Le travail présenté dans cette thèse porte sur la recherche de neutrino stérile à l'aide de désintégrations β dans les expériences SOX et TRISTAN. Le neutrino stérile est une particule hypothétique, solidement établi théoriquement, qui ne prendrait part à aucune interaction fondamentale, gravité mise à part. Étant entendu que le neutrino stérile se mélange avec les neutrinos actifs connus, l'existence de ces premiers peut être étudiée directement en laboratoire. L'expérience SOX a été conçue pour explorer l'existence d'un neutrino stérile d'une masse autour de l'électronvolt (eV). Un neutrino stérile avec une telle masse permettrait d'expliquer plusieurs anomalies observées à courte distance de sources (quelques mètres) lors de mesures d'oscillations de neutrinos de basses énergies (quelques MeV). SOX avait pour projet d'utiliser le détecteur de neutrinos solaire déjà existant Borexino, et d'observer un signal d'oscillation vers le stérile à l'intérieur même du volume actif du détecteur. La source radioactive de 5.5 PBq et positionnée à 8.5 m du centre du détecteur, émettrait des antineutrinos électroniques via la désintégration β du ¹⁴⁴Ce et du ¹⁴⁴Pr. Une des clés de l'observation de cette oscillation, est la connaissance précise de l'activité de la source. Une telle activité peut être déterminée en mesurant la chaleur dégagée par la source. C'est la raison pour laquelle l'INFN Genova et la TUM ont développé conjointement un calorimètre dédié. La chaleur dégagée par la radioactivité est alors captée par un échangeur puis transmise à un circuit d'eau étroitement contrôlé. Le calorimètre a été assemblé, optimisé puis étalonné avec succès. La perte de chaleur du circuit fut déterminée lors des mesures d'étalonnage grâce à un chauffage électrique. Des variations des conditions expérimentales et une isolation thermique sophistiquée ont permis d'opérer avec des pertes de chaleur négligeables. Il a ainsi été démontré que la puissance thermique de la source pouvait être estimée, en 5 jours seulement, avec une précision supérieure à 0,2%. Malheureusement, le programme SOX a dû être annulé. Le projet TRISTAN, quant à lui, tend à démontrer l'existence d'un neutrino stérile avec une masse de l'ordre du kilo-électronvolt (keV). Si le neutrino stérile à l'eV tente d'apporter une réponse aux différentes anomalies observées lors de mesures d'oscillation, le neutrino stérile au keV, en tant que potentiel candidat matière noire. Le projet TRISTAN cherche à mesurer l'empreinte de ce nouvel état de masse sur le spectre du tritium dans le cadre de l'expérience KATRIN. Cette dernière vise à déterminer la masse effective du neutrino (actif) en mesurant l'extrémité du spectre de tritium avec une excellente résolution et un faible taux de comptage. Une fois la mesure achevée, le détecteur de KATRIN sera modifié afin d'effectuer une mesure différentielle et intégrale de l'ensemble du spectre en tritium: c'est le projet TRISTAN. Le détecteur actuel sera remplacé par un nouveau détecteur de silicium à dérive (SDD) de 3500 pixels permettant une résolution de 3% à 6 keV et pouvant supporter un taux de comptage montant jusqu'à 10⁸ coups par seconde, activité maximum attendue. Un prototype a été testé avec succès et une première mesure de tritium a été réalisé au spectromètre de masse neutrino Troitsk afin d'étudier les erreurs systématiques et de développer des méthodes d'analyses pertinentes. Un premier ajustement cohérent du spectre tritium différentiel acquis lors de cette installation, a démontré la faisabilité du projet. TRISTAN lui-même est toujours en cours de développement mais les caractérisations du détecteur et les études de systématiques sont plus qu'encourageantes pour la poursuite du projet. La première investigation de neutrino stérile avec le détecteur de TRISTAN sur le site de KATRIN est prévue après la mesure de masse, en cours à Karlsruhe, aux alentours de 2024. / The work presented in this thesis is about the sterile neutrino search with the two experiments SOX and TRISTAN based on the β-decay. Sterile neutrinos are theoretically well motivated particles that do not participate in any fundamental interaction except for the gravitation. With the help of these particles one could elegantly explain the origin of the neutrino mass, dark matter and the matter-antimatter asymmetry in the universe. As sterile neutrinos can mix with the known active neutrinos, they could be discovered in laboratory searches. The SOX experiment was designed to search for a sterile neutrino with a mass in the eV-range. This particular mass range is motivated by several anomalous observations at short-baseline neutrino experiments that could be explained by an additional oscillation with a length in the order of meters that arises from an eV-scale sterile neutrino. For SOX it was planned to use the existing Borexino solar neutrino detector to search for an oscillation signal within the detector volume. The neutrinos are emitted from a 5.5 PBq electron-antineutrino source made of the β-decaying isotopes ¹⁴⁴Ce and ¹⁴⁴Pr, located at 8.5 m distance from the detector center. For the analysis of the signal it is crucial to know the source activity. This parameter is determined by measuring the decay heat of the source with a thermal calorimeter that was developed by TUM and INFN Genova. The decay heat is measured through the temperature increase of a well-defined water flow in a heat exchanger that surrounds the source. The calorimeter was assembled, optimized and characterized. Heat losses were determined through calibration measurements with an electrical heat source. Adjustable measurement conditions and an elaborate thermal insulation allowed an operation with negligible heat losses. It was proven that the power of a decaying source can be measured with <0.2% uncertainty in a single measurement that lasts ~5 days. Unfortunately the SOX experiment was canceled after a technological problem rendered the source production with the required activity and purity impossible. The TRISTAN project is an attempt to discover sterile neutrinos with masses in the order of keV. In contrast to eV-scale sterile neutrinos that are motivated by several anomalies observed in terrestrial experiments, the existence of sterile neutrinos with masses in the keV range could resolve cosmological and astrophysical issues, as they are dark matter candidates. The TRISTAN project is an extension of the KATRIN experiment to search for the signature of keV-scale sterile neutrinos in the tritium β-spectrum. KATRIN itself is attempting to determine the effective neutrino mass by measuring the end point of the tritium spectrum at low counting rates. The KATRIN setup will be modified after the neutrino mass measurements are finished to conduct a differential and integral measurement of the entire tritium spectrum. This project is called TRISTAN. The current detector will be replaced by a novel 3500-pixel silicon drift detector system that has an outstanding energy resolution of a few hundred eV and can handle rates up to 10⁸ counts per second as they occur when the entire spectrum is scanned. Prototype detectors were successfully tested and first tritium data was taken at the Troitsk ν-mass spectrometer to study systematic effects and develop analysis methods. A successful fit of the differential tritium spectrum proved the feasibility of this approach. TRISTAN itself is still at an early stage, but the detector development and systematic studies are well on track and delivered so far encouraging results. The sterile neutrino search is scheduled after the KATRIN neutrino mass program is finished in ~2024.

Astrophysical observations indicate the existence of dark matter through its gravitational interaction, but since its other interactions remain undetected, its particle nature is still unknown. There are several dark matter candidates, one being a hypothetical particle called axion that can have three types of non-gravitational couplings: electromagnetic, electric dipole moment (EDM), and gradient. This dissertation presents experimental approaches and axionlike dark matter search results from two table-top experiments: Cosmic Axion Spin Precession Experiment (CASPEr-electric) sensitive to EDM and gradient couplings, and Search for Halo Axions with Ferromagnetic Toroids (SHAFT) sensitive to electromagnetic coupling. CASPEr-electric is a resonant search for axionlike dark matter through the induced nuclear spin precession. The experimental approach is measuring nuclear magnetic resonance (NMR) of the heavy atom in a ferroelectric crystal. Experimental setup is characterized using pulsed NMR calibration measurements. Recorded search data that is sensitive to axionlike dark matter is analyzed by optimal filtering and then setting a detection threshold based on the histogram of power spectral density modeled as a Gaussian distribution. The candidates above the threshold are all rejected through statistical fluctuations and scan/re-scan measurements. CASPEr-electric places the upper bounds on the EDM and gradient couplings of axionlike dark matter in the Compton frequency range from 39.1 MHz to 40.2 MHz. SHAFT is a broadband search for axionlike dark matter through the induced oscillatory magnetic field. The resultant magnetic flux is measured with a precision magnetometer called superconducting quantum interference device (SQUID), coupled to a coil placed on the inner surface of a ferromagnetic toroid. After analyzing the search data, all candidates are rejected and SHAFT places a limit on electromagnetic coupling of axionlike dark matter between 3 kHz and 3 MHz Compton frequencies. Finally, coupling limits placed by CASPEr-electric and SHAFT are evaluated in the wider parameter space, and possible future directions that both experiments could take to improve their sensitivities to axionlike dark matter are discussed. / 2022-09-27T00:00:00Z

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We introduce an elastic constitutive model of gravity that enables the interpretation of cosmological observations in terms of established ideas from Solid Mechanics and multiscale modeling. The behavior of physical space is identified with that of a material-like medium called "cosmic fabric," which exhibits constitutive behavior. This cosmic fabric is a solid hyperplate that is broad in the three ordinary spatial dimensions and thin in a fourth hyperspatial dimension. Matter in space is treated as fabric inclusions that prescribe in-plane (three-dimensional) strain causing the transverse bending of the fabric into the fourth hyperspatial dimension. The linearized Einstein-Hilbert action, which governs the dynamics of physical space, is derived from postulating Hooke’s Law for the fabric, and the Schwarzschild metric is recovered from investigating matterabric interactions. At the continuum length scale, the Principle of Relativity is shown to apply for both moving and stationary observers alike, so that the fabric’s rest reference frame remains observationally indistinguishable at such a length scale. Within the Cosmic Fabric paradigm, the structural properties of space at different hierarchical length scales can be investigated using theoretical notions and computational tools from solid mechanics to address outstanding problems in cosmology and fundamental physics. For example, we propose and offer theoretical support for the "Inherent Structure Hypothesis", which states that the gravitational anomalies currently attributed to dark matter may in fact be manifestations of the inherent (undeformed) curvature of space. In addition, we develop a numerical framework wherein one can perform numerical "experiments" to investigate the implications of said hypothesis.

Longstanding evidence from observational astronomy indicates that non-luminous "dark matter" constitutes the majority of all matter in the universe, yet this mysterious form of matter continues to elude experimental detection. This dissertation presents a search for dark matter at the Large Hadron Collider using 139 fb\(^{-1}\) of proton-proton collision data at a centre-of-mass energy of \(\sqrt{s} = 13\,\)TeV, recorded with the ATLAS detector from 2015 to 2018. The search targets a final state topology in which dark matter is produced from the proton-proton collisions in association with a pair of W bosons, one of which decays to a pair of quarks and the other to a lepton-neutrino pair. The dark matter is expected to pass invisibly through the detector, resulting in an imbalance of momentum in the plane transverse to the beam line. The search is optimized to test the Dark Higgs model, which predicts a signature of dark matter production in association with the emission of a hypothesized new particle referred to as the Dark Higgs boson. The Dark Higgs boson is predicted to decay to a W boson pair via a small mixing with the Standard Model Higgs boson discovered in 2012. Collisions that exhibit the targeted final state topology are selected for the search, and an approximate mass of the hypothetical Dark Higgs boson is reconstructed from the particles in each collision. A search is performed by looking for a deviation between distributions of the reconstructed Dark Higgs boson masses and Standard Model predictions for the selected collisions. The data is found to be consistent with the Standard Model prediction, and the results are used to constrain the parameters of the Dark Higgs model. This search complements and extends the reach of existing searches for the Dark Higgs model by the ATLAS and CMS collaborations. / Graduate

<p>Motivated by the future prospects of the Windchime project, we show that even a small prototype array of 7 commercial accelerometers can be used to calculate dark matter limits for the well-known B − L coupled dark photon. As a member of the ultralight sector, the dark photon would be observed in high occupancy as a persistent plane wave characterized by de Broglie wavelength and coupling to the standard model via a hypothesized baryon minus lepton quantum number, g_B−L. Such an interaction can be probed by measuring the differential force or acceleration between two bodies of differing B −L charge-to-mass ratios. This is accomplished for a 7 sensor array of MEMS accelerometers by rigidly coupling all the sensors to a material of known B − L charge. Using a log-likelihood ratio test and Fourier transformed data from the prototype array, we are able to set a limit on g_B−L ∼ 10^−11 within a mass range of 10^{−13}−10^{−12}eV . Setting these noncompetitive limits with real data serves as a proof-of-principle demonstration of the limit-setting procedure used in Windchime future projections for B − L coupled ultralight dark matter. Additionally, this basic setup could be used for future studies on the properties of a detector array. </p>

Although the contribution to the Isotropic Gamma-Ray Background (IGRB) from unresolved extragalactic objects has been studied for many years, its exact composition and origin are as of yet unknown. It is suspected that diffuse processes such as dark matter annihilation contribute to the total IGRB, as well as unresolved gamma-ray emission from Active Galactic Nuclei (AGN), including radio galaxies. Radio galaxies are a source class that emit strongly at radio wavelengths, some of which have also been detected at gamma-ray wavelengths by the Fermi Large Area Telescope (Fermi-LAT), and by very high energy gamma-ray Cherenkov telescopes. It is thought that due to the orientation of their jets, radio galaxies are detected less numerously at gamma-ray energies than blazars. Furthermore, only a small number of radio galaxies have been detected at gamma-ray energies though it is considered that others do as well. It is for these reasons that gamma-ray emitting radio galaxies, an interesting and elusive class of objects, are selected for investigation in this work. In order to reach the goal of better understanding diffuse processes, it is necessary to model the radio galaxy spectral energy distributions (SEDs). As AGN emission is variable with respect to time, it is critical to use simultaneously collected observations. Calculation of the SED based on simultaneous, multiwavelength data across the electromagnetic spectrum produces a reasonably accurate representation of the state of an object in a given time range. The gamma-ray emitting radio galaxies M 87, NGC 1275, Pictor A, and Centaurus A are selected here based on having been detected in very high energy gamma-rays by Cherenkov telescopes, as well as in other wavelengths. A uniquely consistent analysis approach is applied, in which each radio galaxy is analyzed the same way using simultaneously collected data. This approach sets it apart from other studies. Fermi-LAT raw data for each source in the sample is analyzed in time ranges which directly overlap the very high energy gamma-ray Cherenkov observations, as well as several other wavelength ranges. A synchrotron self-Compton (SSC) model is applied, which provides accurate treatment of synchrotron and inverse-Compton processes occurring in the jets of AGN, while estimating physical characteristics of the source. It is found that the spectra of M 87, NGC 1275, Pictor A, and Centaurus A can be well described by the same SSC model, producing values for the physical characteristics such as the doppler factor and magnetic field, which are relatively consistent with each other. In order to characterize the diffuse emission from dark matter self-annihilation, the radio galaxy SEDs are also fit with a dark matter model, resulting in an estimated dark matter particle mass of around 4.7 TeV which lies within predicted ranges. The highly dense regions near the black holes of AGN provide the optimal conditions for detecting these signatures. It is also found here that discrepancies between the expected emission and the observed emission in the spectra of some radio galaxies can be explained using the combined SSC and dark matter model. As emission from dark matter annihilation is expected to remain steady with respect to time, a key feature of this work is the novelty of the combined SSC and dark matter model, and the finding that dark matter characteristics may be revealed through similar multiwavelength analyses during future low emission states of the AGN. The radio galaxy sample is then extended to include all gamma-ray emitting radio galaxies detected by the Fermi-LAT, and a calculation of the core radio, total radio, and gamma-ray luminosities is followed through. A future step in extending this work would be to estimate the gamma-ray luminosity function of radio galaxies and their percent contribution to the total IGRB, based on the widely agreed upon assumption that a reasonable estimate of the gamma-ray luminosity function of a population can be attained by appropriately scaling its radio luminosity function, as gamma-ray luminosities and radio luminosities are strongly linearly correlated. This work has also provided the basis for such a calculation by outlining the theory and initial steps. It is the hope that the vast scope of the gathered data, its simultaneity, and the use of consistent analysis methods across the sample, will provide an improved foundation for a future calculation of the contribution of this population to the IGRB, as well as encourage stricter requirements for multiwavelength studies. / Der Ursprung, sowie die exakte Zusammensetzung des isotropischen Gammastrahlen-Hintergrunds (IGRB), sind trotz jahrelanger Studien über den Einfluss unaufgelöster extragalaktischer Objekte, nicht abschließend geklärt. Es wird für möglich gehalten, dass diffuse Prozesse, wie z.B. die Annihilation dunkler Materie, sowie bisher nicht detektierte Gammastrahlen-Emission aus aktiven Galaxiekernen (AGN), wie zum Beispiel Radiogalaxien, dazu beitragen. Radiogalaxien gehören zu der Gattung der Quellen, die stark im Radiowellenbereich emittieren. Einige dieser Galaxien wurden auch im Wellenlängenbereich von Gammastrahlung mittels des Fermi Large Area Telescope (Fermi-LAT) und für sehr energiereiche Gammastrahlung mittels Cherenkov-Detektoren nachgewiesen. Es wird davon ausgegangen, dass die kleinere Anzahl an nachgewiesenen Radiogalaxien im Gammastrahlenbereich, verglichen mit der Anzahl an nachgewiesenen Blazaren, auf die Orientierung ihrer Jets zurückzuführen ist. Des Weiteren wurde bisher nur eine kleine Anzahl an Radiogalaxien im Energiebereich der Gammastrahlung nachgewiesen, obwohl davon auszugehen ist, dass der Nachweis auch für weitere Galaxien möglich ist. Aus diesen Gründen werden Gammastrahlung emittierende Radiogalaxien, eine interessante und schwer auffindbare Klasse an Objekten, zur Untersuchung im Rahmen dieser Arbeit ausgewählt. Zur Verbesserung des Verständnisses diffuser Prozesse ist eine Modellierung der spektralen Energiedichteverteilung (SED) notwendig. Da die Emission von AGN zeitlich variiert, ist es wichtig simultan aufgezeichnete Daten für die Analyse zu verwenden. Die Berechnung der spektralen Energiedichteverteilung, basierend auf zeitgleich aufgezeichneten Daten für eine Vielzahl an Wellenlängen des elektromagnetischen Spektrums, liefert eine hinreichend genaue Beschreibung des Zustandes eines Objektes innerhalb eines gegebenen Zeitraumes. Diese Arbeit konzentriert sich auf die Gammastrahlung emittierenden Radiogalaxien M 87, NGC 1275, Pictor A und Centaurus A, da diese mittels Cherenkov-Teleskopen im Bereich hochenergetischer Gammastrahlung, sowie auch in anderen Wellenlängenbereichen, nachgewiesen wurden. Es wird eine, in dieser Form erstmals angewandte, konsistente Untersuchung durchgeführt, bei der jede Radiogalaxie auf identische Weise, mittels zeitgleich aufgezeichneter Daten, analysiert wird. Dieser Ansatz unterscheidet diese Arbeit von vergleichbaren Studien. Die Fermi-LAT Rohdaten für jede Quelle werden für die Zeiträume analysiert, in denen diese direkt mit der Beobachtung hochenergetischer Gammastrahlung durch Cherenkov-Teleskope, sowie darüber hinaus mit weiteren Wellenlängenbereichen, überlappen. Das Synchrotron Self-Compton (SSC) Modell wird der Analyse zu Grunde gelegt und ermöglicht eine akkurate Beschreibung, der im AGN Jet auftretenden, Synchrotron Prozesse und inversen Compton-Streuung, sowie die Abschätzung physikalischer Charakteristiken der Quelle. Es stellt sich heraus, dass die Spektren von M87, NGC 1275, Pictor A und Centaurus A mit demselben SCC Modell gut beschrieben werden können und relativ konsistente Werte für physikalische Größen, wie zum Beispiel den Doppler-Faktor oder die Magnetfeldstärke liefern. Zur genaueren Charakterisierung der aus der Annihilation dunkler Materie resultierenden diffusen Emission, werden die SED der Radiogalaxien zusätzlich mit einem Modell für dunkle Materie gefittet. Die daraus resultierende, geschätzte Teilchenmasse für dunkle Materie liegt mit 4.7 TeV innerhalb des vorhergesagten Bereiches. Die hochdichten Regionen in der Nähe der schwarzen Löcher des AGN liefern ideale Voraussetzungen zur Detektion dieser Signaturen. Des Weiteren wurde herausgefunden, dass etwaige Unterschiede zwischen der erwarteten und der beobachteten Emission in den Spektren einiger Radiogalaxien mittels einer Kombination aus SSC Modell und dunkler Materie Modell erklärt werden können. Unter der Annahme, dass die der Annihilation dunkler Materie zu Grunde liegende Emission zeitlich konstant bleibt, stellen zum einen die Kombination des SSC- und dunkler Materie Modells, sowie die Erkenntnis, dass Charakteristiken dunkler Materie durch ähnliche Multi-Wellenlängen-Experimente während zukünftiger, emissionsarmer Zustände gefunden werden können, die wesentlichen Ergebnisse dieser Arbeit dar. Das Sample der Radiogalaxien wird anschließend erweitert, so dass es alle vom Fermi-LAT detektierte und Gammastrahlung emittierende Radiogalaxien umfasst. Im Anschluss daran wird eine Berechnung der aus dem Kernbereich stammenden, und der totalen Radioluminosität, sowie der Gammastrahlungs-Luminosität durchgeführt. Ein künftiger Schritt zur Erweiterung dieser Arbeit wäre die Abschätzung der Gammastrahlungs-Luminositätsfunktion von Radiogalaxien und deren prozentualer Beitrag zum totalen IGRB, basierend auf der weitläufig akzeptierten Annahme, dass eine vernünftige Abschätzung der Gammastrahlungs-Luminositätsfunktion einer Population mittels einer angemessenen Skalierung ihrer Radio-Luminositätsfunktion erreicht werden kann, da die Gammastrahlungs-Luminosität und die Radioluminosität stark miteinander korrelieren. Diese Arbeit hat die hierfür benötigten Grundlagen für diese Art von Berechnung gelegt, indem sie die Theorie und die ersten Schritte darlegt. Es ist die Hoffnung, dass der große Umfang der zusammengetragenen Daten, deren Simultanität, und die Anwendung einer konsistenten Analysemethode für das gesamte Sample eine verbesserte Grundlage für zukünftige Berechnungen des Beitrages dieser Population zum IGRB leistet, sowie strengere Anforderung für Multi-Wellenlängen-Experimente.

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