No abstract available

[en] Nuclear reactors are a strong and pure source of electron antineutrinos. With neutrino experiments close to compact reactor cores new insights into neutrino properties and reactor physics can be obtained. The Nucifer experiment is one of the pioneers in this class of very short baseline projects. Its detector to reactor distance is only about 7 m. The data obtained in the last years allowed to estimate the plutonium concentration in the reactor core by the neutrino flux measurement. This is of interest for safeguard applications and non proliferation efforts. The antineutrinos in Nucifer are detected via the inverse beta decay on free protons. Those Hydrogen nuclei are provided by 850 liters of organic liquid scintillator. For higher detection efficiency and background reduction the liquid is loaded with Gadolinium. Despite all shielding efforts and veto systems the background induced by the reactor activity and cosmogenic particles is still the main challenge in the experiment. The principle of the Nucifer detector is similar to the needs of upcoming experiments searching for sterile neutrinos. Therefore, the Nucifer results are also valuable input for the understanding and optimization of those next generation projects. The observation of sterile neutrinos would imply new physics beyond the standard model.

[en] In neutrino oscillations, a canonical understanding has been established during the last decades after the measurement of the mixing angles θ_1_2, θ_2_3, θ_1_3 via solar, atmospheric and, most recently, reactor neutrinos. However, the re-evaluation of the reactor neutrino theoretical flux has forced a re-analysis of most reactor neutrino measurements at short distances. This has led to an unexpected experimental deficit of neutrinos with respect to the theory that needs to be accommodated, commonly known as the ''reactor neutrino anomaly''. This deficit can be interpreted as the existence of a light sterile neutrino state into which reactor neutrinos oscillate at very short distances. The STEREO experiment aims to find an evidence of such oscillations. The ILL research reactor in Grenoble (France) operates at a power of 58MW and provides a large flux of electron antineutrinos with an energy range of a few MeV. These neutrinos will be detected in a 2000 liter organic liquid scintillator detector doped with Gadolinium and consisting of 6 cells stacked along the direction of the core. Given the proximity of the detector, neutrinos will only travel a few meters until they interact with the scintillator. The detector will be placed about 10 m from the reactor core, allowing STEREO to be sensitive to oscillations into the above mentioned neutrino sterile state. The project presents a high potential for a discovery that would impact deeply the paradigms of neutrino oscillations and in consequence the current understanding of particle physics and cosmology.

[en] We demonstrate that by employing the correspondence between gauge theories in geometric and in deconstructed extra dimensions, it is possible to transfer the methods for calculating finite Casimir energy densities in higher dimensions to the four-dimensional deconstruction setup. By this means, one obtains an unambiguous and well-defined prescription to determine finite vacuum energy contributions of four-dimensional quantum fields which have a higher-dimensional correspondence. Thereby, large kink masses lead to an exponentially suppressed Casimir effect. For a specific model we hence arrive at a small and positive contribution to the cosmological constant in agreement with observations. (author)

[en] Within models containing a very light scalar particle coupled to the 125 GeV Higgs boson, I present the first study of Higgs decays into three of these light scalars. Introduce model-independent conditions which the scalar sector after electroweak symmetry breaking has to satisfy in order for the three-body channel to become relevant. Using a specific model - the real scalar singlet-extension of the Standard Model (SM) - we have identified scenarios, where the rates of scalar three-body Higgs decays are comparable to or even exceed those of the well-studied two-body channel. All those scenarios were shown to be compatible with current experimental and theoretical constraints. I finally argue that scalar three-body Higgs decays lead to exciting new collider signatures with six SM fermions in the final state. In particular, I demonstrate that e.g. six-muon or six-tau final states may be in reach of dedicated searches at the LHC or ILC experiments.

[en] As an intense and pure source of low energy electron antineutrinos, nuclear reactors are one of the most powerful tools to analyse the neutrino oscillations. The Double Chooz experiment aims for a precise determination of the neutrino mixing angle θ_1_3 with the new data from the near detector. In order to reach this precision, a high and accurately known detection efficiency of the inverse beta decay (IBD) signal - the antineutrino interaction - is required. Several methods are available for detector calibration. Cosmic muons and spallation neutron captures are some examples of natural sources that are used. Furthermore, signals created by artificial sources contribute to the calibration with well defined classes of events. LED Light Injection systems in the Inner Detector and the Inner Veto are used to measure PMT gains and time responses. Radioactive sources deployed inside the detector are used to determine the energy scale and the detector stability. The "2"5"2Cf source plays an important role in the detector calibration. In the spontaneous fissions of this isotope neutrons are produced with high multiplicity. An analysis of the neutron interactions in the scintillator can be used to estimate the detection efficiency of the delayed coincidence signal of the IBD reaction. New results from recent calibration campaigns will be presented, providing a crucial input for reactor antineutrino analysis with two detectors.

[en] We investigate the potential to probe new neutrino physics with future experiments measuring coherent neutrino-nucleus scattering. Experiments with high statistics should become feasible soon and allow to constrain parameters with unprecedented precision. Using a benchmark setup for a future experiment probing reactor neutrinos, we study the sensitivity on neutrino non-standard interactions and new exotic neutral currents (scalar, tensor, etc). Compared to Fermi interaction, percent and permille level strengths of the new interactions can be probed, superseding for some observables the limits from future neutrino oscillation experiments by up to two orders of magnitude.

[en] Neutrinoless double beta decay is a lepton number violating process (ΔL=2) whose observation would prove that neutrinos are Majorana particles, i.e. their own antiparticles. The simplest realisation of this process (mediation by light massive Majorana neutrinos) may however interfere with other lepton number violating operators. Therefore, the possibility to reliably extract neutrino parameters from the experimental results may be affected by this interplay. We discuss the effects of various beyond the SM ΔL=2 processes at higher scales on the measurement of the effective Majorana mass and their implications on different parameters in the neutrino sector.

[en] Neutrinoless double beta decay is a very promising experimental test for lepton number violation. The exchange of light Majorana neutrinos is the simplest realization of this decay, but other physics beyond the Standard Model may also mediate neutrinoless double beta decay. We discuss the interplay of different mechanisms and the influence such an interplay has on the extraction of parameters of the neutrino sector from experimental results.

[en] We discuss an A4 flavor symmetry model in the context of the split seesaw mechanism. The model leads to one keV sterile neutrino as the warm dark matter and two heavy right-handed neutrinos being responsible for leptogenesis to explain the observed baryon asymmetry of the Universe. The A4 flavor symmetry predicts nearly tri-bimaximal mixing in the lepton sector. Combining the split seesaw mechanism with the flavor symmetry leads to an interesting correlation between the neutrino oscillation data and the cosmological constraint from X-ray observation.