| 1. |
- Pelssers, Bart Eduard Jan, 1991-
(författare)
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Enhancing Direct Searches for Dark Matter : Spatial-Temporal Modeling and Explicit Likelihoods
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Astronomical and cosmological observations on different scales point to the existence of dark matter. In the current cosmological paradigm this dark matter accounts for about 26% of the energy-density of the universe, yet has not been directly observed. Weakly Interacting Massive Particles (WIMPs) and axions are two candidates among the many theories and particles proposed to explain dark matter. Direct detection experiments aim to detect the scattering or coupling of dark matter to the detector medium. The event rate in such experiments is expected to exhibit an annual modulation due to the motion of the Earth through the Galactic dark matter halo. The XENON collaboration built several experiments that have searched for WIMP dark matter by looking for WIMPs scattering on xenon nuclei. The heart of these detectors consists of a Time Projection Chamber (TPC) which records the scintillation light (S1) and ionization charge (S2) signal following a recoiling xenon nucleus as well as its position and time. Using these ultra-low background detectors, the XENON collaboration has set world-leading exclusion limits on the WIMP-nucleon scattering cross-section. In this thesis several different ways of enhancing direct detection experiments are presented, involving time dependent signal models, event reconstruction and a method enhancing statistical inference. First, during a search for event rate modulation, spanning almost 4 years of XENON100 data, no oscillation was found to be compatible with the expected signature. This thesis presents a verification of the correctness of the test statistic distribution used in this analysis using dedicated simulations. Second, the positions of interactions in XENON detectors are used for detector volume fiducialization as well as for modeling the position dependent detector response. This thesis presents the position reconstruction methods used during the first XENON1T science analysis. Third, a new algorithm for position and energy reconstruction using the likelihood-free paradigm is presented. This simulator-based method increases the accuracy of the previous method by up to 15% and can simultaneously infer the transverse position and size of the charge signal. Fourth, to enhance the physics reach of future dark matter searches using xenon TPCs, a new method for computing differential rates is developed. This method replaces the calculation usually performed by Monte-Carlo simulations with an equivalent analytic expression. This enables the use of higher dimensional explicit (profile) likelihood functions, resulting in better signal-background discrimination. The new method uses time dependent signal models (encoding annual modulation) as well as spatially non-uniform sources such as a radiogenic neutron background and fully accounts for the non-uniform detector response. This method can significantly reduce the exposure needed for a potential dark matter discovery in future detectors such as XENONnT. Lastly, the amplitude of the axion dark matter field is expected to exhibit stochastic behavior. Experiments whose measurements are shorter than the coherence time of the field need to include this effect in their data analysis and inference. This thesis presents an analysis of a simulated axion signal in a CASPEr-ZULF-like detector, showing that exclusion limits on the axion amplitude are too strong by a factor ~4 when not including the axion amplitude fluctuation.
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| 2. |
- Antochi, Vasile Cristian, 1992-
(författare)
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Inference on Dark Matter in Effective Field Theories : From XENON1T towards XENONnT: Chiral effective field theory analysis of nuclear recoils, single electrons and uncommon background modelling
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cosmological and astronomical observations show that most of the matter in the Universe is dark. This dissertation provides an overview of the dark matter evidence, and focuses on the particle dark matter hypothesis, describing possible particle candidates, concentrating on the Weakly Interactive Massive Particles (WIMPs). It describes the main WIMP detection strategies and addresses the subject of WIMP scattering in direct detection experiments. This work analyses the data from the XENON1T experiment, investigating within a Chiral Effective Field Theory (ChEFT) framework the nuclear recoils from possible WIMP interactions. It presents the XENON1T detector, the main backgrounds, the xenon signal emission model and the background studies, and describes the statistical inference adopted in the analysis.The XENON1T detector was a dual-phase Time Projection Chamber (TPC) using a ~2 tonne liquid xenon target to detect scattering particles. WIMPs with masses above ~10GeV/c2 scattering against the xenon nuclei would deposit enough energy to create an observable event.The ChEFT analysis is performed on the XENON1T data from 278.8 days of operation for a total exposure of 1 tonne×year, with a combined likelihood of two science runs. The region of interest for this analysis was extended from [4.9, 40.9] keVnr, in the Spin Independent analysis, to [4.9, 54.4] keVnr, to increase the acceptance of possible models with rates peaking at higher energies (>0keVnr). The analysis shows that the data is consistent with a background only hypothesis and provides constraints on the interaction coefficients and the physics scale for 25 different operators. The analysis is complemented by limits on three benchmark models of interaction using ChEFT. For these models we investigate the effect of isospin breaking interactions, reporting cancellation regions where the limit worsens up to 6 orders of magnitude with respect to the isospin conserving case.The dissertation is complemented with the dark matter-electron scattering study within an EFT framework, analysing the single or few electron emission signals in XENON1T. The analysis provides the first experimental limits on the dark matter-electron effective operators for the magnetic and electric dipole, and anapole interactions.Lastly, the dissertation describes an example of introducing a data-driven background model in an inference framework based on explicit multidimensional likelihood computation. The background modelling is done using calibration data from the XENONnT detector, the next iteration of a dual-phase xenon TPC in the XENON detector family, which is currently in operation.
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| 3. |
- Chiappo, Andrea, 1988-
(författare)
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Dark matter signal normalisation for dwarf spheroidal galaxies : A frequentist analysis of stellar kinematics for indirect Dark Matter searches
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Indirect detection strategies of Dark Matter (DM) entail searching for signals of DM annihilation or decay, typically in the form of excess positrons or high-energy photons above the astrophysical background, originating from (inferred) DM-rich environments. Due to their characteristics, dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are considered very promising targets for indirect particle DM identification. To compare model predictions with the observed fluxes of product particles, most analyses of astrophysical data - which are generally performed via frequentist statistics - rely on estimating the abundance of DM by calculating the so-called J-factor. This quantity is usually inferred from the kinematic properties of the stellar population of a dSph, performing a Jeans analysis by means of Bayesian techniques. Previous works have, therefore, combined different statistical methods when analysing astrophysical data from dSphs. This thesis describes the development of a new, fully frequentist approach for constructing the profile likelihood curve for J-factors of dSphs, which can be implemented in indirect DM searches. This method improves upon previous ones by producing data-driven expressions of the likelihood of J, thereby allowing a statistically consistent treatment of the astroparticle and astrometric data from dSphs. Using kinematic data from twenty one satellites of the Milky Way, we derive estimates of their maximum likelihood J-factor and its confidence intervals. The analyses are performed in two different frameworks: the standard scenario of a collisionless DM candidate and the possibility of a self-interacting DM species. In the former case, the obtained J-factors and their uncertainties are consistent with previous, Bayesian-derived values. In the latter, we present prior-less estimates for the Sommerfeld enhanced J-factor of dSphs. In agreement with earlier studies, we find J to be overestimated by several orders of magnitude when DM is allowed is attractively self-interact. In both cases we provide the profile likelihood curves obtained. This technique is validated on a publicly available simulation suite, released by Gaia Challenge, by evaluating its coverage and bias. The results of these tests indicate that the method possesses good statistical properties. Lastly, we discuss the implications of these findings for DM searches, together with future improvements and extensions of this technique.
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| 4. |
- Conrad, Jan, 1973-
(författare)
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A Search for Neutrinos from Cosmic Point Sources using AMANDA-B10 with Emphasis on Limit Calculation Techniques
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A search for cosmic point sources of neutrinos has been performed using data taken in 1999 with the AMANDA-B10 neutrino telescope. This work describes methods for signal and background separation and the statistical analysis of the final data sample. In particular, the multivariate method Support Vector Machines has been applied to achieve good background rejection while at the same time retaining high signal efficiency. A grid search covering the complete northern hemisphere revealed no statistical significant excess of events over the expected background from mis-reconstructed cosmic ray induced muons and muons induced by atmospheric neutrinos. Thus, no cosmic point sources of neutrinos have been detected. Upper limits on the neutrino flux for each cell of the grid are presented.Twenty potential sources of neutrinos chosen among three classes of astronomical objects (Blazars, Super Nova Remnants and Microquasars) have been preselected. Upper Limits on the flux of cosmic neutrinos from those are presented.The presence of systematic uncertainties makes the calculation of confidence limits an intricate problem. A method is presented which makes it possible to include these uncertainties into the frequentist construction of confidence intervals. Statistical properties of the presented method have been studied.
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| 5. |
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| 6. |
- Garde Lindholm, Maja, 1983-
(författare)
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Dark Matter searches targeting Dwarf Spheroidal Galaxies with the Fermi Large Area Telescope
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I present our recent work on gamma-ray searches for dark matter with the Fermi Large Area Telescope (Fermi-LAT). We have targeted dwarf spheroidal galaxies since they are very dark matter dominated systems, and we have developed a novel joint likelihood method to combine the observations of a set of targets.In the first iteration of the joint likelihood analysis, 10 dwarf spheroidal galaxies are targeted and 2 years of Fermi-LAT data is analyzed. The resulting upper limits on the dark matter annihilation cross-section range from about 10−26 cm3 s−1 for dark matter masses of 5 GeV to about 5 × 10−23 cm3 s−1 for dark matter masses of 1 TeV, depending on the annihilation channel. For the first time, dark matter models with a cross section above the canonical thermal relic cross section (∼ 3 × 10−26 cm3 s−1) are strongly disfavored by a gamma-ray experiment. In the second iteration we include 15 dwarf spheroidal galaxies in the combined analysis, employ 4 years of data and an improved calculation of the dark matter density. The obtained upper limits range from about 10−26 cm3 s−1 for dark matter masses of 2 GeV to about 10−21 cm3 s−1 for dark matter masses of 10 TeV, depending on the annihilation channel.I briefly describe some of the evidence for dark matter, the Fermi-LAT instrument and public data releases, dwarf spheroidal galaxies, likelihood analysis, and results from analyses of Fermi-LAT data. I also document some of the tests made to verify the method and to compare different analysis setups.
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| 7. |
- Hedlund-Åström, Anna, 1957-
(författare)
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Model for End of Life Treatment of Polymer Composite Materials
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Because of increasing environmental demands, especially on dealing with products end of life phase, product manufacturers and designers must consider the future disposal of their products. For conventional materials like steel and aluminium well-functioning recycling methods exists. This is not the case for structures of polymer composites, which are used more extensively, especially for structures like vehicles and vessels. Several techniques do exist but they are not yet commercially available. The current disposal methods of polymer composites are landfill and incineration. Polymer composites are materials, which consist of several materials like fibre, matrix, and additives. In the form of sandwich constructions also foam core material is added. This circumstance complicates the waste treatment of composite materials. In this thesis a model for assessing possible future waste treatment techniques for polymer composites including sandwich structures is presented. The model is meant to be used as an aid for preparing future disposal for end of life products for planning waste treatment and for facilitating communication in contacts with waste receivers. Recommendations for waste treatment have been formed for a number of polymer composites. These recommendations are based on the analysis of costs and environmental effects and they compare different scenarios for mechanical material recycling and energy recovery by waste incineration. The result of this study points out material recycling as the preferable method for the main part of the studied materials. But this recommendation is strongly dependent on type of virgin material replaced by the recycled material. Energy recovery can also be considered if the polymer composite waste replaces coal, which is non renewable. Though incineration will always result in a cost for the waste producer. In the recommendations mentioned above no information concerning implementation of the different waste disposal techniques is included. Therefore, in this study a model for assessing possible waste disposal techniques for polymer composites is presented. The model is based on internal factors, which are related to the waste and to the processes. To implement the model relevant waste properties must be identified in order to fulfil the conditions set by the required processes involved. A case study was carried out using the proposed model for assessing different waste disposal techniques for the hull of the Visby Class Corvette in the Royal Swedish Navy. Six different techniques were studied for the hull structure. Since almost all the important waste properties were known and the waste was assessed to be treatable all the included techniques except one are shown to be usable in the future. Many investigations have pointed out material recycling as the best alternative considering environmental effects. This is also valid for polymer composite materials. Since recycling polymer composites is a complicated process, especially recycling thermoset composite it is important to aquire comprehensive information about the constituents of these materials.
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| 8. |
- Johansson, Jan, 1970-
(författare)
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Material Hygiene : An EcoDesign mindset for recycling of products
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years the end-of-life phase has come into focus. European Union directives have been issued regulating certain product groups and producer responsibility. Vehicles and electronic products are the first to be identified and targeted. EU environmental legislation acts as a driver for increased reuse, recycling and recovery. The overall aim of the presented activities has been to increase the effectiveness of current recycling practices, both in terms of design changes and end-of-life treatment process suggestions. A “pre-step” operation has been suggested, in order to either salvage valuable (or toxic) material or to remove diluting bulk material. As this thesis is focused on the recycling of white-goods specifically dishwashers the suggested prestep would be removal of valuable copper prior to shredding. A life cycle assessment (LCA) study has been conducted. The purpose of this study was to determine if using a pre-step is beneficial from an environmental point of view or not. Furthermore, an experiment on the usability of recycled polymers from waste electrical and electronic equipment (WEEE) has been performed. Based on this work polymer recycling process suggestions are presented. Based on research in the fields of design for recycling, design for disassembly and EcoDesign the material hygiene (MH) concept of design for recycling is formulated. This concept is tested on a disassembly field study carried out at a waste collection facility and a polymer recycling experiment at a refrigerator fragmentation plant. Five MH factors are suggested: MH Mix, MH Identification, MH Resources, and MH Weight and MH Map. Additionally, a MH mind-set is presented.
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| 9. |
- Morå, Knut Dundas, 1989-
(författare)
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Statistical Modelling and Inference for XENON1T
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A great number of astrophysical observations suggests that of the matter in our universe, only a sixth is made up of known matter. The rest, named dark matter, has not been successfully identified. This thesis presents the analysis and statistical inference that was used by the XENON1T collaboration to conduct a search for a particular dark matter candidate; weakly interacting massive particles (WIMPs). XENON1T is a dual-phase time projection chamber that can detect particles scattering in a 2 tonne target of liquid xenon with deposited recoil energies above ~3 keV. This is low enough to observe the elastic recoil between a WIMP and a xenon nucleus for WIMP masses >5 GeV c-2.The results presented in this thesis use 278.8 days of data, with an analysis mass of 1.3 tonne. XENON1T uses models for backgrounds and signals within this volume to construct a combined likelihood for two science data-taking periods as well as calibration data-sets. Fits to simulated data-sets were used to calibrate and validate the confidence interval construction. In addition, analysis choices were evaluated both to optimize the discovery power and expected sensitivity of the search, and to improve the robustness of the analysis. No significant excess was observed in the search for a spin-independent WIMP-nucleon interaction for any WIMP masses between 6 GeV c-2 and 104 GeV c-2 for the 1 ton-year exposure. This analysis produced the strongest constraint on the spin-independent WIMP-nucleon cross-section so far, with a minimum of 4.1 10-47 cm2 for a 30 GeV c-2 WIMP.
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| 10. |
- Scott, Pat, 1982-
(författare)
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Searches for Particle Dark Matter : Dark stars, dark galaxies, dark halos and global supersymmetric fits
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.
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