Benjamin Knepper

Project abstract

Dark matter, constituting 85% of the mass content of the universe, remains to be experimentally observed. Particle dark matter candidates such as dark photons or axions can, however, convert into observable photons at conductive surfaces and under strong magnetic fields. This property motivates direct detection experiments to search for particle dark matter masses via their corresponding electromagnetic frequencies. The Broadband Reflector Experiment for Axion Detection (BREAD) acts like a telescope for dark matter, with an outer cylindrical barrel and inner coaxial parabolic reflector that focuses dark matter-converted photons onto a photosensor, in the frequency range of [0.01, 240] THz. The present work sought to characterize and enhance the detection efficiency in BREAD at the two extremes of this frequency range—both in InfraBREAD and GigaBREAD. InfraBREAD will search for infrared photons using a 1mm^2 single-photon counting quantum photosensor, but at this high-frequency scale photons are emitted incoherently which results in a problem: the focal spot becomes smeared across an area larger than the detector, which experimental misalignments can exacerbate. This first project performed ray-tracing simulations of novel optical configurations of lenses and reflectors to refocus the smeared signal even under millimeter-scale displacement shifts. One configuration with parabolic reflector optics improved the average detection efficiency by 10%, with a maximal 55% increase in one displaced region. GigaBREAD, by contrast, uses a horn antenna photosensor to search for gigahertz frequency photons, which are emitted coherently and form resonant standing waves between the inner reflector and detector. This second project measured the blackbody thermal radiation spectrum of the GigaBREAD apparatus to both calculate its system noise temperature background and confirm sensitivity to detecting gigahertz resonances for its first-ever data taking run. Subsequent radio-frequency measurements and signal processing analyses then led to the world-leading dark photon exclusion limit of 10^-12 in [10.7, 12.5] GHz.

Stefan Knirck, Andrew Sonnenschein