Brazil is participating in the largest neutrino experiment ever

Brazilian researchers may contribute the photodetection system to be used by DUNE, a billion-dollar experiment that will probe the structure of matter and answer nagging questions about the formation of the Universe.

By José Tadeu Arantes  |  Agência FAPESP – Participation by researchers from institutions in Brazil and other South American countries in one of the world’s largest scientific projects was the focus of the DUNE Workshop held at FAPESP’s headquarters in the city of São Paulo, Brazil, on June 1, 2017.

DUNE, short for Deep Underground Neutrino Experiment, is the most ambitious experiment ever undertaken to study neutrinos. Its importance to neutrino research can be considered to be as great as the significance of the Large Hadron Collider (LHC) to research on hadrons and their components. The comparison is more than a rhetorical turn of phrase, since although DUNE is interested in a different class of particles, it is also a billion-dollar initiative to investigate the structure of matter in depth and answer some of the most nagging questions about the formation of the Universe.

DUNE is an international collaboration headed by Fermilab (Fermi National Accelerator Laboratory) in the US and involves 970 scientists affiliated with 164 research institutions in 31 countries. Its cost is estimated at about US$1 billion. Construction of the equipment starts this year, and the experiment is due to go live in 2026.

The workshop held at FAPESP was organized by Ettore Segreto, a professor at the Gleb Wataghin Physics Institute of the University of Campinas (UNICAMP), and Ana Amélia Bergamini Machado, a professor at the Center for Natural & Physical Sciences of the Federal University of ABC (UFABC). The attending researchers from Brazil and other countries included Mark Thomson, Professor of Experimental Particle Physics at Cambridge University and DUNE collaboration co-spokesperson. “DUNE is the next great global project in particle physics. It will do for neutrinos the equivalent of what the LHC did for the Higgs boson,” Thomson told Agência FAPESP.

FAPESP supports Brazil’s participation in DUNE through the Thematic Project “Challenges in the 21st century in neutrino physics and astrophysics”, with Orlando Luis Goulart Peres as the principal investigator, and through the Young Investigator Grants under the “Liquid argon program at UNICAMP”, with Ettore Segreto as the principal investigator.

Brazil is participating not just in the international research team but also in the development and supply of equipment. Segreto and Machado have designed Arapuca, a strong candidate to become DUNE’s photon detector. It is a sort of trap for capturing light, as the name suggests (arapuca, feminine noun meaning “figure-four deadfall trap”). Ingeniously designed and inexpensive, it will be used in the photodetection system for DUNE’s two main precursor experiments: ProtoDUNE, scheduled to begin operation at CERN (the European Organization for Nuclear Research) in October 2018, and the Short-Baseline Neutrino Program (SBN), partially running now at Fermilab and set to be fully installed and operational in 2019. If Arapuca is also to be used in DUNE’s photodetection system, it must win in a competition against two other candidate devices, and it appears highly likely to do so.

Among the many systems required for an experiment as large as DUNE, the photodetection system will play a crucial role, since the scintillation light produced in the detector’s liquid argon tanks will enable the researchers to obtain answers to their questions.

Charge-parity symmetry violation

Postulated by Austrian physicist Wolfgang Pauli (1900-1958) in 1930 to preserve the energy balance in the beta decay of the neutron (outside the atomic nucleus, the neutron decays quickly, giving rise to a proton, an electron and an antineutrino), and initially regarded with a certain skepticism by the scientific community, the neutrino is now one of the principal objects of study in physics. There are at least four highly consistent reasons for this.

First, neutrinos are the most abundant material particle in the Universe. Indeed, among the objects studied by science, they are the second most abundant after photons, the particles responsible for electromagnetic interactions or, put simply, the particles of light.

Second, neutrinos are subject to neither electromagnetic interactions nor the strong nuclear force, so they can pass through huge masses of ordinary matter, and even very compact bodies, without being stopped or diverted.

Third, in the Standard Model of particle physics, neutrinos belong to the lepton family, and for every electrically charged lepton (electron, muon or tau), there is a corresponding kind of neutrino. Hence, there are three types, often called “flavors”: the electron neutrino, the muon neutrino, and the tau neutrino. Neutrinos’ tendency to change from one flavor to another, known as neutrino oscillation and predicted by Italian physicist Bruno Pontecorvo (1913-1993), has been confirmed in experiments performed at the SNO and Super-Kamiokande observatories in Canada and Japan, respectively. This oscillation occurs spontaneously while neutrinos travel through space and may be the key to understanding a phenomenon that particle physicists refer to as “charge-parity violation” (CP violation). According to the hegemonic model of the formation of the Universe, CP violation produced a small surplus of matter over antimatter just after the Big Bang, and this surplus is the Universe we know. 

Fourth, confirming another of Pontecorvo’s predictions, the SNO and Super-Kamiokande experiments demonstrated that neutrinos have mass. Oscillation is possible only because they have mass; zero-mass particles cannot oscillate. Although each neutrino’s mass is tiny, there are so many neutrinos in the Universe that their total mass is highly significant. For this reason, neutrinos are considered “dark matter” candidates, together with other exotic particles. According to the latest estimates, known matter corresponds to only about 4% of the Universe, whereas dark matter accounts for more than 20% and dark energy for more than 70%.

Our planet is regularly traversed by trillions of neutrinos: neutrinos that were produced in the early Universe, neutrinos from extragalactic sources, neutrinos created inside Milky Way stars, neutrinos from the Sun, and neutrinos resulting from collisions between cosmic rays and the Earth’s atmosphere. Neutrinos are also produced on the Earth’s surface through beta decay, a process of radioactive disintegration in which an unstable atomic nucleus changes into another by emitting a beta particle (electron or positron) and an antineutrino or neutrino. Beta decay can be triggered artificially by a particle accelerator, as described below, and occurs as part of the nuclear reactions at a power plant such as the one in Angra dos Reis, for example. However, even though neutrinos are everywhere, they are “probably the most mysterious particles in the Universe”, as Thomson put it in an interview given to Agência FAPESP.

DUNE is designed to elucidate this mystery, at least in part. Scientists also want to analyze the neutrino’s behavior to understand such equally obscure processes as CP violation, among other goals. To this end, the experiment will preferentially use neutrinos produced in Fermilab’s accelerator, comprising the most powerful neutrino beam ever devised.

The process will begin with Fermilab’s Main Injector, which accelerates protons and smashes them into disks of graphite or similar material, where they collide with carbon atoms in the material, producing pions (a pion is a type of meson consisting of a quark and an antiquark). Powerful magnets will confine the positively charged pions in a narrow beam. Thanks to beta decay, the positive pions will decay spontaneously into antimuons (negatively charged leptons similar to positrons but with much less mass) and neutrinos. A concrete and steel barrier will stop the antimuons but not the neutrinos. In short, protons will go in, and neutrinos will come out. This is how the neutrino beam will be formed. 

One of DUNE’s top priorities in connection with studies of CP violation will be a comparison of neutrino and antineutrino oscillation patterns. Antineutrinos are the antiparticles of neutrinos. They differ from neutrinos by the direction of their spin, which is clockwise instead of counter-clockwise when observed along the axis of movement. If these patterns are not rigorously symmetrical, the experiment will give researchers concrete proof of CP violation. However, many scientists believe that the neutrino is its own antiparticle, in which case it would be a concrete example of a hypothetical particle class called Majorana fermions, predicted by the Italian physicist Ettore Majorana in 1937.

Schematic of DUNE, scheduled to go live in 2026. A powerful neutrino beam produced by Fermilab’s particle accelerator in Batavia, Illinois, USA, will travel 1,300 km to a detector in South Dakota holding 70,000 tons of liquid argon and located 1.5 km underground.

70,000 tons of liquid argon

To perform the measurements that will answer these and other questions, DUNE will be equipped with two huge detectors, both in the US, positioned along the beam propagation line. The first will be close to the source at Fermilab in Batavia, Illinois. The second, much larger, detector will be located far underground and 1,300 km from the source at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. It will not be necessary to build a tunnel of this length because neutrinos have no charge and propagate in a straight line without deviation of any kind. They travel at close to the speed of light and will traverse any material on the way to the second detector, so the scientists can be sure that the beam will reach the far detector if they point it precisely in the right direction. The near and far detectors, together with their related equipment, are known as the Long-Baseline Neutrino Facility (LBNF).

“The US will contribute 75% of the funding to build the LBNF, and the rest will come from other countries,” Thomson said. “In the case of DUNE, considering the detectors and other equipment required, the proportions will be reversed, with 25% coming from the US and 75% from other countries.”

The key feature of DUNE’s detection technology will be the use of liquid argon, distinguishing it from detectors using other substances, such as the water used in Japan’s Super-Kamiokande detector. DUNE’s far detector at SURF will basically consist of four giant modules, each holding 17,000 tons of argon kept in a liquid state by refrigeration to minus 184 degrees Celsius. “Many previous experiments with neutrinos used water-filled detectors,” Thomson said. “Liquid argon will enable us to obtain 3D images of the interactions with an unprecedented amount of detail and precision.”

As he explained, the choice of material is based on four virtues of liquid argon: it is inert (recall that argon is part of the noble gases in the periodic table, a group that contains six naturally occurring elements with low chemical reactivity); it is relatively cheap; it can withstand high voltages (necessary for the detection system); and, above all, it is an excellent scintillator.

Located almost 1.5 km underground, the detector will be protected from the cosmic rays that reach Earth and, ideally, will be traversed only by the neutrinos produced at Fermilab, as well as neutrinos from stellar sources. Because neutrinos are not subject to electromagnetic or strong nuclear interactions but instead are influenced by the weak force and gravity only, they cannot be detected directly. The detector will record the showers of particles and light produced when neutrinos interact with argon atoms. Scintillation will occur when the energy bursts caused by the neutrinos eject argon atom electrons from their orbits. 

Proton decay and black hole formation

Thomson highlighted three main research targets: investigating the origins of matter by analyzing the link between neutrino oscillations and CP violation; understanding more about how the four forces, or interactions, in the Universe (the gravitational, electromagnetic, strong nuclear and weak nuclear forces) are unified by detecting signs of proton decay in the liquid argon tanks; and exploring how neutron stars and black holes form by observing neutrino bursts from core-collapse supernovae.

Proton decay does not depend on neutrinos. It is a spontaneous phenomenon that has been predicted by theory but never yet observed. The difficulty of observing it is due to the stability of protons, which have a very long half-life. The half-life is the time a given amount of a material takes to decrease by half as a consequence of decay, during which the material decomposes into other materials with lower energy. In the case of protons, the theory predicts a half-life of greater than 10³³ years, i.e., more than three times the projected life span of the Universe. However, because half-life is a statistical concept, this number refers to the decay of a collection of particles rather than to individual particles. There is no reason to believe that individual protons have not decayed in the past or are not decaying now.

Four huge tanks of liquid argon, containing trillions of protons and with detection systems of unprecedented accuracy, are the ideal setting for an event of this kind to be recorded, if it ever happens. If so, the experiment will furnish empirical proof of supersymmetric models designed to unify the four known interactions – electromagnetism and the strong and weak forces, plus gravity, which has not yet been factored into the models for lack of a quantum theory of gravitation.

As for the third research goal, neutrinos are particularly interesting to anyone who studies the genesis of neutron stars and black holes. Core collapses of supernovae near the center of the Milky Way hurl neutrino pulses with intensities of about 1,000 particles per second toward Earth. Observations of the traces left by these neutrinos in the liquid argon tanks will enable scientists to follow, almost step by step, the transformation of core-collapse supernovae into ultradense bodies such as neutron stars and black holes owing to gravitational compression.

Whatever the goal, precise detection using liquid argon is fundamental. With Arapuca, the photon detector, researchers from Brazilian institutions expect to play an important role in the DUNE international collaboration. They aim to partner with colleagues in other Latin American countries to promote and solidify the region’s capabilities. “In just a year of contacts, our group has linked up with some 60 researchers, including both faculty and students, at UNICAMP, UFABC, São Paulo State University (UNESP), and the Federal Universities of São Carlos (UFSCar), Juiz de Fora (UFJF), Alfenas (UNIFAL), Rio de Janeiro (UFRJ) and Goiás (UFG), as well as Fluminense Federal University (UFF) and the University of Feira de Santana (UEFS),” Machado said. “And we’re collaborating with researchers in Colombia, Peru, Paraguay, Mexico and Argentina, here in Latin America, and with researchers in the US, Italy, France, the UK and the Netherlands.

“Professor Machado and I are responsible for the entire ProtoDUNE photodetection system, which is being built right now at CERN and will be used to test all of the technological and technical solutions to be used later by DUNE. Arapuca is one of the main components of ProtoDUNE,” Segreto said.

“ProtoDUNE will be tested not with neutrinos but with a beam of electrically charged particles produced by one of CERN’s accelerators, directed at a detector filled with about 1,000 tons of liquid argon. It will be the largest detector of its kind until the construction of DUNE’s first module, with a mass of 10,000 tons. Altogether, DUNE will have a mass of 70,000 tons, 40,000 of which will be in the detection tanks proper.”

The other use to which Arapuca is being put is for Fermilab’s SBN, which will have a far less powerful neutrino beam than DUNE and three detectors placed between 100 m and 1,000 m away from the beam exit. In addition to the SBN’s contribution to DUNE via the development of liquid argon detection technology, the program is justified by the knowledge of neutrino physics it will produce. 

What is the neutrino’s mass? Are neutrinos their own antiparticles? Do neutrinos and antineutrinos oscillate differently? Are there other types of neutrinos besides the three known flavors? These are the main questions on SBN’s horizon. A particularly interesting object of investigation is the so-called “sterile neutrino”, whose possible existence would add a fourth flavor.

The sterile neutrino, if it exists, does not participate in the weak force. Its only interaction with known matter is via gravitation, so it is practically impossible to detect it directly. Its presence could be observed through the influence it may have on the neutrino beam’s oscillation patterns. An anomaly noted in previous experiments (LSND and MiniBooNE) suggested this possible presence but did not confirm it. Confirmation is one of the goals set for the SBN.