Commissioning and first results of the SUPAX experiment and prospects for detecting high-frequency gravitational waves with GravNet

Abstract

The axion is a hypothetical particle originally proposed to solve the strong CP problem. Incidentally, its properties also make it a very promising cold dark matter (CDM) candidate. In haloscope experiments, CDM axions are searched for via their conversion into photons in the presence of a strong magnetic field, where the resulting signal is enhanced by a resonant cavity in a cryogenic environment and detected using a low-noise readout chain.

This thesis reports on the first operation and results of the SUPAX experiment, a superconducting axion search experiment, and the proposed GravNet detector network, which intends to utilize existing axion haloscope infrastructure to search for High-Frequency Gravitational Waves (HFGW) originating from another dark matter candidate: Primordial Black Holes (PBH).

During several experimental runs, SUPAX has studied the feasibility of Niobium Nitride (NbN) as a superconducting coating with and without the application of an external magnetic field, and provided an exclusion limit for an axion mass range of $\pm 0.72 \, \mathrm{MHz}$ around $8.26548 \, \mathrm{GHz}$ ($m_a \approx 34.18 \,\mathrm{\mu eV}$) down to $g_{a \gamma \gamma} > 1.67 \cdot 10^{-13} \, \mathrm{GeV}^{-1}$. This limit is recast into a limit on dark photons (DP), another CDM candidate, yielding an exclusion down to $\chi > 1.97 \cdot 10^{-14}$. Another experimental run without an external magnetic field resulted in a limit on the DP coupling down to $\chi > 8.78 \cdot 10^{-15}$ around $8.4712 \, \mathrm{GHz}$ ($m_\mathrm{DP} \approx 35 \, \mathrm{\mu eV}$).

Finally, a software package has been developed to simulate the output of the current data acquisition chain of GravNet for HFGW signals originating from PBH mergers. This way, analysis techniques can be tested and developed before the experimental setup is completed.