A heavy candidate for dark matter

Nearly a quarter of the universe is literally in the shadows. According to the theories of cosmologists, 25.8% of it consists of dark matter whose presence is only reported by its gravitational attraction. What does this substance consist of is a mystery. Hermann Nicolai, director of the Max Planck Institute for Gravitational Physics in Potsdam, and his colleague Krzysztof Meissner of the Warsaw University have proposed a new candidate: a super gravitino gravitino. The existence of this still hypothetical particle stems from a hypothesis that seeks to explain how the observed spectrum of quarks and leptons in the standard model of particle physics could emerge from a fundamental theory. In addition, researchers describe a possible method for tracking this particle.

The standard model of particle physics encompasses the constituent elements of matter and the forces that sustain them. He indicates that there are six different quarks and six leptons grouped into three "families". However, the matter around us and ourselves finally contains only three particles of the first family: the quarks from top to bottom and the electron, which belongs to the family of lepton.

Until now, this long-established standard model has remained unchanged. The Large Hadron Collider (LHC) at CERN in Geneva was commissioned about ten years ago with the main aim of exploring what could go beyond it. However, after ten years of data collection, scientists have not detected any new elementary particles, with the exception of the Higgs boson, despite widely shared expectations. In other words, so far, measurements with the LHC have not provided any indication of "new physics" beyond the standard model. These results contrast sharply with the many proposed extensions of this model that suggest a large number of new particles.

In a previous article published in Letters of physical examinationHermann Nicolai and Krzysztof Meissner presented a new hypothesis that seeks to explain why only the elementary particles already known are basic elements of matter in nature – and why, contrary to what was previously thought, no new particles is to be feared. the range of energy accessible to current or future imaginable experiments.

In addition, the two researchers postulate the existence of supermassive gravitinos, which could be very unusual candidates for dark matter. In a second publication recently published in the journal Physical examination D, they also presented a proposal on how to track these gravitinos.

In their work, Nicolai and Meissner take up an old idea of ​​Nobel laureate Murray Gell-Mann, based on the theory "N = 8 Supergravity". A key element of their proposal is a new type of infinite-dimensional symmetry designed to explain the observed spectrum of quarks and leptons known in three families. "Our hypothesis actually produces no extra particles in ordinary matter that we should then dissipate, because they do not appear in accelerator experiments," explains Hermann Nicolai. "On the other hand, our hypothesis can in principle explain precisely what we see, especially the replication of quarks and leptons in three families."

However, processes in the cosmos can not be fully explained by ordinary matter that we already know. The galaxies are a sign of this: they spin at high speed and the visible material in the universe – which represents only about 5% of the matter in the universe – would not be enough to hold them together. . Until now, however, nobody knows what else is done, despite many suggestions. The nature of dark matter is therefore one of the most important unanswered questions of cosmology.

"It is generally expected that dark matter is composed of an elementary particle and that it has not yet been possible to detect this particle because it interacts with ordinary matter almost exclusively by the force of gravitation, "explains Hermann Nicolai. The model developed in collaboration with Krzysztof Meissner offers a new candidate for a dark matter particle of this type, even though it presents properties completely different from those of all candidates discussed so far, such as axions. or WIMP. These only interact very weakly with the known material. The same goes for the very light gravitinos that have been repeatedly proposed as dark matter candidates in connection with low energy supersymmetry. However, the present proposal goes in a completely different direction, in that it no longer assigns a major role to supersymmetry, even though the scheme descends from the maximum supergravity of N = 8. "Our scheme in particular predicts the existence of super-heavy gravitinos which, unlike the usual candidates and previously considered light gravitinos, would also interact strongly and electromagnetically with ordinary matter, "explains Hermann Nicolai.

Their large mass means that these particles can only appear in a very diluted form in the universe; otherwise, they would "overload" the universe and thus lead to its early collapse. According to researcher Max Planck, he would not actually need many of them to explain the dark matter content of the universe and our galaxy: one particle per 10,000 cubic kilometers would be enough. The mass of the particle postulated by Nicolai and Meissner lies in the Planck mass region, that is, about one hundred millionth of a kilogram. In comparison, protons and neutrons – constituent elements of the nucleus of the atom – are about ten billion times (ten million billion) lighter. In the intergalactic space, the density would still be much lower.

"The stability of these heavy gravitinos depends on their unusual quantum numbers (loads)," Nicolai said. "Specifically, there is simply no end state with the corresponding charges in the standard model in which these gravitinos could disintegrate – otherwise, they would have disappeared shortly after the Big Bang."

Their strong and electromagnetic interactions with known materials can make these particles easier to detect despite their extreme rarity. One possibility is to search for them with dedicated measurements of flight time in the underground depths, because these particles move much more slowly than the speed of light, unlike ordinary elementary particles from cosmic radiation. Nevertheless, they would penetrate the Earth effortlessly because of their large mass, like a cannonball that can not be stopped by a swarm of mosquitoes.

This fact gives researchers the idea of ​​using our planet itself as a "paleo detector": the Earth orbits across interplanetary space for about 4.5 billion years, period during which many of these massive gravitinos had to penetrate. In the process, the particles must have left long, straight traces of ionization in the rock, but it can be difficult to distinguish them from traces caused by known particles. "Ionizing radiation is known to cause lattice defects in crystal structures.It may be possible to detect traces of such traces of ionization in crystals that remain stable for millions of years," explains Hermann Nicolai. Due to its long "exposure time", such a search strategy could also be effective if the dark matter was not homogeneously distributed in galaxies but subject to local density fluctuations – this which could also explain the failure of searches for more conventional candidates to dark matter up to now.