Swiss Research Team Explores Potential Fifth Force of Nature

Physicists at ETH Zurich have embarked on a groundbreaking investigation into the existence of a potential fifth fundamental force of nature, utilizing advanced methodologies to probe atomic interactions. Their research, focused on calcium isotopes, aims to uncover evidence that may extend beyond the established four forces: gravity, electromagnetism, and the strong and weak nuclear forces. This innovative approach, which deviates from traditional particle accelerator experiments, employs atomic spectroscopy to detect minute energy shifts within atoms, potentially indicating the presence of a new particle responsible for previously unexplained phenomena.

The Standard Model of particle physics has served as the prevailing framework for understanding the universe and its constituent forces. However, significant gaps remain, particularly concerning dark matter, an enigmatic substance that constitutes a majority of the universe's mass yet eludes direct detection. "The Standard Model is currently the best explanation of the universe, but we know it cannot explain everything," asserted Dr. Diana Prado Lopes Aude Craik, an assistant professor at ETH Zurich and a leading figure in this research initiative.

To investigate the potential fifth force, the research team concentrated on five stable isotopes of calcium, differing only in their neutron counts. By conducting precision measurements using an ion trap—an instrument that holds charged atoms in place through electromagnetic fields—they were able to excite the isotopes with lasers and analyze the emitted light during energy transitions. Their measurements achieved unprecedented precision, detecting shifts as small as 100 millihertz, significantly surpassing previous capabilities.

The collaborative effort included partners from institutions in Germany and Australia, who provided complementary analyses of the calcium isotopes. Researchers at the Physikalisch-Technische Bundesanstalt and the Max Planck Institute for Nuclear Physics contributed vital data, allowing for a comprehensive understanding of the isotopes under varying states of charge and mass. "We can’t say that we’ve discovered new physics here, but we know how strong the new force can be at most because we would have seen it otherwise in our measurements," Craik noted, acknowledging the limitations of their findings.

Despite the challenges, the researchers successfully established new constraints on the mass and electric charge of a hypothetical boson, a particle that could mediate this fifth force. These boundaries will guide future investigations in a more focused direction, enhancing the understanding of atomic interactions and their broader cosmic implications.

Looking ahead, the team plans to measure additional energy transitions within the calcium isotopes, aiming for even greater precision. This ongoing research is expected to culminate in a three-dimensional King plot, a visualization tool that will aid physicists in determining whether observed energy shifts align with existing theoretical frameworks or suggest the presence of new physical phenomena.

The implications of this research extend far beyond theoretical physics; it could reshape our understanding of dark matter and challenge existing models of the universe's fundamental forces. As the field of high-precision atomic spectroscopy continues to evolve, the potential for discovering new forces of nature represents a pivotal frontier in the quest to unravel the universe's mysteries. The study, titled "Nonlinear Calcium King Plot Constrains New Bosons and Nuclear Properties," was published in *Physical Review Letters*, marking a significant milestone in the scientific exploration of atomic interactions and their role in the cosmos.