Could supermassive gravitinos be the key to dark matter?

Dark matter has bewildered scientists for centuries. It makes up most of the universe's matter, but it can't be seen directly. Now, a group of scientists worldwide think they've found an unexpected culprit—superheavy particles called gravitinos that are also electrically charged. If these strange particles exist, they would reveal themselves not by deadly crashes but by glitteringly glowing filaments of light in gigantic underground detectors.

The feat has been accomplished by Warsaw University's Adrianna Kruk and Michał Lesiuk, Warsaw University's Krzysztof A. Meissner, and the Max Planck Institute for Gravitational Physics' Hermann Nicolai. Their work describes how future-generation neutrino observatories could be used as dark matter detectors.

Gravitinos, according to their hypothesis, would be the only other dark matter candidate, being different from all the other dark matter contenders. While the neutral WIMPs or axions would have masses ranging from practically unimaginable 10^18 giga-electronvolts, trillions and trillions more than a proton, and infinitesimally small electric charge of ±2/3, the particles would possess. Although they are charged, due to their rarity, they would not be detectable by telescopes and therefore classify as dark matter candidates.

This proposal has a larger mystery that has confused physicists for many decades: unifying the Standard Model of particle physics with gravities. Nobel laureate Murray Gell-Mann, in 1981, noted that the number of quarks and leptons in the Standard Model equaled the spin-1/2 states of a mathematical construct known as N=8 supergravity.

Decades after, Meissner and Nicolai extended this idea and showed how the quark and lepton charges could be adjusted to match measurements. Their paper added a higher hidden symmetry, K(E10). As one of its consequences, eight heavy gravitinos were predicted—six with charges of ±1/3 and two with charges of ±2/3. Since these won't decay, the ±2/3 charge pair appeared as a brazen dark matter candidate.

The challenge, of course, is to find something that happens so rarely. In the solar system, there could be only one gravitino per 10,000 cubic kilometers. Researchers believe detectors such as China's Jiangmen Underground Neutrino Observatory (JUNO) and the US's Deep Underground Neutrino Experiment (DUNE) have the best chances.

JUNO will go online in late 2025. It's a 40-meter ball filled with 20,000 tons of strange liquid called linear alkylbenzene (LAB). It has more than 17,000 photomultiplier tubes outside and will be listening for the faint glows of light. Even though JUNO is really going to watch for antineutrinos, its giant size makes it possible for it to do work on other things as well. DUNE, using liquid argon, will use the same concept when complete.

If a gravitino passed through one of these detectors, its electric charge would disturb the electrons in passing molecules. At a timescale of femtoseconds, these electrons would be pushed to higher energy shells and release their energy as photons—a soft radiation that would last from microseconds to hundreds of microseconds. A gravitino would produce a slowly decaying beam of light, unlike the fast, short flashes from neutrinos or cosmic rays.

The computer simulations performed by the team show that such a signal would be impossible to confuse with any present background activity. Even the random close approaches within atomic nuclei or the weak interactions among molecules in LAB are negligible to alter the result. The bright light would be the clear signal that something strange had gone through.

What is most amazing in this study is how it merges two disciplines that do not often speak with each other. In order to be able to make calculations about what would happen within JUNO or DUNE, the researchers applied advanced quantum chemistry techniques more commonly used to study molecules to a question of particle physics. They applied those techniques to a problem in particle physics and combined chemistry and cosmology in a new but successful way.

"The simulations had to account for everything," the scientists explained, including radioactive decay inside the detector and how light travels within the liquid. The final findings are a distinct, unequivocal glow that would be identifiable from any other kind of event.

If scientists ever observe such a glow, the implications would be vast. It would establish that dark matter is not totally dark but made up of partially charged, superheavy particles. It would constitute the first experimental evidence of physics at the Planck scale, many orders of magnitude beyond the capability of any particle accelerator today. Perhaps most importantly, it could be a step towards a unification of gravity and particle physics, one of the most fervent longings in science.

Even if gravitinos are never found, the search is honed. By ruling out possibilities, physicists inch closer to understanding what dark matter really is.

JUNO and DUNE sit quietly in the earth, hundreds of thousands of electronic eyes into the void, waiting to catch the faintest whiff of a glow. If gravitinos exist, their detection would shine physics in a new light in a matter of time.

If it turns out to be true, then supermassive gravitinos would revolutionize physics entirely. They would explain some part of dark matter and relate the Standard Model to supergravity, potentially having a means of unifying all forces.

For mankind, it could unlock completely new technologies by opening up physics at energy scales never before seen.

Even if gravitinos prove as evasive as will-o-the-wisps, the methodology used here—intermingling particle physics and chemistry—can help to imagine new detectors and bolster the search for dark matter, and ultimately settle one of science's greatest enigmas.

Research findings are available online in the journal Physical Review Research.

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