surplus of matter that constitutes the universe.
“If θ13 were zero, it would be impossible to measure CP violation in oscillations. Double Chooz has shown that its value is not zero, so future experiments will be able to measure CP violation. These next-generation experiments are necessary because the asymmetry could be null even with a non-zero θ13,” Chimenti said. His confirmation of conventional measurements via the Bayesian method was applauded by his peers.
“Bayesian analysis has rarely been used as a statistical method in high-energy physics because it requires a computational capacity that barely existed until 20 years ago. Now, very powerful computers operating at low cost have enabled more frequent use of the technique. The results I achieved are perfectly compatible with those already obtained by the Double Chooz collaboration using different techniques. We said the same things in different words, as it were,” Chimenti said.
In speaking of next-generation experiments, Chimenti refers specifically to the Deep Underground Neutrino Experiment (DUNE), a billion-dollar international mega-science project designed to discover new properties of neutrinos.
DUNE’s first stage is scheduled to go live in 2018, followed by stage two in 2021. The project calls for the construction of an underground source emitting the world’s most intense neutrino beam at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, in the United States.
The neutrinos created by the underground beamline will be intercepted by two detectors: one 600 m underground at Fermilab, and a larger detector 1.47 km below the surface at the Sanford Underground Research Facility in Lead, South Dakota, 1,300 km away from their source.
Brazil is also participating in DUNE, with contributions from researchers at the University of Campinas (UNICAMP), UFABC, the Brazilian Physics Research Center (CBPF), the Federal University of Goiás (UFG), the Federal University of Alfenas (UNIFAL) at Poços de Caldas, and the University of Feira de Santana (UEFS). FAPESP is providing support through the Thematic Project “Challenges in the XXI century in Physics and Astrophysics of neutrinos”, with Orlando Luís Goulart Peres as principal investigator, and through Young Researcher Grants for the “Liquid Argon Program at UNICAMP”, led by Ettore Segreto.
Neutrinos are the second most abundant particles in the universe, behind only photons. Because they have no electric charge and do not participate in electromagnetic interactions or the strong nuclear interaction, they can pass through ordinary matter and even compact bodies without hindrance and without being noticed by human beings. These properties give them a unique role in physics. Until the end of the 1990s, they were believed to have no mass, but experiments performed at Japan’s Super-Kamiokande and Canada’s Sudbury Neutrino Observatory (SNO) showed they do indeed have mass, albeit very small. This discovery led to the awarding of the 2015 Nobel Prize in Physics to Japanese physicist Takaaki Kajita and Canadian astrophysicist Arthur McDonald (read more on the subject in Portuguese at agencia.fapesp.br/22019).
In the Standard Model of particle physics, the neutrino is part of the lepton family. One type of neutrino corresponds to each electrically charged lepton (electron, muon and tau). The experiments performed at Super-Kamiokande and SNO demonstrated the neutrino’s strange ability to transform among the three flavors. This is possible only if neutrinos have mass.
This proof that neutrinos have mass and the awarding of the Nobel Prize to Kajita and McDonald have made the study of neutrinos one of the most promising fields of physics today.
Our planet is constantly being bombarded by trillions of neutrinos: neutrinos that were produced during the first instants of the universe, neutrinos from extragalactic sources, neutrinos created inside the Milky Way’s billions of stars, neutrinos originating in our Sun, and neutrinos resulting from collisions between cosmic rays and the Earth’s atmosphere. In addition to these, there are also the neutrinos produced on Earth’s surface by beta decay, a radioactive process that is frequently used in nuclear power plants – these are the neutrinos being measured by the Double Chooz experiment.
In the beta decay process, an unstable nucleus decays into a nucleus of a different element by emitting a beta particle (an electron or a positron). In beta-minus decay, a neutron is converted into a proton while emitting an electron and an electron antineutrino. In beta-plus decay, a proton is converted into a neutron while releasing a positron and an electron neutrino. In addition to these two kinds of decay, analogous atomic reactions can also occur in the form of electron capture. In this case, an electron in the atom’s inner shell is drawn into the nucleus, where it combines with a proton to form a neutron and an electron neutrino. The neutrino is ejected from the nucleus.
“The phenomenon is significant at Chooz because of its powerful nuclear reactors,” Chimenti said. “The Double Chooz experiment was designed to measure conversions of electron neutrinos into other neutrinos as they travel away from their source. The experiment is set to continue for one more year. It has already provided very important measurements of the θ13 mixing angle, and this raises great expectations for research on the matter-antimatter asymmetry problem. CP violation could explain why we observe matter and not antimatter in the universe.”