Phd in PHYSICS

Thesis title: The demonstrator of the BULLKID detector for dark matter and neutrinos

One of the open problems of particle physics in this century is represented by Dark Matter. Postulated nearly a century ago, the existence of Dark Matter is supported by many astrophysical and cosmological observations to the point that a precise measurement of its abundance is available. During the last decades many experiments where designed and run to attempt Dark Matter characterization, either via direct or indirect detection, or by producing it in colliders. In particular, many experiments focusing on direct detection aim to reveal the presence of Dark Matter candidates such as Weakly Interacting Massive Particles (WIMPs) by measuring the nuclear recoil that follows the elastic WIMP-nucleon scattering. As many experiments designed to detect WIMPS of mass above 10 GeV did not find positive evidence, the scientific community is also aiming to probe lighter dark matter candidates by increasing the sensitivity to lower recoil energies, with many collaborations choosing to deploy low threshold cryogenic sensors. As a consequence of their excellent resolution, cryogenic solid state particle sensors also find application in the quest for the precise measurement of Coherent Elastic Neutrino-Nucleus Scattering (CEνNS). This process involves the elastic scattering of a low energy neutrino with a target nucleus mediated by the exchange of a neutral current, and can be detected by observing the recoil energy of the target nucleus, similarly to WIMPS. The process, observed for the first time in 2017 by the COHERENT collaboration, is of particular interest both as a test for standard model parameters and for civil applications such as nuclear waste remote monitoring: more precise measurements of the cross section are needed. The focus of this thesis revolves around the design, development and characterization of BULLKID (BULky and Low-Threshold Kinetic Inductance Detectors), a cryogenic and highly scalable nuclear recoil detector for low threshold applications such as WIMP direct detection and observation of reactor antineutrino CEνNS. The detector is an array of silicon targets sensed by multiplexed Kinetic Inductance Detectors (KIDs), the detection principle consists in sensing athermal phonons produced in the crystal by particle interactions. In this thesis we discuss the design and operation of a demonstrator of the detector, designed to prove the feasibility of achieving mass scalability by combining many identical arrays stacked one on top of the other. In Chapter 1 are discussed the Dark Matter candidates and the neutrino interaction that BULLKID is designed to reveal. In Chapter 2 are introduced the fundamentals of superconductivity and the working principles of KIDs, while Chapter 3 describes the BULLKID array design, its principle and the data analysis leading to the characterization the prototype of the device. The performance of the array is evaluated in terms of energy resolution, demonstrating the acquisition of above ground background data down to an analysis threshold of 160 eV. In Chapter 4 is explored the scaling of the array from a single wafer device to a vertical stack of identical units, with the fabrication and operation of a first demonstrator consisting of 3 wafers. Lastly, in Chapter 5 we discuss the fabrication process of the phonon detectors, as well as additional optimization of the sensors such as modification of the their design to further improve the energy resolution, scaling of the array from 3-inch wafers (76.2 mm) to 100 mm wafers and porting the BULLKID technology on germanium substrate to potentially assemble a multi-target payload.