A wide array of cosmological and astrophysical observations support the existence of dark matter. More precisely, temperature anisotropy measurements of the cosmic microwave background (CMB) estimate that the current dark matter mass density is about five times that of the visible Universe. However, the nature of dark matter is not yet understood, inspiring numerous theoretical candidates. One popular candidate is the weakly-interacting massive particles or WIMPs that interact with standard model particles on the electroweak scale and could have the correct relic abundance today. Experiments such as XENON1T and XENONnT are designed to search for WIMPs on Earth using the dual-phase liquid xenon Time Projection Chamber (LXeTPC) technology. The XENON1T experiment operated until Dec. 2018 and had made the world-leading upper limits for WIMP-nucleus interactions at the time. Its successor, the XENONnT experiment, has been commissioned since 2021 and has taken data for its first science run. This thesis presents the commissioning data and the first science-run data analysis. This thesis describes an essential facet of the XENON1T and XENONnT experiments: how, step by step, the most elementary signals of single photons are reconstructed into events. Each event represents a particle interaction in the detector, including those from rare physical processes. This includes several technical developments with signal processing and simulation software that enable accurate reconstruction of signals and precisely evaluate the effect of various types of remaining miss-reconstruction. Furthermore, this thesis will present two analyses developed to search for rare events in XENON1T, only possible with an accurate and precise understanding of the event reconstruction. One is to search for ⁸𝐁 Solar neutrino events via 𝐂𝐄𝜈𝐍𝐒 process and low mass WIMPs by characterizing reconstruction efficiency and additional background at a lower energy threshold. The spin-independent DM-nucleus interaction is improved in the mass range between 3𝐆𝐞𝐕𝑐² and 11𝐆𝐞𝐕𝑐² by as much as an order of magnitude from the previous world-leading result, using data from the XENON1T experiment. The other is the search for the neutrinoless double-beta decay at its 𝑄-value, 𝑄_𝛽𝛽 = (2457.83$\pm$0.37)\,keV. The analysis demonstrated that the relative energy resolution at one 𝝈/𝝁 is as low as (0.80±$0.02) % in its one-ton fiducial mass, and for single-site interactions at 𝑄_𝛽𝛽, a world-leading resolution in 𝐋𝐗e experiment that enhance the experimental sensitivity to the neutrinoless double-beta decay events.

The General theory of Relativity was first introduced by Albert Einstein. There have been many attempts to unify General Relativity with the Standard Model of Physics and many of these try to do so by modifying General Relativity slightly. One way to do this is to add a mass to the graviton. Such a theory was proposed by Fierz and Pauli. However, a massive gravity theory suffers from the vDvZ discontinuty where taking the masss of the graviton to zero does not reproduce the results of General Relativity exactly. This can, to some extent, be resolved via the Vainshtein mechanism, where General Relativity can be reproduced within a certain radius from a source, called the Vainshtein radius. Another modification that can be imagined, is to add a second metric. However, doing this results in extra degrees of freedom which manifest as a Boulaware Deser ghost. The bimetric action which avoids the Boulaware Deser ghost was first introduced by Hassan and Rosen in 2011. In this theory, only one of the metrics couples to standard model matter to avoid the ghost. In this scenario, the propagating massless and massive spin-2 modes turn out to be linear combinations of the two metrics, just as in neutrino mixings. In this thesis, we review some works which investigate the oscillations between the massless and massive modes and the implications for gravitational waves. In particular we consider the bounds on the parameters of the theory based on the fact that evidence for such oscillations have not been observed by LIGO. We use a new LIGO result to extend these bounds. We also review an investigation which explores the possibility that the dark matter particle could be the massive particle of bimetric gravity. / Den allmänna relativitetsteorin introducerades först av Albert Einstein.Många har försökt förena allmän relativitetsteori med partikelfysikensstandardmodell och många av dessa försök gör detta genom att lägga tillen massa för gravitonen. En sådan teori föreslogs av Fierz och Pauli.Massiv gravitation lider dock av vDvZ-diskontinuiteten där gränsen närgravitonmassan går mot noll inte reproducerar allmän relativitetsteori.Detta kan, till viss del, lösas genom Vainshteinmekanismen, där allmänrelativitetsteori kan reproduceras inom ett visst avstånd från källan,kallat Vainshteinradien. En annan modifikation som kan komma på fråga äratt lägga till en andra metrik. Att göra detta leder dock till nyafrihetsgrader som yttrar sig som ett Boulaware-Deser-spöke. Denbimetriska verkan som undviker Boulaware-Deser-spöket introduceradesförst av Hassan och Rosen år 2011. I denna teori kopplar enbart en avmetrikerna till standardmodellen vilket gör att spöket kan undvikas. Idetta scenario visar sig det masslösa och det massivapropagationsegentillstånden vara linjärkombinationer av de tvåmetrikerna i analogi med neutrinoblandning. I detta arbete går vi igenomnågra arbete som undersöker oscillationerna mellan de två metrikerna ochimplikationerna för gravitationsvågor. Speciellt kommer vi att betraktade begränsningar som finns på teoriparametrarna baserat på det faktumatt LIGO inte observerat några bevis för sådana oscillationer. Vianvänder också nya LIGO-resultat för att utöka dessa begränsningar. Vidiskutera också möjligheten att mörk materia skulle kunna bestå av denmassiva gravitonen i bimetrisk gravitation.

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This thesis describes research I conducted within the XENON program of dark matter searches. In particular, I focus on contributions I made to the development of a novel system for purifying liquid xenon employed in XENONnT, to the reconstruction and modeling of electronic and nuclear recoil signals by fitting calibration data, and in the employment of these tools to world-leading physics searches for spin-dependent DM-nucleus scattering and coherent neutrino-nucleus scattering from boron-8 solar neutrinos.

Everything that humans have ever touched, created or built something from consists of a type of matter that only makes up 15 percent of the total matter in the universe. The remaining 85 percent is attributed to dark matter, a so far not discovered and non-luminous type of matter. In this thesis a potential dark matter particle candidate has been studied by investigating an extension of the SU(2) symmetry into a dark gauge sector, where the new sector is connected to the standard model through a vector-like fermion portal. In order to understand how such an extension is made, the Lagrangian density of the standard model and its different gauge sectors were derived. The cross sections of the process of pair production of dark matter particles and tau leptons in the final state due to proton-proton collisions at the LHC was simulated with the software \texttt{MadGraph}. The cross sections were used to draw significance contours for the exclusion and discovery regions for parts of the parameter space of the new model, for current and projected luminosities of the LHC. The projected luminosity scans also consider how lowering the uncertainty in the number of background events through hypothetical improvements to detectors would impact the exclusion and discovery contours. The significance contours were combined with relic density constraints, derived from comparisons between measurements of the Planck telescope and calculations from the software \texttt{MicrOMEGAs}. The resulting graphs show that there are non-forbidden regions of the parameter space that are significant for exclusion and discovery for luminosity of current searches. Increasing the luminosity while keeping the uncertainty in the number of background events the same yielded only minor increases to the exclusion and discovery contours. Combining the projected luminosities with improvements to the background uncertainty instead produced exclusion and discovery regions that were significantly larger than those for the current luminosity.