Dark Matter continues to puzzle physicists. Despite numerous theories (like axions and WIMPs) and four decades of exhaustive experiments, the nature of Dark Matter remains elusive. A few years ago, a groundbreaking theory emerged that united particle physics with gravity, introducing new Dark Matter candidates: superheavy charged gravitinos. A recent paper in Physical Review Research by scientists from the University of Warsaw and the Max Planck Institute for Gravitational Physics reveals that new underground detectors, especially the JUNO detector, which will begin collecting data soon, are well-suited to detect these charged Dark Matter gravitinos. Their simulations, which merge elementary particle physics with cutting-edge quantum chemistry, indicate that the signal from gravitinos in the detector should be both unique and clear.
In 1981, Murray Gell-Mann, a Nobel laureate known for introducing quarks as fundamental matter components, observed an intriguing point: the particles from the Standard Model—quarks and leptons—are contained in a purely mathematical theory called N=8 supergravity, established two years earlier. This theory, recognized for its maximal symmetry, includes not only Standard Model matter particles with spin 1/2 but also a gravitational component: the graviton (with spin 2) and eight gravitinos with spin 3/2. If the Standard Model connects to N=8 supergravity, it could offer a solution to one of the most challenging issues in fundamental physics: unifying gravity with particle physics. The matter content of N=8 supergravity includes exactly six quarks (u, d, c, s, t, b) and six leptons (electron, muon, tau, and neutrinos), prohibiting the existence of any other matter particles. Despite 40 years of thorough accelerator research yielding no new matter particles, this theoretical framework remains the only known explanation for the number of quarks and leptons in the Standard Model. However, its direct relationship with the Standard Model had challenges, notably the misalignment of electric charges for quarks and leptons, where the electron’s charge was calculated to be -5/6 instead of -1. Several years ago, Krzysztof Meissner from the University of Warsaw and Hermann Nicolai from the Max Planck Institute revisited Gell-Mann’s concept and advanced it beyond N=8 supergravity, achieving the correct electric charges for Standard Model matter particles. Their modification points toward an infinite symmetry, K(E10), which is relatively obscure mathematically, thus replacing the common symmetries of the Standard Model.
A surprising result of this modification, discussed in their papers in Physical Review Letters and Physical Review, is the discovery that the gravitinos have extremely large masses, close to the Planck scale (about a billion billion times the mass of a proton) and are electrically charged, with six of them having charges of ±1/3 and two having ±2/3. Although these gravitinos are vastly massive, they cannot decay as there are no lighter particles for them to transform into. Meissner and Nicolai proposed that two gravitinos, each with charges of ±2/3, could serve as Dark Matter candidates, differing significantly from typical candidates like axions (which are very light) and WIMPs (which are of intermediate mass) that are electrically neutral—hence their designation as ‘Dark Matter.’ However, extensive searches over 40 years have failed to identify new particles beyond the Standard Model.
Gravitinos present a refreshing alternative. Despite their electric charge, they qualify as Dark Matter candidates due to their immense mass, making them exceedingly rare and thereby “invisible” in the cosmos, which allows them to sidestep strict regulations on the charge of Dark Matter constituents. Moreover, their electric charges prompted new strategies for proving their existence. A 2024 paper by Meissner and Nicolai, published in Eur. Phys. J., suggested that neutrino detectors employing scintillators other than water could effectively search for Dark Matter gravitinos. However, the challenge remains due to their extreme scarcity—estimates indicate just one gravitino exists per 10,000 km³ in the Solar System—which limits detection with current technologies. Fortunately, plans for massive underground detectors using oil or liquid argon are in progress, opening realistic pathways for these particles’ discovery.
Among all detectors, the Jiangmen Underground Neutrino Observatory (JUNO) in China, currently under construction, is especially suited for this search. Designed primarily to study neutrinos (or more accurately, antineutrinos), its large volume—20,000 tons of synthetic oil-like liquid housed in a 40-meter-diameter spherical vessel and equipped with over 17,000 photomultipliers—enables it to detect even the faintest signals. JUNO is set to commence measurements in the latter half of 2025.
The recent paper in Physical Review Research by Meissner, Nicolai, and their collaborators—Adrianna Kruk and Michal Lesiuk from the Faculty of Chemistry at the University of Warsaw—provides a comprehensive analysis of the specific signals that could be generated by gravitinos interacting with JUNO and potential future detectors like the Deep Underground Neutrino Experiment (DUNE) in the U.S. The study outlines both the theoretical framework and detailed simulations of possible signatures linked to a gravitino’s velocity and trajectory as it travels through the oil. This endeavor demands advanced knowledge in quantum chemistry alongside time-intensive computations. The simulations also considered multiple potential background signals, including radioactive decay of C14 in the oil, dark count rates, photomultiplier efficiency, and photon absorption in oil. The results indicate that, with proper software, the passage of a gravitino through the detector will yield a distinct signal that cannot be mistakenly attributed to any known particle. This analysis sets new interdisciplinary benchmarks by integrating theoretical and experimental particle physics with sophisticated quantum chemistry methodologies.
Detecting superheavy gravitinos would represent a significant advancement in our quest for a unified theory of gravity and particle physics. Given that gravitinos are anticipated to exhibit masses near the Planck mass, their detection would mark the first direct evidence of phenomena occurring at this scale, offering valuable experimental insights into the unification of all fundamental forces in nature.
Summary: Dark Matter continues to baffle physicists, with existing theories failing to explain its nature. Newly proposed superheavy charged gravitinos offer a fresh perspective. A recent study highlights that underground detectors like the JUNO, designed for neutrino research, are also capable of identifying these elusive particles. With innovative simulations and interdisciplinary collaborations between physics and quantum chemistry, the potential discovery of gravitinos could be a groundbreaking step toward unifying gravity and particle physics.
