Centuries-old physics test could help detect millicharged particles

A team of theoretical physicists has proposed that a centuries-old experiment known as the Cavendish test might offer a novel approach to detecting millicharged particles (mCPs), hypothetical entities carrying a minuscule fraction of the electric charge of an electron. This research, led by scientists from Fermi National Accelerator Laboratory, Stanford University, and the University of Delaware, was detailed in a study published in Physical Review Letters. The findings suggest that the Cavendish test, originally devised to confirm Coulomb’s law, could achieve higher sensitivity in identifying mCPs compared to modern particle accelerator-based methods. Millicharged particles are considered among the simplest extensions of the Standard Model of particle physics. They are theorized to interact extremely weakly with ordinary matter and electromagnetic fields, making them nearly impossible to detect through traditional means. Some models propose that mCPs could constitute a minor component of dark matter, the mysterious substance believed to permeate the universe yet remain undetected due to its minimal interaction with visible matter. Harikrishnan Ramani, a senior researcher and co-author of the study, emphasized the lack of stringent constraints on mCPs despite extensive investigations. He noted that current data allows for scenarios where these particles exhibit interactions similar to those of standard model particles, including production via cosmic ray collisions and potential influence on Earth’s magnetic and electric fields. The proposed method involves modifying the classic Cavendish setup to detect subtle deviations from Coulomb’s law, which governs the electrostatic force between charged bodies. Traditionally, the Cavendish experiment used two masses suspended by a torsion wire to measure gravitational forces, confirming Newton’s law of universal gravitation. However, the adaptation suggested by Ramani and colleagues involves enclosing the apparatus within an electric trap, allowing for the detection of minute electric fields generated by mCPs. According to the researchers, the presence of mCPs could lead to observable effects akin to a non-zero mass for photons, a phenomenon previously studied using similar experimental setups. Both conditions, nonzero photon mass and mCPs, could produce signals that deviate from Gauss’s law, a cornerstone of electromagnetism. By measuring these anomalies, the modified Cavendish test could potentially reveal the existence of mCPs. The study outlines how the Cavendish test could surpass the sensitivity of existing and planned particle accelerators in probing mCPs. This is attributed to the unique configuration of the experiment, which isolates the system from external influences while maintaining high precision. The proposed device would oscillate the electric charges of mCPs and measure the resulting weak electric fields, offering a new pathway to explore these elusive particles. The implications of this research extend beyond the detection of mCPs. If successful, the Cavendish test could provide insights into the nature of dark matter and the validity of fundamental physical laws under extreme conditions. The method also highlights the enduring relevance of historical experiments in addressing contemporary scientific challenges. The team plans to refine the design of the experimental setup and conduct further simulations to validate the feasibility of detecting mCPs using this approach. Researchers are currently evaluating the practical aspects of implementing the modified Cavendish test, including the necessary shielding against external interference and the calibration of measurement instruments to ensure accuracy. As the search for dark matter continues, alternative methods like the Cavendish test may prove essential in uncovering new phenomena that lie beyond the reach of conventional particle accelerators. The proposed experiment represents a bridge between classical physics and cutting-edge astrophysical inquiry, demonstrating how foundational principles can still yield valuable insights into the unknown.