Why is our Universe made of matter? Why does everything exist as we know it? These questions are related to one of the most important unsolved problems in particle physics. This problem is that of the nature of the neutrino, which could be its own antiparticle, as the ill-fated Italian genius Ettore Majorana ventured almost a century ago. If this were so, the mysterious cosmic asymmetry between matter and antimatter could be explained.
Indeed, we know that the Universe is made almost exclusively of matter. However, the Big Bang theory predicts that the early Universe contained the same number of particles of matter and antimatter. This prediction is consistent with the “small Big Bangs” that form in proton collisions in CERN's gigantic LHC accelerator, where a symmetrical production of particles and antiparticles is always observed. Where, then, did the antimatter of the early Universe end up? A possible mechanism points to the existence of heavy neutrinos that were their own antiparticle and therefore could disintegrate both matter and antimatter. If a second phenomenon occurs, called charge and parity violation (that is, if the neutrino slightly favors the production of matter over antimatter in its decays), then it could have injected an excess of the first over the second. After all matter and antimatter in the universe were annihilated (with the exception of this small excess), the result would be a cosmos made only of matter, from the leftovers of the Big Bang. We could say that our Universe is the remains of a shipwreck.
It is possible to demonstrate that the neutrino is its own antiparticle by observing a rare type of nuclear process called neutrino-free double beta decay (bb0nu), in which simultaneously two neutrons (n) in the nucleus become protons (p) and also they are emitted two electrons (e) that escape outside the atom. This process can occur in some rare isotopes, such as Xenon-136, which has 54 p and 82 n in its nucleus, in addition to 54 e in its neutral form. The NEXT experiment (conducted by JJ Gómez-Cadenas, from DIPC and IKerbasque and D. Nygren, from the University of Texas at Arlington), located in the Canfranc Underground Laboratory (LSC), looks for these disintegrations using high-pressure gas chambers.
When an Xe-136 atom undergoes a spontaneous decay bb0nu, the result of the process is the production of a doubly charged Barium-136 (Ba²+ ) ion, with 54 e and a nucleus consisting of 56 p and 80 n, and two electrons (Xe → Ba²+ + 2e).
The NEXT experiment has so far focused on observing these two electrons, whose signal is very characteristic of the process. However, the desired decay is extremely rare and the expected signal is on the order of a bb0nu decay per tonne of gas per year of exposure. This weak signal can be completely masked by background noise due to the ubiquitous natural radioactivity. However, if, in addition to observing the two electrons, the ionized barium atom is detected, the background noise can be reduced to zero, since natural radioactivity does not produce this ion. Observing a single Ba²+ ion in a large bb0nu detector is so extremely difficult that until recently it was considered impractical. But a series of recent works, including the one recently published in the journal Nature, shows that the feat could be achieved in a reasonable period of time.
The mentioned work, conceived and directed by the F.P. Cossío, Professor at the University of the Basque Country (UPV / EHU) and scientific director of Ikerbasque, and J.J. Gómez-Cadenas, Professor Ikerbasque of the Donostia International Physics Center (DIPC), has a team that includes scientists from the DIPC, the UPV / EHU, Ikerbasque, the Optics Laboratory of the University of Murcia (LOUM), the Physics Center of Materials (CFM, CSIC - UPV/EHU Mixed Center), POLYMAT, and the University of Texas at Arlington (UTA). Gómez-Cadenas highlighted that “the result of this interdisciplinary collaboration that combines, among other disciplines, particle physics, organic chemistry, surface physics and optics, is a clear example of the DIPC's commitment recently to open new lines of investigation. The objective is not only to generate knowledge in other fields, different from the usual ones of the center, but also to look for hybrid lands and create interdisciplinary projects that, in many occasions, like this one, can be the most original”.
The study starts from the idea, proposed by one of the authors of the article, the prestigious scientist D. Nygren (UTA), inventor of technology of temporal projection cameras (TPCs) on which numerous experiments in particle physics are based (among them NEXT). In 2016 Nygren proposed the possibility of capturing Ba²+ with a molecule capable of forming a supramolecular complex with it and of providing a characteristic signal when this occurs, as a molecular indicator. In later work, Nygren and his group have designed a type of indicator called "switches" capable of brightening when they capture a Ba²+ ion. The group of Cossío and Gómez-Cadenas has followed a different strategy, designing an indicator capable of selectively capturing Ba²+ and that not only shines brighter when trapping the ion, but changes color, thus contributing to a very clear observation of the signal on background noise. The synthesis of this bicolor molecular indicator, called FBI (Fluorescent Bicolor Indicator), has been carried out under the leadership of researcher I. Rivilla of the DIPC. If a barium-free FBI molecule is illuminated with ultraviolet light, it emits fluorescence in the green light range, with a narrow emission spectrum of around 550 nm. In contrast, when this molecule captures Ba²+ , its emission spectrum shifts to blue (420nm). This makes it possible to identify the presence of Ba²+ from observation of a blue FBI molecule.
It is noteworthy that the experimental multiphoton microscopy systems used in the LOUM by P. Artal's group for green/blue spectral detection are based on those previously developed to obtain images of the cornea of the human eye in vivo. It is an example of intertwining using a unique technology in the world for biomedical applications in a fundamental problem of particle physics.
As Cossío explained, “the most complicated part of the chemical part of the work was designing a new molecule that would meet the strict (almost impossible) requirements imposed by the NEXT experiment. This molecule had to shine a lot, capture barium with extreme efficiency (bb0nu is a very rare event and no cation could be wasted) and emit a specific signal that would allow the capture to be detected without background noise. Furthermore, the chemical synthesis of the new FBI sensor had to be efficient in order to have ultrapure samples in sufficient quantity for its installation in the detector. The most gratifying part was verifying that, after much effort by this multidisciplinary team, our specific and ultra-sensitive FBI sensor was working as intended.”
In addition to the FBI design and characterization, the article offers the first demonstration of the formation of supramolecular complexes in a dry medium. This milestone has been achieved by preparing a layer of FBI molecules on a compressed silica tablet and evaporating on this layer a barium perchlorate salt. Z. Freixa, professor Ikerbasque assures: “the preparation of the FBI on silica has been a quick solution for this proof of concept. A little bit of home alchemy never hurts.” Evaporation in a vacuum has been carried out by the CSIC scientist at the CFM, C. Rogero and her doctoral student P. Herrero-Gómez. Rogero, an expert in surface physics, says: “It was one of those Eureka moments, when we realized that we had the know-how to demonstrate for the first time that a molecule is capable of trapping a dication in a dry medium. We got down to business and it went well almost on the first try.”
The next step in this project will be to build an FBI-based detector for the detection of double beta decay without neutrinos or bb0nu, for which Gómez-Cadenas and F. Monrabal of the DIPC together with D. Nygren and their collaborators from UTA are already developing the conceptual proposal. This future experiment, which could be up and running in a few years, would be able to search for bb0nu events free of background noise thanks to the identification of the two electrons and the barium atom produced in the reaction and would have great potential to discover if the neutrino is its own antiparticle, which would allow answering fundamental questions about the origin of the Universe, including why we are here.
For further information: https://www.nature.com/articles/s41586-020-2431-5
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