Recent discoveries made by the international Double Chooz collaboration regarding neutrino physics have been published by the journal Nature physics, one of the titles of Nature magazine dedicated to the dissemination of research in Physics. Created in 2006, the experiment Double Chooz revealed in an unprecedented way, still in 2011, the value of the quantum mixing angle θ13, one of the properties of the neutrino that allows investigations ranging from the activity of reactors in nuclear power plants to hypotheses regarding the Big Bang. Now the group announces a new way of detecting neutrinos, which expands the possibilities for research in the area. Among the members from several countries who are part of the collaboration are Unicamp researchers Ernesto Kemp, professor at the Gleb Wataghin Institute of Physics (IFGW) and Luis Fernando Gomes Gonzalez, volunteer researcher at the university.
Neutrinos: where do they come from, where do they go?
Neutrinos are one of the so-called elementary particles, one of the elements from which all things are made. "Elementary" means that, according to what science knows to date, it is not possible to divide a particle into smaller units. The neutrino is the second most abundant particle in the universe, surpassed in quantity only by the photon, the particle of electromagnetic radiation, generally associated with light. The neutrino was proposed by Wolfgang Pauli in the 1930s as a solution to the unexpected energy behavior of beta radiation. Its existence was proven in 1956 by Americans Frederick Reines and Clyde Cowan. Until then, the neutrino was already used in calculations and studies, but only as a hypothesis in nuclear physics and elementary particles.
One of its most peculiar characteristics is that the neutrino does not form matter itself, that is, it does not integrate the internal structure of protons, neutrons and electrons, which come together to form atoms, molecules and so on. However, the existence and stability of atomic nuclei is governed by two fundamental forces of nature, which act differently on protons and neutrons: the "strong" force and the "weak" force. The names are due to their range. The strong force has a greater range, the size of the nucleus, while the weak force acts at distances the size of protons and neutrons. Simply put, the strong force tries to hold protons and neutrons together while the weak force tends to transform neutrons into protons and vice versa.
The neutrino is always involved when the weak force comes into play. In other words, in nuclear fusion reactions, such as the one that generates the energy of stars, including the Sun, and in fission reactions, such as those that occur in radioactive elements in nuclear reactors and bombs. Thus, in every nuclear reaction that forms new elements, neutrinos are emitted. It is through this constant participation in the interactions that form matter that the study of neutrinos can reveal important information about the entire origin and dynamics of natural elements. This reality, taken into account in research in the area that describes the fundamental laws that form matter, is described by the so-called "standard model" of elementary particles.
"The standard model describes the physical reality that we observe regarding the constitution of matter, radiation and everything else. However, despite being a model with gigantic success in describing particles and how they interact, it has flaws, mostly due to lack of knowledge than for any other reason. It presents gaps, open points, which we still need to explain. And these open points can be explored using neutrinos", explains Ernesto Kemp, professor at IFGW. According to him, there are phenomena that have not yet been fully explained by the standard model and neutrinos would play a role. Therefore, understanding neutrinos opens up ways to understand these phenomena and the interactions involved, which can update the standard model itself.
Because they arise from interactions that result in modifications in the nuclei, the study of neutrinos is closely related to nuclear reactors, since the basis for their operation is, precisely, the obtaining of energy from nuclear fission. Ernesto Kemp explains that the study of neutrinos is also possible from other artificial sources, such as particle accelerators. However, accelerators require very complex and expensive techniques for the generation of neutrinos to be successful, which makes the use of nuclear reactors simpler and less expensive.
"In each nuclear fission that occurs within a reactor, in the division of an atom into two smaller ones with the release of energy, neutrinos are involved. The radiation emitted, in general, is accompanied by neutrinos. Therefore, since nuclear reactors , or nuclear reactions, are abundant sources of neutrinos, much research on them is carried out with nuclear reactors", comments the professor. It is with this objective that the Double Chooz collaboration was installed at the Chooz Nuclear Power Station, a French city close to the border with Belgium.
In search of θ13
The Double Chooz international collaboration arose from the union of scientists from several countries who proposed to develop research on neutrinos linked to nuclear reactors, each in their own country. Due to the complexity of this research, there was a consensus that a union around a smaller number of projects would be more productive. They then re-organized around three experiments: Double Chooz, in France, which has Brazilian researchers; Daya Bay, in China, and REINDEER, in South Korea. In addition to the researchers from Unicamp, Double Chooz members from the Brazilian Center for Physics Research also participate in Double Chooz (CBPF), Rio de Janeiro, and the State University of Londrina (UEL).
Double Chooz's main goal was the discovery of θ13, one of the three quantum mixing angles of neutrinos. These values are parameters used to describe the oscillation of neutrinos. When they are produced, neutrinos come in three different species and, between their production and detection, an oscillation between these three species can occur. One of Double Chooz's greatest contributions was, in 2011, the unprecedented publication of a non-zero measurement value for θ13, proving its existence. Until then, studies assigned the angle only an upper limit, which meant that it could even be null, equal to zero. The lack of knowledge of a specific value left many possibilities open in describing how interactions between neutrinos occur.
The precise identification of these properties of neutrinos opened up space for even deeper questions, such as explaining and measuring the difference between matter and antimatter in the universe, a gap in the Big Bang theory itself. According to her, during the formation of the universe, particles and antiparticles existed in equal quantities and were supposed to annihilate each other, leaving behind a sea of radiation. The answer to the emergence of the universe as it is observed today would lie in explaining why the amount of matter and antimatter became different, breaking this balance.
"If θ13 were zero, there would be a symmetry in how neutrinos and antineutrinos interact with matter, making observations that would help complete the Big Bang model impossible. The question that remains is: at what moment in the Big Bang did this symmetry, this mirror, occur? broke, and allowed the emergence of matter in different quantities than antimatter? This is what gave rise to this universe that we know, so answering this question, in itself, already has existential merit", analyzes Kemp.
The most recent advance made at Double Chooz, which resulted in publication by Nature Physics, was a new technique for detecting neutrinos from nuclear reactors. Because they have minimal mass and have no electrical charge, neutrinos are particles that are difficult to detect. Therefore, they are observed from the daughter particles produced in their interactions in the detectors, including the neutron. Thus, one of the detection techniques is the use of elements that reveal the neutrons produced by neutrinos. Until then, in reactor neutrino experiments, Hydrogen and Gadolinium were traditionally used. Now international collaboration has demonstrated that it is possible to also use Carbon, an option that had not yet been explored by any reactor experiments previously. This experimental advance helped reduce the experimental error of previous θ13 measurements, improving their accuracy.
Another advance in the research carried out by Double Chooz disclosed in the article is the exact calculation of the fission cross section, a number that indicates the probability of neutrinos being produced in nuclear fissions that occur inside reactors. Until then, the number was a standard reference value, adopted as a relative measure to allow comparisons between experiments in different reactors. Thanks to a favorable geometry, Double Chooz's detectors were installed in such a way that each one receives the same amount of radiation from the reactors, something unique among existing experiments in the world.
Experience applied in Brazil
The research carried out at the reactors at the Chooz Nuclear Power Plant has come to an end and the detector installed at the site is already in the dismantling phase. However, the experiments yielded researchers a large amount of data yet to be processed. This indicates that new discoveries about neutrinos can still be announced by the group.
"I contributed a lot to the hardware part, Unicamp helped a lot in the construction of one of the detector's data acquisition electronics modules. I also worked on organizing the data collection, and we always actively participated in the operation shifts, so remote and right there at the plant in Chooz. At that time we also contributed to data analysis, but the main thing was in the construction and operation of the detector", recalls Professor Ernesto. For him, the participation of Brazilians in Double Chooz was important not only to give visibility to studies on the topic carried out in the country, but also to bring to Brazil the necessary expertise to install a reactor neutrino detector here. In 2018, a reactor neutrino monitor was installed at the Angra dos Reis Nuclear Power Plant, the implementation of the first initiative to carry out experimental neutrino physics within Brazil.
"At the time, the decision was based on a symbiotic strategy, of entering into the Double Chooz collaboration and, as the detector was being built, we would acquire familiarity, delve deeper into data analysis techniques and, in parallel, build the detector in Brazil. It was very healthy, we brought many techniques to build the detector in Angra and our contribution with the Double Chooz electronics is our main data acquisition electronics here in Brazil. It was like having tested there for today to be our official electronics" , account.
While the focus of investigations at Double Chooz is the θ13 angle, at the Angra dos Reis plant the goal is to open up space for broader research. In addition to Unicamp, CBPF and UEL, the Federal Universities of Bahia (UFBA), Federal District of Juiz de Fora (UFJF) and State of Feira de Santana (UEFS). Ernesto Kemp also mentions that this is a milestone for neutrino research in the country, as other institutions can count on the research infrastructure that was installed there. "There is another neutrino experiment, CONNIE, conducted by an international team led by UFRJ, operating in the Angra dos Reis laboratory, but to study neutrino interactions, a more fundamental research than ours, which is more applied in nature" , exemplifies.
He also highlights the possibility of monitoring reactor activities independently of the plant. According to him, in the eyes of international regulatory agencies, it is something positive for the country. "The International Atomic Energy Agency has a series of rules for the operation of nuclear plants, precisely so that there are no undeclared diversions of waste, which can be used for non-peaceful purposes. In Brazil this risk does not exist as much because the only nuclear plant Here it is state-owned, the possibility of undeclared deviations is much less likely, it is something that involves the State. But in the United States, for example, nuclear plants managed by public authorities are rare, the overwhelming majority are private plants", he analyzes the teacher.
