Introduction

The Standard Model of particle physics provides a 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 experiment must align a strong magnet with the Sun, focus any converted photons onto a small detector area, and identify a possible excess of keV X-rays above a 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 the IAXO experiment [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 studies the IAXO-D0 and IAXO-D1 Micromegas prototypes to inform the BabyIAXO detector design. 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.

The central experimental task is to identify the sources of X-ray-like background, evaluate how reconstructed observables reject these events, and measure the X-ray acceptance of the resulting selection. This requires following an interaction through charge collection, waveform reconstruction, and event selection. The relevant observables include energy, position, track multiplicity and shape in the TPC, together with prompt signals, delayed activity and panel multiplicity in the veto. They connect the detector measurements to the background prediction.

In this thesis, we address this task through three studies. First, we present methods for comparing measured and simulated events, including our contributions to REST-for-Physics, its Geant4 interface restG4, detector-response models, and large Monte Carlo production. Second, we study cosmic-ray backgrounds, particularly neutrons, and a multilayer plastic-scintillator–cadmium veto. We cover its optimization, waveform observables, construction, commissioning, and comparison with data. Third, we build a background inventory by source for IAXO-D1 in argon–isobutane using material radioactivity, environmental measurements, cosmic-ray models, and detector-response simulations. Predicting the BabyIAXO background additionally requires its xenon–neon response, focusing optics, final veto, and radiation fluxes and material activities at DESY.

These studies mainly use REST detector-response processes. We also developed a complementary model coupling Geant4, Heed, and Garfield++ to the same experimental readout and native REST reconstruction. Its refined version follows electrons through a local Micromegas field and assigns their anode arrivals to readout channels. We report numerical checks of this refined model separately from calibrated comparisons of earlier versions with data, examining reconstructed observables and selection acceptance.

The background simulations use these reconstructed observables whenever a detector-response calculation is available. This treatment is especially important for surface operation, where muons, high-energy neutrons, and secondaries produced in the shielding can leave X-ray-like deposits despite the passive shielding [17].

The chapters follow the progression from the physics motivation to detector technology, simulation methods, and background studies. Section 1 connects the axion and axion-like-particle motivation, the strong $\text{CP}$ problem, and the main experimental search methods to the solar helioscope technique and the IAXO experiment. The solar signal, conversion probability, helioscope figure of merit, and role of BabyIAXO establish the detector requirements and the experimental context of this work. Section 2 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. It closes by following detector signals to the X-ray candidates used in the analysis. Section 3 defines the common reconstructed observables, TPC baseline and multivariate selection method before comparing selection acceptance or applying the analysis to the veto and background studies. It also describes the simulation and reconstruction framework, detector-response chain, data production, visualization, and related software developments. Section 4 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. Section 5 applies the common selection to the IAXO-D1 argon source-response model, including intrinsic, environmental, cosmic, and neutron-induced activation contributions, 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.