Ettore Segreto, professor at the Physics Institute at Unicamp, will lead the consortium that is part of the Dune mega-experiment
the physical Ettore Segreto, professor at the Gleb Wataghin Institute of Physics, at the State University of Campinas (Unicamp), was appointed leader of the Dune Photon Detection System, one of the five international consortia that are part of the mega-experiment Dune – Deep Underground Neutrino Experiment.
Based at Fermilab (Fermi National Accelerator Laboratory), United States, Dune is the most ambitious experiment ever designed to study neutrinos (read more about the project at http://agencia.fapesp.br/25451/).
The international collaboration responsible for the five consortia responsible for taking it forward already brings together 970 researchers, from 164 institutions, from 31 countries. For the success of the venture, the photodetection system (Dune Photon Detection System) is crucial, as it will be through the scintillation produced by the passage of neutrinos through gigantic tanks of liquid argon that researchers hope to obtain fundamental information about the formation of the Universe and the structure of the material world.
With a doctorate in physics from the Università degli Studi dell'Aquila, a traditional Italian public university, founded in 1596, Segreto came to Brazil encouraged by his wife and collaborator, Ana Amélia Bergamini Machado, currently professor at the Center for Natural and Physical Sciences, at the Federal University of ABC (UFABC). It was Machado who convinced him to apply for a teaching position at Unicamp. Approved in the competition, Segreto requested and received financial support from Fapesp, through porgrama Support for Young Researchers in Emerging Centers.
“Fapesp’s support has been fundamental, both for me and for all the other scientists from Brazilian institutions involved in Dune”, said the researcher. Fapesp supports Brazilian participation through the Thematic Project “Challenges for the 21st Century in Neutrino Physics and Astrophysics”, coordinated by Orlando Luis Goulart Peres, and research assistance within the scope of Young Researchers “Liquid argon program at Unicamp", coordinated by Segreto himself. Recently, he also supported the holding of the “DUNE Workshop”, organized by Segreto and Machado.
Trap to detect photons
In a creative flow, while traveling by car along an Italian road, the research couple conceived an ingenious and inexpensive device for detecting photons. Called Arapuca, the equipment is being analyzed by the international consortium and has a great chance of being adopted as one of the main components of the Dune photodetection system. The decision should be announced in two or three months.
The Arapuca is a kind of trap to capture the light. “One of the challenges for Dune's photodetection system is that the argon tanks where the scintillations are expected to occur are very large and the available light sensors are very small. In particular, the silicon sensors that will be used have a collecting surface on the order of just one square centimeter [1 cm2]. The function of Arapuca is to increase the collection area and trap the collected photons inside a box, to make them available to sensors”, said Segreto.
The researcher explained, step by step, how this is done: “The interaction of particles generated by neutrinos with liquid argon from large tanks produces light with a wavelength of 128 nanometers. Using a filter, we modify the wavelength to 350 nanometers. As the Arapuca window is transparent for this wavelength, photons can enter. However, once inside, we use a second filter to return the [wavelength to] 128 nanometers. And the photons can't get out, because the window is opaque for that wavelength. Trapped, they bounce off the highly reflective walls of the box, until they are picked up by sensors placed inside.”
These filters, generically called wavelength shifters (wavelength shifters), are made up of organic materials (polycyclic aromatic hydrocarbons) that absorb photons in one frequency band and re-emit them in another. In this case, para-terphenyl and tetraphenyl butadiene will be used. Arapuca has already been incorporated into the ProtoDune photodetection system, a large-scale prototype of Dune, which is being built and is expected to come into operation at CERN (European Organization for Nuclear Research) in October 2018. Responsible for the photodetection system of the ProtoDune, Segreto and Machado are currently at CERN, where the researcher spoke to the Fapesp Agency.
“ProtoDune’s role is to test all technological and technical solutions that will be used later in Dune”, informed the researcher. “The test will not be carried out with neutrinos, but with a beam of electrically charged particles, produced by one of CERN's accelerators, and aimed at a detector with around a thousand tons of liquid argon. Dune will use, in total, 70 thousand tons of liquid argon, 40 thousand of which will make up the detection tank itself,” he continued.
The objectives, history and modus operandi of Dune were described in detail in a previous report by Fapesp Agency [read here]. In short, Fermilab's accelerator will produce the most powerful beam of neutrinos ever studied. This beam will be detected twice: first, very close to the source, at Fermilab itself, in the state of Illinois; then, 1.300 kilometers from the source, in the state of South Dakota.
The second detector is the giant filled with 70 tons of argon, kept in a liquid state by refrigeration at minus 184 degrees Celsius. What it will record are the showers of particles and light produced when super-energetic neutrinos rip electrons from argon atoms from their orbits. One of Dune's main targets is to compare, through the two detections, the oscillation patterns of neutrinos and antineutrinos (neutrino antiparticles). If these patterns are not strictly symmetric, this will provide researchers with concrete proof of “charge-parity symmetry violation” (CPV).
CPV is a fundamental ingredient of the so-called standard model. And it explains why a universe that, at the beginning, had identical amounts of matter and antimatter turned into a universe in which matter is largely predominant. If the composition had remained strictly symmetrical, matter and antimatter would have annihilated each other. But, according to the model, the symmetry violation generated a small surplus of matter in relation to antimatter. And it was this surplus that resulted in the material universe: galaxies, stars, planets, life, humanity.
In addition to the violation of symmetry, researchers from the international collaboration hope to be able to record, in the gigantic argon tank, another phenomenon, which does not depend on neutrinos: the decay of the proton, predicted by theory, but never observed. If this occurs – and there is great expectation that it will occur – the experiment will have provided empirical proof of the predictive capacity of supersymmetric models that seek to unify three of the four known interactions: electromagnetic, strong nuclear and weak nuclear.
The third target of the experiment is to improve models about the formation of neutron stars and black holes, through the observation of neutrinos from the collapse of supernovae.
US$ 1 billion of investments
The three objectives are highly relevant. And they justify the investment of around US$ 1 billion and the attribution of importance to Dune as great as that of the LHC (Large Hadron Collider). Focusing on different phenomena (neutrinos, in the case of Dune, and hadrons, in the case of the LHC), the two experiments, as well as the supertelescopes currently under construction, are the inaugural milestones of a new era in the human effort to understand the Universe.
Segreto's appointment opens a great door for the participation of scientists from Brazilian institutions in the venture. And he makes a point of emphasizing the role played by Ana Amélia Bergamini Machado in this regard. “She sought out contacts in Brazil and other Latin American countries, attracted researchers, and committed herself to creating a very integrated and dynamic community. We are starting to reap the fruits of this work,” he acknowledged.
As for his personal agenda, the prognosis is for a lot of work in the coming years. “I will now spend a few months at CERN, participating in the construction of ProtoDune. Afterwards, I must return to Brazil, because a large part of the development of the photodetection system will be carried out in Latin America. I will have to travel several times a year to Fermilab, but the development center will be at Unicamp,” he said.
Watch below an interview with Segreto, carried out on the occasion of the “DUNE Workshop” at Fapesp, in which he talks about the objectives of ProtoDune and the photodetection system he proposes.
More about the trap
www.unicamp.br/unicamp/ju/676/arapuca-que-aprisiona-particulas-de-luz
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