| 1. |
- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 2. |
- 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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| 3. |
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| 4. |
- Acharya, B. S., et al.
(författare)
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Introducing the CTA concept
- 2013
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 43, s. 3-18
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. (C) 2013 Elsevier B.V. All rights reserved.
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| 5. |
- 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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| 6. |
- Antochi, Vasile Cristian, 1992-
(författare)
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WIMP searches in an Effective Field Theory Framework from XENON1T to XENONnT
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Observations from cosmology and astronomy suggest that most of the matter in the Universe is comprised of dark matter. In this thesis the evidence for dark matter and the possible candidates are explored, focusing on the Weakly Interacting Massive Particles (WIMPs) hypothesis. The various ways WIMPs can be detected are shown, with particular regard to WIMP scattering in direct detection experiments.Since traditional Spin Independent (SI) and Spin Dependent (SD) searches for WIMPs have produced very tight constraints on the interaction cross section, we investigate the WIMP scattering through a model independent Effective Field Theory (EFT) framework. The chosen approach for this work is Chiral EFT (ChEFT), which studies the nuclear responses of possible WIMP interactions including quantum chromo-dynamics (QCD) effects using chiral symmetry, starting from fundamental interactions with quarks and gluons in atomic nuclei.This thesis presents the statistical analysis and the inference used in the XENON1T experiment to perform a ChEFT analysis of WIMP interactions. Furthermore it shows the work towards the upgrade of the XENON experiment, XENONnT, with particular regard to the photomultiplier tube (PMT) testing performed at Stockholm University.XENON1T was a dual-phase time projection chamber using a 2 tonne liquid xenon target to detect scattering particles. WIMPs with masses above 10 GeV scattering against the xenon nuclei would deposit enough recoil energy to create an observable event. The XENONnT detector is a new detector that will have a target volume of ~6 tonnes of liquid xenon and is now being commissioned.The ChEFT analysis is done considering the XENON1T data from 278.8 days of operation and a fiducial volume of 1.3 tonne and it uses the full XENON1T combined likelihood, constructed with background models and signal models slightly extended in the analysis space with respect to the previous XENON1T searches, to increase the signal acceptance for the various EFT models.Simulated data sets were used to validate the inference and to compute the expected sensitivities for 25 different ChEFT responses.This work shows that XENON1T can constrain the physics scale Lambda of WIMP interactions within the ChEFT framework to regions of O(10ˆ2-10ˆ5 GeV) for most of the channels.
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| 7. |
- Aprile, Elena, et al.
(författare)
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Analysis of the XENON1T data for WIMP search : Background Models and Statistical Inference
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The XENON1T experiment searches for dark matter recoils within a $2$ tonne liquid xenon target. The detector is operated as a dual-phase time projection chamber, and reconstructs the energy and position of interactions in the active volume. In the central volume of the target mass, the lowest background rate of a xenon-based direct detection experiment so far has been achieved. In this work we describe the detector response modelling, the background and signal models, and the statistical inference procedures used in a search for Weakly Interacting Massive Particles (WIMPs) using 1\,tonne$\times$year exposure of XENON1T data.
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| 8. |
- Aprile, E., et al.
(författare)
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Effective Field Theory and Inelastic Dark Matter Results from XENON1T
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this work we expand on the XENON1T nuclear recoil searches and study the individual signals of Dark Matter interactions from operators up to dimension-eight in a Chiral Effective Field Theory (ChEFT) as well as a model of inelastic Dark Matter using data from the two science runs of the detector totalling 1 tonne*year exposure. For these analyses we extended the region of interest from [4.9, 40.9]keVnr to [4.9, 54.4]keVnr to enhance our sensitivity for signals that peak at nonzero energies. We show that the data is consistent with a background only hypothesis, with small excesses in the models which peak between 20 and 50keVnr, obtaining a maximum local discovery significance of 1.7 for the VVs ChEFT model for a WIMP mass of 70GeV/c2, and 1.8 for an iDM particle of 50GeV/c2 with a mass splitting of 100keV/c2. For each model we report 90% confidence level upper limits. We also report limits on three benchmark models of WIMP interaction using ChEFT for which we investigate the effect of isospin breaking interactions, reporting up to 6 orders of magnitude weaker limits with respect to the isospin conserving case driven by cancellations in the expected rate.
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| 10. |
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