A search for galactic axions with a tunable dielectric cavity in the QUAX–aγ experiment

The axion of quantum electrodynamics (QED), which was proposed by Peccei and Quinn to solve the strong charge-parity problem in the standard model (SM) of particle physics, is currently the best motivated cold dark matter (CDM) candidate. The sensitivity required to probe its existence needs the utilisation of resonant detectors, and in particular an high quality factor microwave cavity, which is readout by a very low noise amplifier at the first stage of the electronic amplification chain. The cavity is a cylinder cavity hosting a sapphire shell, designed to obtain high effective volumes at high frequency and quality factors larger than those achievable with empty copper cavities. Furthermore, the need to scan over a wider frequency range requires the cavities to be tunable, via the opening of the copper cylinder with a clamshell mechanism. The receiver that will be employed to readout the cavity is based on a traveling wave parametric amplifier (TWPA), a superconducting amplifier devised to introduce minimum noise, barely exceeding the standard quantum limit (SQL), the level allowed by quantum mechanics. In this thesis work I show the characterisation of the QUAX dielectric cavity and the working principle of its tuning system at cryogenic temperature. The cavity is then mounted at the lowest stage of the QUAX dilution refrigerator, immersed in a 8T magnetic field, and operated to search for axions with a mass of about 42 μeV. After the characterization of the electronic amplification chain, the experiment is run for a two-week period in two different ignitions: one used as a check for the tuning mechanism and the second as the effective data taken. This procedure allowed a scan of circa 3 MHz per day. After the run I analyzed the data using a procedure similar to the HAYSTAC experiment for both the preventive analysis and the upper limit estimation for the coupling constant gaγγ.