Towards the development of the ferromagnetic axion haloscope

The axion is an hypothetical beyond the Standard Model particle, first introduced in the seventies as a consequence of the strong CP problem of QCD. Axions can be the main constituents of the galactic Dark Matter halos. Their experimental search can be carried out with Earth-based instruments immersed in the Milky Way’s halo, which are therefore called “haloscopes”. Nowadays haloscopes rely on the inverse Primakoff effect to detect axioninduced excesses of photons in a microwave cavity under a static magnetic field. This thesis describes the process leading to the successful peration of a ferromagnetic axion haloscope, which does not exploit the axion-to-photon conversion but its interaction with the electron spin. The study of the axionspin interaction and of the Dark Matter halo properties yields the features of the axionic signal, and is fundamental to devise a proper detector. A scheme of a realistic ferromagnetic haloscope is drawn to realize the challenges of its development. It emerges that there are a number of requirements for a this setup to get to the sensitivity needed for a QCD-axion search. These are kept in mind when designing the prototypes, to overcome the problems without compromising other requirements. A state-of-the-art sensitivity to rf signals allows for the detection of extremely weak signals as the axionic one. The number of monitored spins is necessarily large to increase the exposure of the setup, thus its scalability is a key part of the design process. A ferromagnetic haloscope consists in a transducer of the axionic signal, which is then measured by a suitable detector. The transducer is a hybrid system formed by a magnetic material coupled to a microwave cavity through a static magnetic field. Its two parts are separately studied to find the materials which match the detection conditions imposed by the axionsignal. The detector is an amplifier, an HEMT or a JPA, reading out the power from the hybrid system collected by an antenna coupled to the cavity. A particular attention is given to the measurement of the noise temperature of the amplifier. As it measures variation in the magnetization of the sample, the ferromagnetic haloscope is configured as a spin-magnetometer. Three different prototypes of increasing sensitivity make up the part of the thesis dedicated to physics results, namely limits on the axionic Dark Matter field. For every prototype it is verified that larger sample dimensions do not compromise the signal transduction or increase the noise. The working temperature of the haloscope ranges from the 300 K of the first device, to 90 mK of the last one. In every step the noise temperature is also decreased. The final prototype reached the sensitivity limit imposed by quantum mechanics, the Standard Quantum Limit, and can be improved only by quantum technologies like single photon counters. The haloscope embodies a large quantity of magnetic material, i. e. ten 2 mm YIG spheres, and is designed to be further up-scaled. The quantum-limited ultra cryogenic prototype meets the expectations, and, to present knowledge, is the most sensitive rf spin-magnetometer existing. The minimum detectable field results in 5.5 × 10−19 T for 8 h integration, and corresponds to a limit on the axion-electron coupling constant gaee ≤ 1.7 × 10−11. This result is the best limit on the DM-axions coupling to electron spins in a frequency span of about 150 MHz, corresponding to an axion mass range from 42.4 µeV to 43.1 µeV. The efforts to enhance the haloscope sensitivity include improvements in both the hybrid system and the detector. The deposited axion power can be increased by means of a larger material volume, possibly with a narrower linewidth. To overcome the standard quantum limit of linear amplifiers one must rely on quantum counters. Novel studies on microwave photon counters, together with some preliminary results, are reported. Other possible usages of the spin-magnetometer are eventually discussed.