Detection of neutrinos and dark matter are the focus of an international event

Researchers and students from several countries are participating in an intensive course organized in Brazil by the International Center for Theoretical Physics - South American Institute for Fundamental Research.

By José Tadeu Arantes  |  Agência FAPESP – According to the latest estimates, known matter accounts for only some 4% of the content of the Universe, whereas “dark matter” corresponds to more than 20% and “dark energy” to more than 70%. 

Theorized decades ago as an indispensable component of the gravitational balance of the Universe on various scales, dark matter has not yet been detected. Equally elusive and mysterious is the neutrino, a fundamental particle postulated by Wolfgang Pauli in 1930 but not experimentally detected until 1956, by Clyde Cowan and Frederick Reines.

Dark matter and neutrinos have much in common. They are extremely abundant – the neutrino is the most abundant massive particle in the Standard Model. They interact very little with ordinary matter and are therefore hard to detect. Moreover, scientists now believe a certain type of neutrino – called sterile because it does not interact with other matter via the weak nuclear force – may be part of dark matter, alongside other exotic particles. 

Today there is a huge research effort in both areas, including mega-projects such as DUNE and the DarkSide program. Training a new generation of physicists in Brazil and other Latin American countries to work theoretically and/or experimentally in both fields of investigation is the aim of the School on Dark Matter and Neutrino Detection, being held until August 3 by the South American Institute for Fundamental Research (ICTP-SAIFR) at the Theoretical Physics Institute of São Paulo State University (IFT-UNESP) in São Paulo City. The event ends on August 3. 

ICTP-SAIFR is a collaboration between UNESP and the International Center for Theoretical Physics (ICTP), based in Trieste, Italy, and has funding from FAPESP.

“The School features internationally renowned researchers, from both Brazil and elsewhere, who work on both topics in an integrated manner combining theory and experimentation. In fact, this is a fundamental and innovative characteristic of the School: the first week was devoted to theoretical groundwork, and the second is focusing on detection methods and associated experiments, both ongoing and under construction or being planned,” Martin Makler, who chairs the local organizing committee, told Agência FAPESP.

Organized by researchers from the Brazilian Center for Research in Physics (CBPF), UNESP, the University of São Paulo (USP), the Federal University of Goiás (UFG) and the Federal University of Rio de Janeiro (UFRJ), the School is supported by FAPESP and the Ministry of Education’s Office for Faculty Development (CAPES).

“The School aims to give the participants a sound basis by summarizing the state of the art and pointing out the major questions that motivate theoretical and experimental research in both fields. In addition to general presentations, there are discussion sessions that enable students and professors to dialog more directly. We also have experimental work in small groups to illustrate the techniques used in the large-scale research projects,” Makler said.

“A strong expectation of the scientific community is that the big experiments like DUNE will help us answer important questions, such as why the Universe is made up of matter rather than antimatter.”

WIMPS and axions

In the Standard Model of particle physics, the neutrino is a member of the lepton family. For every electrically charged lepton (electron, muon or tau), there is a corresponding neutrino (electron neutrino, muon neutrino or tau neutrino). 

“Neutrino oscillation”, where one type (or flavor) of neutrino changes into another, occurs spontaneously during the particle’s propagation through space. The main goals of DUNE include comparing neutrino oscillation patterns with antineutrino oscillation patterns. Antineutrinos are the antiparticles of neutrinos, distinguished from the latter by having clockwise rather than counterclockwise spin. 

If the patterns are not strictly symmetrical, the experiment will furnish concrete evidence of the violation of matter-antimatter symmetry known as charge-parity (CP) violation. CP violation is thought to have produced a small surplus of matter over antimatter just after the Big Bang. This surplus still presumably comprises the entire observable universe.

“As for dark matter, the whole of astrophysics points to its existence. Indeed, research performed recently at UNESP’s Physics Institute and published in the journal Nature Physics measured the amount of dark matter in the Milky Way. This evidence, obtained by calculating our Galaxy’s gravitational balance, has also been found for other galaxies and galaxy clusters, and for the large-scale structure of the Universe. In principle, one particle could explain all this. But that particle hasn’t been detected yet,” Makler said.

All ongoing and planned experiments assume dark matter must involve interaction of a different kind to gravitational interaction. In the past decade, several experiments have been designed to detect a hypothetical particle known as the WIMP, short for weakly interacting massive particle, but so far they have not succeeded, making way for new theoretical models and new experiments. One proposal that has come to the fore concerns a particle called the axion.

“The WIMP is thought to be a particle with a large mass compared to the known elementary particles, and produced in a very hot and dense primordial Universe by a mechanism similar to the mechanism that is believed to have produced the known particles. Cooling of the Universe fixed the number of WIMPs in existence and they haven’t been detected because they don’t participate in electromagnetic interaction. Axions are hypothetical particles thought to be extremely light and cold, produced by a different mechanism, and capable of responding to a very strong magnetic field,” Makler said. 

Currently there are models that postulate direct interaction between dark matter and known matter via the weak nuclear interaction, more complex models that assume the existence of a mediating particle such as the “dark photon”, and even models in which dark matter forms bound states such as atoms and molecules.

All these different scenarios are part of the scope of the School on Dark Matter and Neutrino Detection. While the first week is designed to give students a theoretical grounding, the second features leading names in detection of both neutrinos and dark matter.

Speakers include Stefan Soldner-Rembold and Ettore Segreto on neutrinos; Andrew Sonnenschein on WIMPs and axions; Enectali Figueroa-Feliciano on sterile neutrinos; and Juan Estrada, on charge-coupled device (CCD) detection technology for neutrinos and dark matter. 

Students from Argentina, Brazil, Canada, Chile, Colombia, Mexico and Peru are attending the event. The opening theoretical lectures were delivered by Fabio Iocco (dark matter) and André de Gouvea (neutrinos). The complete program can be found at ictp-saifr.org/school-on-dark-matter-and-neutrino-detection/.