Axion Dark Matter Experiment at High Frequency (ADMX-HF)

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Multiple astronomical observations have established that about 85% of the matter in the universe is not made of known elementary particles. Deciphering the nature of this so-called Dark Matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. One of the most exciting quests is the search for new particles beyond the Standard Model of Particle Physics, which describes all the known elementary particles and the interactions between them. Extensions of the model predict not only new particles with large masses but also some with very small masses. Such a candidate for the latter is the axion, which has been introduced to explain the smallness of the violation of the Charge-Parity (CP) symmetry in Quantum Chromodynamics, the theory describing the action of the strong force, for example the interactions between protons and neutrons that make up atomic nuclei. The axion turns out to also be a prime candidate for a constituent of the dark matter in the universe. Axions constituting the dark matter of our Milky Way halo may be detected through their conversion into a narrow Radio-Frequency signal in a microwave cavity permeated by a magnetic field. Searching for this signal is the goal of this project.R&D on superconducting thin-films together with squeezed-state and single-photon detection in quantum electronics will certainly produce unanticipated spin-offs. ADMX-HF at Yale, has already proven to be a great attractor for young talent, with roughly half being women. The experiment provides a focal point for the PI's outreach activities with high school teachers in the greater New Haven area. It is planned to establish a permanent exhibit on dark-sector science at the Yale Peabody Museum, one of the foremost natural history museums in the world.This experiment represents the inaugural use of both Josephson Parametric Amplifiers (JPAs) in the microwave cavity search, and operation with a dilution refrigerator. The ADMX-HF PIs have laid out a future strategy for improving sensitivity, showing that, for above 10 GHz, an optimized search will require transitioning from linear amplifiers to photon detection schemes that can evade the Standard Quantum Limit (SQL) noise. The experiment's plans include: a) reducing the system noise below the SQL, initially by incorporating a JPA-based squeezed-vacuum state receiver; eventually, photon detector systems based on superconducting qubits will be developed and deployed; b) implementing a Photonic Band Gap (PBG) structure as an alternative to a traditional microwave cavity; if successful it will completely eliminate the forest of TE modes interfering with the TM mode of interest, which have been a bane to fast and efficient mass coverage; and c) boosting the cavity Q by one of two avenues: Incorporating multilayer structures of thin-film Type-II superconductors into the cavity design, or the use of Distributed Bragg Reflector cavities based on nested dielectric cylinders. R&D on the fabrication and characterization of planar thin-films of NbTiN, a Type-II superconductor is showing great promise. Technical deliverables expected are to (a) extend the mass range of the search to 12 GHz (50 micro eV), and (b) deliver an improvement in power sensitivity by a factor of five, and thus approach sensitivity to DFSZ axions. This will dramatically improve our discovery potential in the most promising mass region.