The Standard Model of particle physics has enjoyed unprecedented success in predicting experimental results. However, evidence from astrophysical observations points to the existence of a dark sector of particles that interact only very weakly with the Standard Model. In this work, we search for dark sector signatures in X-ray telescope data. Much of this work concerns a class of hypothetical particles, the axion-like particle (ALP). ALPs are a theoretically well-motivated extension of the Standard Model. If ALPs exist, they may lead to intriguing astrophysical signatures: in the presence of a background magnetic field, ALPs and photons can interconvert. We could detect ALPs by searching for photon to ALP conversion. For example, photons produced by point sources in or behind galaxy clusters may convert to ALPs in the cluster's magnetic field. This could lead to observable spectral anomalies. Using this strategy, we place world leading bounds on the ALP-photon coupling. One potential signal of dark matter is an anomalous line in the spectra of galaxies and galaxy clusters. In 2014, an anomalous line was found at an energy of 3.5 keV. The nature and cause of this line is still under discussion. We analyse a scenario in which the 3.5 keV line arises from dark matter decay to ALPs, which interconvert with 3.5 keV photons in astrophysical magnetic fields. We further report an anomalous deficit at 3.5 keV in the spectrum of the Active Galactic Nucleus at the centre of the Perseus galaxy cluster. This motivates the study of a new model in which both features are caused by “fluorescent dark matter” which resonantly interacts with 3.5 keV photons. We analyse observations of Perseus at 3.5 keV to date, and show that they are well explained by this model. Further theoretical and experimental work is needed to discover or exclude fundamental physics effects in X-ray spectra.

No description available.

In this thesis, we investigated the generation of the higher derivative Chern-Simons term, as well as the ether-like term, both at finite temperature. We also examined the question of the large gauge invariance, with respect to the Chern-Simons term and the hight derivative one. We observed that the coefficient of the Chern-Simons term vanishes, when we taken into account the limit of high temperature. This happens to occur also for the other high derivative terms. With respects to the Chern-Simons term, we observed that the action of the Chern-Simons term and the high derivative one are invariant under the large gauge transformation. For this, we calculated the exactly effective action, however, for a specific gauge choice. Finally, with respects to the ether-like term, we observed that the calculation at finite temperature is ambiguous, as well as it is observed for the calculation at zero temperature. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese, investigamos a geração do termo de Chern-Simons de derivada superior, assim como o termo tipo éter, ambos à temperatura finita. Também estudamos a questão da invariância de gauge ampla, com o termo de Chern-Simons e Chern-Simons de derivada superior. Constatamos que o coeficiente do termo de Chern-Simons de derivada superior anula-se, quando tomamos o limite de altas temperaturas. Esse resultado parece se repetir para os demais termos de derivada superior. Dentro ainda do contexto de Chern-Simons, observamos que a ação de Chern-Simons e a ação de Chern-Simons de derivada superior são invariantes sob transformação de gauge ampla. Para isso, conseguimos calcular a ação efetiva exata, contudo, para uma escolha de gauge específica. Finalmente, com relação ao termo tipo éter, constatamos que o cálculo à temperatura finita e ambíguo, assim como observado no cálculo à temperatura zero.