Millicharged Dark Matter Detection with Ion Traps

We propose the use of trapped ions for the detection of millicharged dark matter. Millicharged particles will scatter off the ions, giving a signal either in individual events or in the overall heating rate of the ions. Ion traps have several properties that make them ideal detectors for such a signal. First, ion traps have demonstrated significant isolation of the ions from the environment, greatly reducing the background heating and event rates. Second, ion traps can have low thresholds for the detection of energy deposition, down to approximately the nanoelectronvolt range. Third, since the ions are charged, they naturally have large cross sections for scattering with the millicharged particles, this being further enhanced by the low velocities of the thermalized millicharged particles. Despite ion-trap setups being optimized for other goals, we find that existing measurements put new constraints on millicharged dark matter that are many orders of magnitude beyond previous bounds. For example, for a millicharged dark matter mass m_Q=10 GeV and charge 10⁻³ of the electron charge, ion traps limit the local density to be n_Q≲1 cm⁻³, a factor of approximately 10⁸ better than current constraints. Future dedicated ion-trap experiments could reach even further into unexplored parameter space.