Introduction

The Standard Model of particle physics provides an exceptionally successful description of the known elementary particles and their interactions. Nevertheless, several observations and theoretical questions point to physics beyond this framework. Among them, the nature of dark matter and the absence of observed $\text{CP}$ violation in the strong interaction remain two of the most compelling open problems. The axion was originally proposed as a dynamical solution to the strong $\text{CP}$ problem, but it also emerged as a well-motivated dark-matter candidate [1–7]. More generally, axion-like particles appear naturally in many extensions of the Standard Model and can be searched for through their weak couplings to photons, electrons, and nucleons [8–10].

Solar axion helioscopes exploit one of the most direct experimental signatures of these particles. If axions are produced in the solar interior, they can traverse the Sun and the interplanetary medium essentially unattenuated. Inside a strong transverse laboratory magnetic field, a small fraction can convert coherently into X-ray photons. The experimental task is therefore conceptually simple but technically demanding: point a powerful magnet toward the Sun, focus any converted photons onto a small detector area, and identify a possible excess of keV X-rays above an extremely low background [11, 12].

The International Axion Observatory (IAXO) is designed as the next major step in this technique, building on the experience of previous helioscopes and especially on the CERN Axion Solar Telescope (CAST) [13]. BabyIAXO is the intermediate stage of this program [14]. It is intended to validate the main technologies required for IAXO while also operating as a competitive helioscope in its own right. The March 2026 outdoor working baseline brings low-background X-ray detection, solar tracking, and mechanical integration together in a surface environment [15]. Its detector development must therefore address cosmic-ray backgrounds under realistic operating conditions.

This thesis focuses on the IAXO Micromegas detector program, spanning IAXO-D0, IAXO-D1, and the separate BabyIAXO projection. Microbulk Micromegas detectors are well suited to helioscope searches because they combine low intrinsic radioactivity, good energy response in the keV range, topological discrimination, and compatibility with compact shielding and focusing optics [16, 17]. However, the expected signal rate is extremely small. The physics reach of the experiment therefore depends not only on detector performance, but also on the reliability of the background model, the realism of the detector-response simulation, and the effectiveness of the shielding and active veto strategy.

The work presented here addresses these requirements from three connected directions. First, it describes contributions to the software and simulation infrastructure used by the collaboration, with particular emphasis on REST-for-Physics, its Geant4 interface restG4, and the production workflows needed for large Monte Carlo campaigns. Second, it develops a source-resolved background inventory for IAXO-D1 in argon–isobutane, combining radiopurity information, environmental measurements, cosmic-ray models, and detector-response simulations. A BabyIAXO prediction additionally requires its xenon–neon response, focusing optics, final veto, and site-specific DESY source environment. Third, it studies the surface-level cosmic-ray-induced background and the corresponding active veto system, including the optimization of a multilayer plastic-scintillator and cadmium design, waveform-level veto observables, construction and commissioning aspects, and comparison with experimental data.

A central theme of the thesis is the transition from idealized background estimates to analysis objects that can be compared with real detector data. The relevant question is not only whether a simulated particle deposits energy in the detector volume, but whether the resulting event would pass the same energy, topology, timing, and veto selections applied to the experimental data. For this reason, the simulations are propagated through a detector-response chain whenever possible, and the veto studies are expressed in terms of prompt signals, delayed activity, channel multiplicity, and reconstructed observables. This approach is especially important for surface operation, where muons, high-energy neutrons, and secondary particles produced in the shielding can generate backgrounds that are not adequately described by passive shielding arguments alone [17].

The structure of the thesis follows this logic. Chapter 1 introduces the axion and axion-like-particle motivation, the strong $\text{CP}$ problem, and the main experimental approaches used in axion searches. Chapter 2 describes the IAXO program, the helioscope figure of merit, the role of BabyIAXO, and the experimental context in which the detector work is carried out. Chapter 3 presents the Micromegas detector technology, the IAXO-D0 and IAXO-D1 detector prototypes and their relation to BabyIAXO, and the associated gas, high-voltage, slow-control, data-acquisition, and calibration systems. Chapter 4 describes the computational framework used in the thesis, including ROOT, REST-for-Physics, restG4, data production, visualization, and related software developments. Chapter 5 studies the shielding and veto system, with emphasis on cosmic-ray-induced backgrounds, passive-shielding limitations, the active scintillator–cadmium veto concept, and the comparison between simulations and prototype data. Chapter 6 presents the IAXO-D1 argon source-response model, including intrinsic, environmental, cosmic, and neutron-induced activation contributions, the common selection, the generated-primary denominators, and the unresolved inputs needed for an absolute total. The conclusions assess the established results and prioritize the response and source-validation studies needed for IAXO-D1 and the subsequent BabyIAXO projection.