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Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

We investigate the sensitivity of tritium β-decay experiments for keV-scale sterile neutrinos. Relic sterile neutrinos in the keV mass range can contribute both to the cold and warm dark matter content of the universe. This work shows that a large-scale tritium beta-decay experiment, similar to the KATRIN experiment that is under construction, can reach a statistical sensitivity of the active-sterile neutrino mixing of sin 2 θ ∼ 10 −8 . The effect of uncertainties in the known theoretical corrections to the tritium β-decay spectrum were investigated, and found not to affect the sensitivity significantly. It is demonstrated that controlling uncorrelated systematic effects will be one of the main challenges in such an experiment.is one of the most intriguing questions of modern physics, since the Standard Model of elementary particle physics (SM) does not provide a suitable dark matter candidate. Such a candidate should be electrically neutral, at most weakly interacting, and stable with respect to the age of the universe.Relic active neutrinos, forming hot dark matter (HDM), are firmly ruled out as the dominant dark matter component. At the time of structure formation, light neutrinos had relativistic velocities and a large free streaming length, leading to a washing-out of smallscale structures, which is in disagreement with observations [16,17]. Consequently, the most most favored candidate was thought to be a cold dark matter (CDM) particle, the so-called weakly interacting massive particle (WIMP). Its freeze-out in the early universe occurs at non-relativistic velocities, preventing the washing-out of small-scale structures. Furthermore, the existence of WIMPs is independently motivated by theories extending the SM, such as Supersymmetry [18]. WIMPs are actively sought in direct and indirect measurements, but no solid evidence for their existence yet has been reported [19].Relic sterile neutrinos, with a mass in the keV range, are a candidate for both warm and cold dark matter (WDM and CDM) [8][9][10][11][12][13][14]. WDM and CDM scenarios fit the largescale structure data equally well [20]. On the galactic scale WDM scenarios predict a smaller number of dwarf satellite galaxies and shallower galactic density profiles than CDM, resolving tensions between observations of galaxy-size objects and specific CDM model simulations [21][22][23][24][25][26][27][28][29][30][31].Astrophysical observations constrain the sterile neutrino mass m s and active-sterile mixing angle θ. A robust and model-independent lower bound on the mass of spin-one-half dark matter particles is derived by considering the phase-space density evolution of dwarf spheroidal satellites in the Milky Way, leading to a mass limit of m s >1 keV [32,33]. Another sensitive observable is the X-ray emission line at half of the neutrino mass, arising from the decay of a keV-scale sterile neutrino into an active neutrino and a photon, which can be searched for with appropriate X-ray Space Telescopes, such as XMM-Newton [34] and Chandra [35]. A combination ...

The Karlsruhe Tritium Neutrino (KATRIN) experiment is a large-scale effort to probe the absolute neutrino mass scale with a sensitivity of 0.2 eV (90% confidence level), via a precise measurement of the endpoint spectrum of tritium β-decay. This work documents several KATRIN commissioning milestones: the complete assembly of the experimental beamline, the successful transmission of electrons from three sources through the beamline to the primary detector, and tests of ion transport and retention. In the First Light commissioning campaign of autumn 2016, photoelectrons were generated at the rear wall and ions were created by a dedicated ion source attached to the rear section; in July 2017, gaseous 83mKr was injected into the KATRIN source section, and a condensed 83mKr source was deployed in the transport section. In this paper we describe the technical details of the apparatus and the configuration for each measurement, and give first results on source and system performance. We have successfully achieved transmission from all four sources, established system stability, and characterized many aspects of the apparatus.

The KATRIN experiment will probe the neutrino mass by measuring the β-electron energy spectrum near the endpoint of tritium β-decay. An integral energy analysis will be performed by an electro-static spectrometer ("Main Spectrometer"), an ultra-high vacuum vessel with a length of 23.2 m, a volume of 1240 m 3 , and a complex inner electrode system with about 120 000 individual parts. The strong magnetic field that guides the β-electrons is provided by super-conducting solenoids at both ends of the spectrometer. Its influence on turbo-molecular pumps and vacuum gauges had to be considered. A system consisting of 6 turbo-molecular pumps and 3 km of non-evaporable getter strips has been deployed and was tested during the commissioning of the spectrometer. In this paper the configuration, the commissioning with bake-out at 300 • C, and the performance of this system are presented in detail. The vacuum system has to maintain a pressure in the 10 −11 mbar range. It is demonstrated that the performance of the system is already close to these stringent functional requirements for the KATRIN experiment, which will start at the end of 2016.