Brazilian participation in mega-project to study neutrinos is fundamental

Researchers at UNICAMP and other Brazilian universities, as well as 20 companies, are participating in the DUNE experiment. They are responsible for the argon purification and photon detection processes, without which the international mega-project would be impossible.

By José Tadeu Arantes | Agência FAPESP – With significant Brazilian participation, a crucial stage of the Deep Underground Neutrino Experiment (DUNE) has just been completed. This hugely ambitious mega-project seeks to revolutionize our understanding of neutrinos and their role in the universe.

On August 15, after three years of work, excavation of three long caverns more than 1.6 km underground for construction of the Long-Baseline Neutrino Facility (LBNF) was completed at Lead, South Dakota, in the United States. Two of the caverns are huge – over 150 m long and as high as a seven-story building. They will contain two particle detector modules each, while the third will house equipment and utilities to keep the detectors running. The detectors will be filled with 17,000 tons of highly purified liquid argon chilled to minus 184° C. The scintillation light and electrical charges produced in the liquid argon tanks by the passage of the neutrino beam will tell scientists about the changes undergone by these particles after they have traveled through 1,300 km of earth from the beam’s source at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, near Chicago to the LBNF Far Site in South Dakota.

A ribbon-cutting ceremony was held to celebrate completion of the three-year excavation of the caverns. It was led by Fermilab Director Lia Merminga and attended by many US scientists, officials and members of Congress, as well as representatives of partner institutions from around the world, including a delegation from Brazil: Carlos Américo Pacheco, CEO of FAPESP; Antonio José de Almeida Meirelles, known as Tom Zé, Rector of the State University of Campinas (UNICAMP); Maria Luiza Moretti, Vice Rector of UNICAMP; and Pascoal Pagliuso, full professor at the Gleb Wataghin Physics Institute (IFGW-UNICAMP), and principal investigator for the argon purification scientific project and the purification system construction project.

 Hema Ramamoorthi (Fermilab), Maria Luiza Moretti, Tom Zé and
Pascoal Pagliuso attended the ribbon-cutting ceremony to mark completion
of the caverns (photo: Carlos Américo Pacheco/FAPESP).

By investigating the phenomenon known as “neutrino oscillation”, the scientists involved in the DUNE experiment seek answers to at least three crucial questions: Why did matter predominate over anti-matter in the formation of the Universe? How do explosions of supermassive stars create black holes? And could neutrinos be a key component of dark matter, which makes up more than 20% of the Universe?

The neutrino beam produced by Fermilab’s particle accelerator is the most powerful in the world. It will travel 1,300 km underground and pass through two detection systems: one close to the source at Fermilab in Illinois, and another much larger detector at the LBNF in South Dakota. It will not be necessary to construct a tunnel of that length because neutrinos do not interact with electromagnetic forces or the strong nuclear force, and pass unimpeded through any amount of ordinary matter, even in highly compact objects.

They travel in a straight line at near-light speed without stopping to avoid any obstacles in their path. The differences between the measurements taken in the first and second detection systems will serve as a basis for understanding their oscillation and could provide answers to the above questions.

Argon purification and detection of scintillations

The DUNE collaboration comprises more than 1,400 scientists and engineers affiliated with over 200 institutions in 36 countries. Brazil’s participation is important, because it is responsible for the highly sophisticated argon purification system and for X-Arapuca, the ingenious device specially created to detect the scintillation light produced in the detection system’s argon tanks. These contributions are being led by UNICAMP, with robust financial support from FAPESP (projects 21/13757-9 and 19/11557-2) and the National Scientific and Technological Development Fund (FNDCT). More than 20 companies are also participating. 

“The argon filtration and refrigeration project associates extraordinarily advanced scientific research with the production of highly sophisticated instrumentation involving complex technological challenges. The work is being done by physicists from UNICAMP, other institutions, and some 20 Brazilian companies. A hub will be structured in Campinas for access to the experiment, and in future, this will facilitate interaction by the Brazilian and Latin American scientific community with data produced by the project, along similar lines to the ongoing cooperation between CERN [the European Organization for Nuclear Research] and Fermilab in the US. It complements the progress already made on the design and fabrication, also funded by FAPESP, of the X-Arapuca, which is the best photon detector ever built and will be part of the neutrino detection instrumentation that’s being installed in South Dakota,” Pacheco said.

FAPESP is investing around BRL 88.6 million in argon purification, cryogenics and regeneration for the DUNE enterprise. A matching amount has been allocated by the FNDCT, a government fund set up in 1969 to finance innovation and the nation’s scientific and technological development. The fund is managed by FINEP, the Brazilian Innovation Agency (an arm of the Ministry for Science, Technology and Innovation, MCTI).

“The FNDCT financing, which will be delivered in two installments, one in 2024 and the other in 2025, is part of our ‘More Innovation’ program to foster innovation for industrialization on a sustainable basis in Brazil. It’s worth recalling that the entire process relating to the argon used at the LBNF will be conducted by Brazilian institutions and companies,” said Carlos Alberto Aragão de Carvalho Filho, FINEP’s Director of Scientific and Technological Development.

Integration of the process is being managed by IFGW-UNICAMP and two companies, Akaer and Equatorial Sistemas, which have entered into a scientific agreement with Fermilab to do research, development, testing, modeling and prototyping for the purposes of purifying, regenerating, circulating and condensing the argon used in the LBNF’s underground facilities.

Impurities of less than 100 parts per trillion

“It’s very important to note this question of purity. In order for the experiment to work, an enormous amount of argon is needed, as neutrinos interact very little with anything, and the argon has to be extremely pure, meaning impurities of less than 100 parts per trillion. Argon as pure as that isn’t available on the market, and we’ve developed a highly original purification method,” Pagliuso said.

The argon is purified partly while it is a gas and partly while it is in a liquid state. It passes through specially designed pressure vessels with a series of filters resembling trays bearing a sophisticated adsorbent material made up of small alumina-ceramic spheres doped with chemicals. The gas or liquid is forced through the spheres’ microscopic pores, and specific impurities are trapped in the spheres by adsorption.

The project that made development of this system possible won the 2023 National Innovation Prize (PNI) awarded by the National Confederation of Industry (CNI) and the Small Business Support Service (Sebrae). The award was given to Pagliuso as team leader in the category of “researchers working with midsize companies”. 

The impurities are basically water, oxygen and nitrogen. Removing the water is relatively easy, but capturing the oxygen and nitrogen is much harder, so the system developed at UNICAMP was deeply appreciated by the people at Fermilab.

“We not only designed a successful innovative purification system, but also innovated by using commercial materials to capture the nitrogen using a methodology unknown to Fermilab. Moreover, we developed alternative materials for oxygen purification. All this qualified us to take charge of producing the system itself. It also enabled Brazil and UNICAMP to make the third most important contribution to the project, behind only the US Department of Energy [DOE] [which runs Fermilab] and CERN,” Pagliuso said.

It bears repeating that more than 200 institutions worldwide are participating in the DUNE collaboration. Being responsible for the third most important contribution after those of such giants as the DOE and CERN is a significant achievement for the Brazilian scientific community. “Another point worth noting is that we developed a prototype of our argon purification cryostat to demonstrate the efficiency of our methods. It’s called PuLArC [Purification Liquid Argon Cryostat] and it’s installed at UNICAMP’s Physics Institute.

It’s been used in tests conducted by our team or requested by Fermilab. Based on the methods we’ve developed, a second prototype with a capacity for 3,000 liters, the Iceberg, has been used at Fermilab to reproduce the results from PuLArC. PuLArC and Iceberg demonstrate the efficiency of our method for capturing nitrogen in liquid argon,” Pagliuso said. “In September, with joint support from FAPESP and FNDCT, we begin the second stage of the project, which is construction proper.”

Technological returns to Brazilian industry

The project calls for advanced high-vacuum low-temperature technology, which will be supplied by Brazilian companies. They will be certified to produce material based on these parameters for the US market and for CERN. Two patent applications (BR102023024694-0 and BR102023026705-0) were filed in 2023 relating to the innovative methods and filtration mediums for adsorption of oxygen and nitrogen. The work is expected to create 100-150 jobs directly, and possibly more than 500 indirectly, in both the industrial sector and universities.

UNICAMP and Akaer were mobilized at an event organized by FAPESP and Fermilab in 2019, and are playing a strategic role in the process. For Fernando Ferraz, Akaer’s chief operating officer, participating in the project is a far greater challenge than building a ship in a bottle. 

“Once the solutions have been developed on the outside, we’ll have to lower the entire plant, piece by piece, to a depth of 1,600 m below the surface. The elevator is fast but takes more than half an hour to complete the trip. In addition, filling the tanks will take a huge amount of argon, equivalent to three years of global production. The purity of the argon must be 1,000 times higher than the best commercial standard in existence. And it has to be kept cold at a stable temperature of minus 184° C. If this colossal mass starts to warm up or cool further, it will be very difficult to take any kind of action. So everything must be very tightly controlled,” Ferraz said.

The underground facility will be subdivided into blocks, he explained: one for purification, another for condensation, and a third for regeneration. In accordance with parameters defined at UNICAMP and other universities, his company designed the pressure vessels through which liquid argon or argon gas will pass in order to be filtered. “We’re talking about some 100 pressure vessels connected by 14 km-15 km of piping, all of it to be fabricated in Brazil. The components will be packaged, placed in containers and shipped to the US. They will be taken by road to the entrance to the caverns and descend by elevator to the underground facility, where they will be assembled piece by piece. Each container will also function by design as a crane to place the items in the elevator and as a positioning tool for use in the final assembly operation,” he said.

Ferraz compared the DUNE experiment to CERN’s Large Hadron Collider (LHC). Their designs and purposes are very different but the technology involved is equally sophisticated. LHC is at the forefront of research on hadrons (particles held together by the strong nuclear force), and DUNE is spearheading research on leptons (particles not affected by the strong nuclear force but only by the weak force). “It’s important to recall that CERN’s research required a great many technological solutions that have since become part of our day-to-day lives. That will also be the case with the research done at DUNE,” he said.

An example of these possible technological spinoffs is refrigeration of green hydrogen for use as vehicle fuel. “Everyone says Brazil has great potential for green hydrogen production, but this is a highly flammable product in its gaseous form. It will have to be stored and transported. There are several possible solutions. The one that appears most interesting from the energy balance standpoint is cryogenics – cooling the gas to convert it into a much smaller volume of liquid so that it can be stored and transported with optimal efficiency and safety. Trouble is, this involves the extremely low temperature of minus 253° C. We’re acquiring the necessary capabilities via what we’re doing in the LBNF project. So we're very close to this solution. It’s a collateral benefit that should significantly extend the company’s technological base and the scope of its offering,” he said.

X-Arapuca: highly efficient photodetection

Another fundamental contribution by Brazilian science to the DUNE mega-project will be X-Arapuca, the main component of the photodetection system. X-Arapuca (“deadfall trap” in Portuguese) is basically a small box with a window and highly reflective inner walls. Photons can get in but not out. The device is intended to detect photons highly efficiently in large-scale systems such as DUNE’s liquid argon tanks.

The device was designed by husband-and-wife team Ettore Segreto and Ana Amélia Bergamini Machado, both of whom are currently based at UNICAMP. For this feat, they won the 2019 DPF Instrumentation Early Career Award. This annual award is bestowed by the American Physical Society to recognize exceptional contributions to the field of particle physics instrumentation. 

“Neutrinos scarcely interact with matter, and when they do, they can’t be perceived directly because they lack electrical charge. However, their passage through liquid argon produces charged particles that make the argon scintillate. X-Arapuca's job is to detect this scintillation. Light is produced by the atoms of liquid argon with a wavelength of 127 nanometers [nm]. At the entrance to the device, we put a filter made up of organic materials that modifies the wavelength to 350 nm. Because the window in X-Arapuca is transparent to this wavelength, the photons can get in, but once they’re inside the box, a second filter changes the wavelength to 430 nm and the photons can’t get out,” Segreto explained.

X-Arapucas are already up and running. “In the Short-Baseline Neutrino Program, a smaller-scale program to detect neutrinos at Fermilab, 192 X-Arapucas with two windows have been installed and are making their first detections right now. In addition, 160 X-Arapucas with six windows have been delivered to Proto-DUNE, a large prototype installed at CERN in Switzerland. All of them were produced by Brazilian companies and assembled and tested at UNICAMP,” Machado said.

X-Arapuca was chosen as the photon detection device for two giant detectors that will be installed in the caverns at LBNF. Some 300 scientists are involved in the development and fabrication process. Construction will begin this year in Brazil, and installation at the South Dakota site is scheduled for 2026.

Data processing center

These achievements give international prominence to Brazilian science and engineering, and especially to UNICAMP. “Ettore’s and Ana Amélia’s creative ideas about the photodetection process, and later the contributions of Pagliuso’s group to purification and cryogenics, have brought us this far. All these initiatives had support from FAPESP. Now there’s also support from FNDCT, because we’ve completed what we call the ‘phase one’ research and in ‘phase two’ we’ll be coordinating the construction of all these items of equipment to scale. The rigor required is huge: keeping liquid argon free of oxygen and nitrogen, with a contamination level below 100 parts per trillion, when we live in an atmosphere that basically consists of oxygen and nitrogen, is a major challenge. In addition, there's the cryogenics and the need to ship the equipment entirely knocked down and assemble it 1,600 m below the surface,” said UNICAMP Rector Tom Zé.

Fermilab and the DOE have encouraged UNICAMP, in partnership with the National Center for Research on Energy and Materials (CNPEM), to install a data center to process the data from DUNE. “This would mean we’d have here a Brazilian and Latin American center with access to the primary data collected by the experiment, and could make it available to scientists in Brazil and Latin America. In other words, we’d be a long-term regional hub for this experiment. The first step consists of building the structure of the experiment, but the structure and the access it gives us to the experiment conducted by Fermilab will provide a firm foundation for research in theoretical physics. Early-career researchers, postdocs and PhDs will have promising research horizons. Indeed, that’s already happening to Ettore, Ana Amélia and their team,” he added.

FAPESP has supported the entire process from the word go, Tom Zé stressed, not just financially but also by mobilizing potential participants. Once the design and fabrication of Arapuca, later refined as X-Arapuca, were well underway, the possibility of cooperation with Fermilab led to a US visit to the institution by a delegation from FAPESP including CEO Pacheco, Scientific Director Marcio de Castro Silva Filho and Sylvio Canuto, an advisor to the Scientific Director.

“We realized there were more important possibilities,” said Canuto, full professor at the University of São Paulo’s Physics Institute (IF-USP) and a former pro-rector for research at USP with affiliations to several scientific institutions in Brazil and abroad.

He was tasked with presenting a special project to FAPESP’s Board of Trustees to request the institution’s support for the advanced instrumentation required for the liquid argon purification and photodetection processes for LBNF-DUNE. “The presentation highlighted the importance to science of the study of neutrinos, how neutrinos can be detected, the contribution São Paulo could make to this undertaking, and the benefits and prospects that would flow from doing so. It was well received by the board, and this enabled the work to begin,” he recalled.

Why neutrinos

The neutrino’s existence was first postulated in 1930 by Austrian physicist Wolfgang Pauli (1900-1958), one of the leading contributors to quantum theory. 

Pauli was trying to solve the problem of energy balance conservation in neutron beta decay. Physicists now accept that neutrons decay quickly outside the atomic nucleus, with each neutron giving rise to a proton, an electron and an anti-neutrino, but the idea of the neutrino then seemed a mere mathematical artifice and was received with skepticism by the scientific community. A few of Pauli’s colleagues liked it at once, however: one of them was the then young Brazilian physicist Mário Schenberg (1914-1990), who worked with Pauli in Switzerland, heard about the concept of the neutrino in a lecture delivered by Italian-American physicist Enrico Fermi (1901-1954) in São Paulo, and later, in the US, took the particle on board to correct the energy balance proposed by Ukrainian-born George Gamow (1904-1968) for supernovas.

The neutrino’s existence was confirmed in 1956, in an experiment conducted by American physicists Clyde Cowan Jr (1919-1974) and Frederick Reines (1918-1998), who received the 1995 Nobel Prize in Physics in his own name and that of Cowan, who had died in 1974. 

Today the neutrino is one of the main objects of study in physics. It is the most abundant material particle in the Universe and ranks second in abundance among the objects studied by science, after the photon (the particle responsible for electromagnetic interaction, or, in simplified terms, the light particle). Because neutrinos are not affected by the electromagnetic force that acts on charged particles or by the strong nuclear force, they pass through vast masses of ordinary matter and even the most compact bodies without hindrance.

In the so-called Standard Model of Particle Physics, the neutrino belongs to the lepton family. A type of neutrino corresponds to each electrically charged lepton. Thus there are three known types (or “flavors”) of neutrino: the electron neutrino, muon neutrino, and tau neutrino. Transformation of neutrinos from one flavor to another is known as neutrino oscillation, as predicted by Italian physicist Bruno Pontecorvo (1913-1993) and confirmed by Japan’s Super-Kamiokande neutrino detector and Canada’s Sudbury Neutrino Observatory (SNO). 

Neutrino oscillation occurs spontaneously while neutrinos are traveling through space and could provide the key to understanding a phenomenon called charge parity violation. According to the hegemonic model for the formation of the Universe, it was this symmetry violation that produced a small surplus of matter over antimatter shortly after the Big Bang, and this matter-antimatter imbalance explains the Universe as we know it.

The SNO and Super-Kamiokande experiments also confirmed that neutrinos have mass, as Pontecorvo believed. In fact, neutrino oscillation is only possible because they have mass, since only massive particles can oscillate. Although the mass of each neutrino is very small, there are so many neutrinos in the Universe that their total mass is significant. For this reason, neutrinos are currently considered possible components of dark matter, together with other exotic particles.

Understanding lepton charge parity violation and investigating the composition of dark matter are among the main aims of the DUNE mega-project. In addition, the experiment may answer many other questions, including some that relate to the formation of black holes.

The study of charge parity violation at DUNE will include comparing the pattern of neutrino oscillation with the pattern of anti-neutrino oscillation. Anti-neutrinos have right-handed spin (opposite to their linear momentum), whereas neutrinos have left-handed spin. If these patterns are not strictly symmetrical, the experiment will give scientists concrete proof of the violation. On the other hand, there is also a possibility that the neutrino is its own anti-particle, in which case it would belong to a hypothetical class of particles known as “Majorana fermions”, predicted theoretically by Italian physicist Ettore Majorana in 1937.

Curiosities of neutrinos
1. Neutrinos have zero charge and very little mass (at least six orders of magnitude less than electrons).
2. Owing to their tiny mass, they travel at very nearly the speed of light.
3. The three known types interact only with the weak force and gravity.
4. They are the most abundant particles with mass in ordinary matter.
5. They are produced during the beta decay process in various sources, from particle accelerators and nuclear reactors to stars. Human beings also emit neutrinos!
6. Given the low probability of interaction, only one neutrino from the Sun is expected to interact with a person in their entire life, even though the solar neutrino flux at Earth’s surface is about 100 billion per square centimeter per second. 
7. There are currently three known types or flavors of neutrino. Individual neutrinos can oscillate among these as they travel through space.
8. More flavors could exist. In particular, theoretical predictions point to the existence of “sterile neutrinos”, which interact only via gravity.