Direct detection of electromagnetically interacting ultraheavy dark matter

The universe is filled with invisible matter that holds galaxies together, yet we have never seen a single particle of it. Scientists call this "dark matter," and for decades, the leading theory has been that it is made of heavy, slow-moving particles that barely interact with ordinary light or matter. However, a quieter possibility has lingered: what if some of this dark matter carries a tiny, faint electric charge? If it does, it would interact with the photons of light, however weakly, creating a bridge between the hidden sector and the visible world we inhabit. This idea, known as "millicharged" dark matter, suggests that these particles might be light enough to zip through detectors or heavy enough to punch through them, depending on their mass. The question remains whether we can catch them, and if so, how heavy they might be before our current tools simply cannot see them.

A team of physicists has now mapped out the limits of our ability to find these elusive particles, specifically focusing on the heaviest candidates imaginable. They examined data from a wide array of underground experiments designed to catch dark matter, including massive tanks of liquid xenon, chambers filled with liquid argon, and bubble chambers that detect tiny flashes of boiling liquid. Their goal was to determine how heavy these particles could be before the detectors themselves become blind to them. The researchers found that for particles interacting through electric charge, there is a hard limit to how much charge they can have before they are rejected by the very safety systems designed to protect the experiments. This limit, or "ceiling," depends entirely on the type of detector used.

The study reveals a surprising advantage for one specific type of detector: the bubble chamber. In most experiments, such as those using liquid xenon, the detectors are so sensitive that they would see a flood of signals from electrons if the dark matter had even a tiny electric charge. To avoid being overwhelmed by this noise, these experiments automatically discard any event that looks like an electron interaction. Consequently, if the dark matter is too charged, it triggers this veto system and is thrown away, leaving the experiment unable to see it. The researchers calculated that for liquid xenon and semiconductor detectors, this creates a strict upper bound on the charge of the dark matter they can search for. However, bubble chambers work differently. They are designed to ignore the faint signals left by electrons and only react to the heavy, forceful hits from atomic nuclei. Because they do not have a system that automatically rejects electron signals, they do not suffer from this same ceiling.

This difference allows the bubble chamber experiment known as PICO-60 to explore a vast region of possibilities that other detectors cannot reach. The team found that PICO-60 can constrain the electric charge of dark matter to two orders higher than liquid xenon experiments, and it can do so for particles with masses up to one order greater. This discovery effectively closes a large gap in our knowledge, ruling out a wide range of heavy, millicharged dark matter that was previously unconstrained. For particles with masses below a certain threshold, this result bridges the divide between what direct detection experiments can see and what neutrino experiments have already ruled out.

The researchers also looked at other ways dark matter might interact with light, such as having a magnetic or electric dipole moment, or a specific internal size known as a charge radius. For these types of interactions, the "ceiling" problem does not exist because the signals they produce do not overwhelm the detectors in the same way. The team calculated the limits for these scenarios as well, extending the known constraints for dark matter masses up to 10^17 GeV. This is an incredibly high mass, far beyond what current particle colliders can produce, pushing the boundaries of what we know about the heaviest possible dark matter candidates.

Looking ahead, the study outlines what future, even larger detectors will be able to achieve. Experiments planned for the coming years, such as the multi-tonne scale DarkSide-20k and DARWIN, will be able to probe even deeper into this parameter space. The researchers suggest that by refining how these detectors analyze their data, particularly by adjusting the rules for what counts as a background signal, the current limits on millicharged dark matter could be pushed even higher. While the Earth's crust and the atmosphere might stop some of these heavy particles before they reach the underground labs, the calculations show that for the heaviest candidates, this effect is negligible. The work provides a clear roadmap for where the search for the heaviest, faintly charged dark matter stands today and where it must go tomorrow.