Mathematics has long served as a guiding tool in theoretical physics. Symmetries, mathematical 'beauty' and the notion of 'naturalness' have long been successfully used to predict new phenomena in physics to be verified experimentally later. Although we may never know whether the universe actually cares about our funny intuitions, in the lack of empirical evidence to the contrary we tend to stick to such approaches. Arguably in the infinite space of mathematical avenues physics could express itself, this may be considered an application of Occam's razor to physics. In a sense it underlines the intersection between philosophy, mathematics and physics and indicates that the term 'philosophy of nature' is not actually wholly inapplicable to modern physics. And so we arrive at one of our current representations of this in the form of an angle \(θ\) in quantum chromodynamics. Measurements seem to indicate this angle is essentially zero. However, we tend to reject the idea that our universe simply is such that \(θ\) happens to be close to zero. Instead we derive the simplest explanations as to why this might be the case. And who can blame us when the result of accepting \(θ = 0\) by nature would lead us precisely nowhere? Finding an explanation spawns a new hypothetical friend in our zoo of particles, the axion. The point of this thesis is to continue the search for this zoo member by way of staring into the core of the Sun. With the help of a very large number of virtual photons we will attempt to entice some axions to become real X-rays, directly detectable by us. And if we fail in this quest, we can put our philosopher's hat back on and muse about how little our axion friends want to dance with our virtual photons.

After a short side note about this thesis as a document in chapter 3, we introduce the theoretical foundation of axion physics in chapter 4. From a historical standpoint as to why axions were invented in the first place to the avenues of detection and related, the expected (model dependent) solar axion flux. We will see that the Sun acts as a strong source of axions in the soft X-ray energy range.

This leads to chapter 5, which fully introduces the concept of an axion helioscope as a way to potentially detect axions of a solar origin. A large magnet is used as a solar telescope in an attempt to reconvert solar axions into photons, X-rays. In particular it introduces the CERN Axion Solar Telescope (CAST) as the experiment at the center of this thesis. Its successor, the International AXion Observatory (IAXO) will also be introduced.

With an understanding of possible detection mechanisms for axions, we will focus next on the required hardware to actually measure axions indirectly. That is, via gaseous detectors for X-ray detection thanks to the axion energy spectrum from the Sun. In chapter 6 we cover the relevant physics related to X-ray interactions with matter and gaseous detector physics.

Next we introduce the detector used in this thesis, the 'Septemboard' 7-GridPix detector, in chapter 7. Here we first introduce the concept of a 'Micromegas detector', which our GridPix detectors are a type of. We will motivate the different detector features this 7-GridPix detector has over the single GridPix detector used previously.

From the hardware of the detector the reader may optionally go over to the data acquisition software and the monitoring tools used during the CAST data taking campaign, in appendix 17.

With a fully operational detector in mind, we then introduce the software to reconstruct and analyze data taken with this detector in chapter 9. We discuss cluster finding in the GridPix data, calculation of geometric properties of clusters and reconstruction of FADC spectra.

Then it is finally time to show the installation of the Septemboard detector at the CAST experiment in chapter 10, talk about potential issues encountered which affect data quality and summarize the total data taken, which will later be used for a limit calculation.

Data taken at CAST still needs to be processed and further calibrated to be useful, done in chapter 11. This includes mitigating effects of slight detector instabilities and calibrating the data in energy.

At that point we have everything ready to try and filter the entire dataset to the most X-ray like clusters. When applied to the background dataset this yields our irreducible background rate of events that are either real X-rays due to non-axion sources or other type of X-ray like data. Applied to the axion-sensitive solar tracking dataset the same techniques yield a set of axion candidates. The different classification techniques and how all detector features are used for this purpose is explained in chapter 12.

This finally brings us to chapter 13, in which we introduce our method to evaluate our axion-sensitive solar tracking dataset against our background dataset. This is done using a Bayesian extended likelihood approach. We will mainly compute a limit on the axion-electron coupling constant \(g_{ae}\). But secondarily, we will also consider the axion-photon coupling constant \(g_{aγ}\) and the chameleon-photon coupling \(β_γ\), a separate hypothetical particle. We will compare each obtained limit with the current best limits.

As a concluding outlook we will discuss potential improvements on different levels possible for future detectors and physics searches in chapter 14. The lessons learned in this thesis will be summarized to give ideas about which aspects should be emphasized more for future data taking campaigns and which techniques might be worthwhile to investigate for possible improvements to the background rate and similar. This will be placed into context for a potential Timepix3 based detector in the future.

We finally summarize the results and conclude in chapter 15.