A Simple Search for Tiny Charges
One candidate for the mysterious dark matter believed to pervade the Universe is a hypothetical form of matter called millicharged particles (mCPs), which carry a tiny fraction of the charge on an electron. A research team has now proposed that such particles, if they exist, might be detected by letting them accumulate in simple laboratory-scale devices already used for creating and measuring electric charge [1, 2]. The team has shown that previous measurements made with such devices can be used to set new limits on the properties of mCPs.
The standard model of particle physics accommodates the 17 particles that make up regular, visible matter, but researchers are seeking to extend it to include gravity or dark matter or both. Dark matter seems to be demanded by astronomical observations and—aside from its gravitational interactions—should interact minimally, if at all, with light and with other matter.
Millicharged particles are among the hypothesized particles lying outside the standard model. In principle, they could have any charge less than that of an electron, and any mass. Several experiments have sought evidence for them [3, 4], including some that used particle accelerators [5]. But those efforts failed both to detect the particles and to place strong constraints on their properties.
Five years ago, particle physicist Harikrishnan Ramani of the University of Delaware argued that, if they exist, mCPs should interact with and scatter from ordinary atoms in the terrestrial environment [6]. This scattering would slow the particles and allow them to accumulate around objects. Negatively charged mCPs might become bound inside atomic nuclei, while positively charged ones would just execute random walks in the ambient environment.
Now Ramani and co-workers have shown that, if these particles are indeed present in our surroundings, it should be possible to concentrate them further in simple instruments. One such instrument is a Van de Graaff generator, which was invented in the 1920s. In this machine, friction is used to build up charge on a hollow metal globe, creating enormous voltages [1]. A typical device incorporates a spherical metal shell with a concentric metal globe inside, separated by an air gap. If the outer shell is charged to a high negative voltage, positive mCPs in the environment would be drawn into the shell and lose energy. “These particles rush [into the device] because of the electric field,” Ramani says, “but when they propagate through the metal, they are slowed down and do not have enough energy to escape.”
Ramani and colleagues considered the full range of plausible mCP charge and mass. They calculated how much such an instrument would be able to concentrate the particles relative to their density in the environment. This factor could reach 1012 for the most optimistic case, they found. “We are currently working on an experimental proposal where we run the accumulator for a year outdoors and then bring it in to run the experiment on the accumulated mCPs,” Ramani says.
In additional theoretical work [2], he and his colleagues have shown that another well-established instrument used for testing electrostatic laws can act as an accumulator. They also show that data from such tests performed in the past can be reanalyzed to set limits on mCP properties. The instrument was devised in the 18th century by Henry Cavendish, who used it to investigate the nature of electricity. In its simplest form, it consists of a hollow metal globe that is given a surface charge by an oscillating voltage. According to Coulomb’s law, the electric field inside the shell should average to zero. A nonzero value in a Cavendish test would imply that Coulomb’s law is not exactly true. But Ramani and colleagues show that such deviations would also be expected if mCPs accumulated inside the shell.
No such deviations have ever been observed. But Ramani and colleagues use the results of Cavendish tests conducted in the 1930s [7] and the 1970s [8] to calculate limits on the mCP charge and mass. The calculation extends the limits previously obtained from accelerator experiments and astrophysical observations to cover much more of the range of possibilities for mCP masses of between around 0.001 to 105 giga-electron-volts (GeV/c2) and charges down to around a billionth that of an electron. (The mass of an electron is about 0.0005 GeV/c2.) The researchers also show how these limits could be improved further by relatively simple improvements to the Cavendish test.
“I think it’s a creative insight that data taken for a different purpose—to test Coulomb’s law—nearly 100 years ago are able to constrain whether dark matter could carry a tiny effective electric charge,” says particle physicist David Moore of Yale University. “It is also impressive that in certain parameter regimes these relatively simple technologies can outperform the most precise modern sensors.”
However, mCPs are not among theorists’ most favored dark matter candidates, not least because they are not really “dark,” says quantum physicist Jake Taylor of the University of Maryland, College Park. (Ramani says that his team’s work does not hinge on whether mCPs would be a component of dark matter; they might exist in a different capacity.) Taylor adds that the new analysis makes many strong assumptions about how mCPs interact with and scatter from ordinary matter. Even so, he says, given that the accumulation experiments are quite cheap to conduct, “it’s worth giving them a try.”
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
References
- A. Berlin et al., “Electric accumulation of millicharged particles,” Phys. Rev. D 114, 015016 (2026).
- A. Berlin et al., “Cavendish tests of millicharged particles,” Phys. Rev. Lett. 137, 021804 (2026).
- D. Budker et al., “Millicharged dark matter detection with ion traps,” PRX Quantum 3, 010330 (2022).
- P. C. Kim et al., “Search for fractional-charge particles in meteoritic material,” Phys. Rev. Lett. 99, 161804 (2007).
- S. Alcott et al., “Search for millicharged particles in proton-proton collisions at = 13.6 TeV,” Phys. Rev. Lett. 135, 121802 (2025).
- M. Pospelov and H. Ramani, “Earth-bound millicharge relics,” Phys. Rev. D 103, 115031 (2021).
- S. J. Plimpton and W. E. Lawton, “Very accurate test of Coulomb’s law of force between charges,” Phys. Rev. 50, 1066 (1936).
- D. F. Bartlett et al., “Experimental test of Coulomb’s law,” Phys. Rev. D 2, 483 (1970).