UC Davis

An abundance of astrophysical and cosmological data convincingly suggests that a largefraction of the Universe is comprised of cold matter, which has no known interaction with any of the forces of the standard model apart from gravity. This dark matter (DM) plays a central role in a wide range of phenomena that span cosmic time, from the anisotropies of the cosmic microwave background temperature to the formation of galaxies. Yet, despite its fundamental significance in our Universe and approximately 50 years of theoretical and experimental efforts, the source of this DM remains unknown. Recently, many non-baryonic DM candidates have been proposed. One popular candidate, the weakly interacting massive particle (WIMP), was thought to be the perfect DM candi- date for the past four decades. To date, an enormous WIMP-detection program has found nothing. Working under the guiding principle of “leave no stone unturned”, other, more exotic candidates are now being explored. To that end, new detector technology must be developed in order to probe the vast parameter spaces of many such DM candidates. This dissertation describes the progress of the Dark E-field Radio (DER) experiment in searching for one such candidate, the dark photon (DP). The DP is an extension of the Standard Model (SM) of particle physics. It is a gauge boson of a new U (1), which could today have a relic density from the early Universe. To explore the DP DM scenario, the DER experiment was designed to efficiently probe a wide range of DP parameter space, leveraging a wide-bandwidth E-field antenna moved to multiple positions in a shielded room and a low noise amplifier. Following this, a real- time, wide-band spectrum analyzer serves as the data acquisition system. Borrowing from the field of Electromagnetic Compatibility (EMC), the concept of statistical uniformity in a reverberation chamber is introduced. Moving the antenna and averaging the spectra together averages over the resonances of the system, allowing for a much more stable calibration. The thermal noise emerging from the antenna results in a power spectrum that features prominent variations with frequency. The source of these variations is explored and tested. Detailed system characterization was performed, including characterization of the anten- na/room response, noise contributions (primarily due to the blackbody radiation of the room-temperature walls), shielding effectiveness of external radio frequency interference and various tests of the spectrum analyzer. To verify system performance, a small signal injection test was performed. The signal was detected after the predicted amount of averaging, con- firming the effectiveness of the entire experiment — from the antenna/room system through data analysis. So far, Run 1A of the DER experiment has been completed. It has been peer-reviewed and published (Levine et al. 2024). Many aspects of this data run and analysis are described herein, including data acquisition, reduction, storage and analysis, followed by the necessary calibration to process the null power spectrum into an exclusion limit on SM photon/DP coupling ε in the 0.2-1.2 μeV mass range (equivalent to the 50-300 MHz frequency range). A 95% exclusion limit on ε between 6 × 10−15 and 6 × 10−13 is reported, tracking the complex resonant mode structure in the shielded room. Run 1A extends the results of our 2021 pilot experiment (Godfrey et al. 2021), which was designed to demonstrate the feasibility of the DER technique. The pilot experiment was conducted over the same mass range as the experiment reported here, but did not use the calibration techniques to approximate statistical uniformity, nor did it fully account for the resonant enhancement of the cavity. This detection technique can be applied at higher DP masses, with planned runs up to 60 μeV. Statistical uniformity can be improved and automated using a mode stirrer, a com- mon practice in the field of EMC. Additionally, a dual-conversion superheterodyne receiver has been designed and tested, greatly extending the maximum frequency of the spectrum analyzer. The estimated reach for these runs is described. New antennas and microwave electronics could enable sensitivity at an additional order of magnitude in mass beyond this. Additionally, smaller cryogenically cooled cavities and low noise amplifiers could improve sensitivity to ε by an order of magnitude. Considerations for designing these types of exper- iments are described. This work was supervised by Professors J. Anthony Tyson, S. Mani Tripathi, and Brian H. Kolner