This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Theoretical physicists have long predicted the existence of millicharged particles (mCPs), hypothetical particles that carry a very small electric charge. These particles are expected to interact very weakly with ordinary matter and electromagnetic fields; hence, they would be difficult to detect in conventional particle physics experiments.
Some studies hypothesized that millicharged particles also make up a small fraction of dark matter in the universe. Dark matter is an elusive type of matter that does not emit, absorb or reflect light and that interacts weakly with ordinary matter. These characteristics have so far prevented physicists from determining its composition and properties.
Researchers at Fermi National Accelerator Laboratory, Stanford University and the University of Delaware recently showed that the so-called Cavendish test, one of the oldest precision experiments performed by physicists, could also be used to search for millicharged particles. Their paper, published in Physical Review Letters, suggests that this experiment may reach greater sensitivities than particle accelerator–based experiments. "mCPs, new particles that possess an electric charge that is a small fraction of the electron charge, are one of the simplest extensions of the Standard Model of particle physics," Harikrishnan Ramani, senior author of the paper, told Phys.org.
"However, they are poorly constrained despite decades of scrutiny. "The constraints are so poor that there is allowed parameter space where these charges couple so strongly with the Standard Model that they share properties of Standard Model particles, such as being produced in cosmic ray collisions, having subsequent tiny mean free paths that lead to local thermalization and interacting with the large-scale electric and magnetic fields of Earth that lead to buildup over the edge of Earth." Ramani and his co-authors, Asher Berlin, Zachary Bogorad and Peter W. Graham, have been exploring the possibility of searching for mCPs predicted to accumulate naturally near Earth.
In their recent paper, they proposed developing a device that can oscillate the particles' electric charges and measure the weak electric field that it produces during this process. "Historically, such a device was used to set limits on the photon's mass," Ramani explained. "It turns out that the nonzero nature of the photon's mass and the presence of mCPs, although somewhat unrelated, can both cause the same signal—deviations from Gauss' law." The researchers introduced a new possible route for detecting mCPs or setting new constraints on their properties using a classical Cavendish test.
Notably, their projections suggest that this test could be three times more sensitive than existing or future particle accelerator–based experiments. A Cavendish test is a precision experiment designed to measure Coulomb's law, a fundamental principle describing the electric force between charged objects. The test relies on a large Faraday cage, a shell made of conductive material that blocks external electric fields and electromagnetic radiation.
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