| 1. |
- Persson, Daniel, 1978
(författare)
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Arithmetic and Hyperbolic Structures in String Theory
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of an introductory text followed by two separate parts which may be read independently of each other. In Part I we analyze certain hyperbolic structures arising when studying gravity in the vicinity of spacelike singularities (the BKL-limit). In this limit, spatial points decouple and the dynamics exhibits ultralocal behaviour which may be mapped to an auxiliary problem given in terms of a (possibly chaotic) hyperbolic billiard. In all supergravities arising as low-energy limits of string theory or M-theory, the billiard dynamics takes place within the fundamental Weyl chambers of certain hyperbolic Kac-Moody algebras, suggesting that these algebras generate hidden infinite-dimensional symmetries of gravity. We investigate the modification of the billiard dynamics when the original gravitational theory is formulated on a compact spatial manifold of arbitrary topology, revealing fascinating mathematical structures known as galleries. We further use the conjectured hyperbolic symmetry E10 to generate and classify certain cosmological (S-brane) solutions in eleven-dimensional supergravity. Finally, we show in detail that eleven-dimensional supergravity and massive type IIA supergravity are dynamically unified within the framework of a geodesic sigma model for a particle moving on the infinite-dimensional coset space E10/K(E10). Part II of the thesis is devoted to a study of how (U-)dualities in string theory provide powerful constraints on perturbative and non-perturbative quantum corrections. These dualities are typically given by certain arithmetic groups G(Z) which are conjectured to be preserved in the effective action. The exact couplings are given by moduli-dependent functions which are manifestly invariant under G(Z), known as automorphic forms. We discuss in detail various methods of constructing automorphic forms, with particular emphasis on a special class of functions known as (non-holomorphic) Eisenstein series. We provide detailed examples for the physically relevant cases of SL(2,Z) and SL(3,Z), for which we construct their respective Eisenstein series and compute their (non-abelian) Fourier expansions. We also discuss the possibility that certain generalized Eisenstein series, which are covariant under the maximal compact subgroup K(G), could play a role in determining the exact effective action for toroidally compactified higher derivative corrections. Finally, we propose that in the case of rigid Calabi-Yau compactifications in type IIA string theory, the exact universal hypermultiplet moduli space exhibits a quantum duality group given by the Picard modular group SU(2,1;Z[i]). To verify this proposal we construct an SU(2,1;Z[i])-invariant Eisenstein series, and we present preliminary results for its Fourier expansion which reveals the expected contributions from D2-brane and NS5-brane instantons.
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| 2. |
- 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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| 3. |
- Dall` Olio, Daria, 1981
(författare)
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Magnetic fields around massive protostars as traced by masers and dust emission
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is not fully clear how the magnetic field acts during the first stages of star formation. A possible way to clarify its role is to observe the polarized light coming from masers and thermal dust emission. By measuring linear polarization angles and Zeeman splitting of different maser species it is possible to study the magnetic field morphology and strength in different parts of the protostar. Polarized emission of thermal dust has also been used extensively to probe the magnetic field at the onset of star formation. In this thesis we study the magnetic field properties of two well-known sources: the massive protostar IRAS18089-1732, showing a hot core chem- istry and a disc-outflow system, and the high-mass star forming complex G9.62+0.19, presenting several cores at different evolutionary stages. We also investigate the polarization properties of selected methanol masers, con- sidering newly-calculated methanol g-factors and hyperfine components. We compare our results with previous maser observations and we evaluate the contribution of preferred hyperfine pumping and non-Zeeman effects. We make use of MERLIN and ALMA observations and we analyse the polarized emission by 6.7 GHz methanol masers and thermal dust. Simulations were run using the radiative transfer code CHAMP for different magnetic field values, hyperfine components and pumping efficiencies. We observe that the large scale field probed by dust continuum emission is consistent with the small scale magnetic field probed by masers. Moreover, in the G9.62+0.19 complex we resolved several cores showing polarized emission. We propose an evolutionary sequence of magnetic field in this complex, where the less evolved stellar embryo exhibits a magnetic field stronger than the more evolved one. From our simulations, we find that preferred hyperfine pumping can explain some high levels of linear and circular polarization. We also notice that non-Zeeman effects need to be considered in magnetic field studies. In conclusion, our work indicates that there is a link between the magnetic field at different scales. More masers observations will help in evaluating the relevance of non-Zeeman effects and obtain good estimates of magnetic fields close to the protostar. Future multi-wavelength and multi-scale observations, aimed at detecting polarized light from masers, thermal dust and thermal molecular lines, will help to constrain magnetic field properties around massive protostars.
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| 4. |
- Urdshals, Einar, 1995
(författare)
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Dark matter electron interactions in detector materials
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dark Matter (DM) makes up 85% of the matter content of the universe, and its gravitational effects are seen on scales ranging from that of cosmology to that of galactic astrophysics. The nature of DM is, however, unknown. Study- ing DM in the lab with a class of experiments called direct detection (DD) experiments is key to understanding its properties. For decades, experiments have been attempting to do this through searches for DM induced nuclear recoils. These have not been found, and a possible reason for this is that the hypothetical DM particle is too light to induce nuclear recoils. Therefore, in the last decade experiments have been built to study DM through electron recoils instead. As the electron is 4 orders of magnitude lighter than the nu- cleus, electron recoils can be induced by DM down to 4 orders of magnitude lighter than the lightest DM particle probeable with nuclear recoils. In order to understand current and upcoming results from experiments searching for DM induced electron recoils, a theoretical understanding of DM electron scatterings in detector materials is needed. When modelling such electron interactions, one need input both from DM and material physics. This thesis improves the theoretical understanding by both improving the material description using density functional theory (DFT), and by extending the DM description using non-relativistic effective theory (NR-EFT) tools. The improvement gives not only a more accurate description of the DM- electron interactions that the experiments are expected to see; it also vastly extends the forms of DM that can be studied in direct detection experiments. Before this extension, one typically focused on a benchmark case of DM, the so called dark photon model. With this extension, one can cover all forms of gravitationally bound DM with spins of 0, 1/2 or 1. In the included works, advances are made in the description of DM-electron interactions in common detector materials such as liquid xenon, silicon and germanium, as well as to materials in the research and development phase, such as graphene and carbon nanotubes (CNTs).
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| 5. |
- Zema, Vanessa, 1991
(författare)
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Unveiling the nature of dark matter with direct detection experiments
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The desire of discovery is an anthropic need which characterises and connects the human being over the eras. In particular, observing the sky is an instinctive drive exerted by the curiosity of the mysteries which it retains. At the present time, the tremendous advances in the exploration of space have opened even more challenges than back in the days. One of the most urgent question is unveiling the nature of dark matter (DM). As stated by Neta A. Bahcall (Professor at Princeton University), "Cosmology has revealed an amazing universe, filled with a "dark sector" that composes 95% of the energy density of our cosmos [...]" ( Dark matter universe , PNAS, 2015). About one-third of this dark sector is associated to an invisible and still undetected form of matter, the so-called dark matter, whose gravitational effect manifests at all cosmological scales. Both theoretical and experimental observations based on ordinary gravity reinforced the evidences for the existence of DM, since its first appearance in the pioneering calculations of F. Zwicky (1933). This PhD project explores the hypothesis that DM is made of new particles beyond the standard model. More specifically, it focuses on those DM particles which are trapped into the galactic gravitational field and populate the galactic halo. If DM interacts with ordinary particles, extremely sensitive detectors operating in very low-background environments, are expected to detect galactic DM particles scattering off their target material. This widely employed experimental technique is known as DM direct detection and it is the focus of my studies, where I consider the further hypothesis that DM interacts with atomic nuclei. The research I conducted during my PhD program consists of two main parts: the first part focused on purely phenomenology aspects of the DM direct detection (namely on the DM annual modulation treated using a non-relativistic effective theory and on the scattering of spin-1 DM particles off polarised nuclei) and the second one is more closely connected to experimental applications. The latter has been strongly stimulated by my collaboration with the two DM direct detection experiments CRESST and COSINUS. For CRESST, I compute the DM-nucleus cross-section for the conventional spin-dependent interactions, used to analyse the data collected with a prototype Li-based detector module, and I derive some prospects for a time dependent analysis of CRESST-III data, using a statistical frequentist approach based on Monte Carlo simulations. For COSINUS, I provide a significant extension of the pulse shape model currently used by CRESST and COSINUS in order to explain experimental observations related to the COSINUS detector response. Finally, I contribute to ongoing studies on the phonon propagation in NaI crystals based on solid state physics. This PhD thesis has been oriented to fill the gap between theoretical and experimental efforts in the DM field. This approach has facilitated the exchange of expertise, has driven the trend of my research and has stimulated the development of the ideas and methods described in this PhD thesis.
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| 6. |
- Bao, Ling, 1980
(författare)
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Aspects of Wrapped Branes in String and M-Theory
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of an introductory text together with five appended research papers. The Ariadne's thread through the whole thesis is various effects coming from high-dimensional p-branes in various subsectors of string and M-theory.The low energy effective actions in string and M-theory consists of a classical supergravity together with quantum corrections. In particular the non-perturbative correction terms arise from instanton effects, which are interpreted as p-branes wrapping supersymmetric cycles. The general structure of the full effective action is the result of a complicated interplay between supersymmetry and U-duality. Requiring the action to be invariant under U-duality leads to mathematical functions called automorphic forms. Both perturbative and non-perturbative corrections seem to be captured by these functions. The U-duality groups can be found by analyzing the algebraic structures of the moduli space after toroidal compactification. Using this line of thinking, some simple examples of higher order derivative corrections in pure gravity are investigated.Compactification on manifolds with special holonomy is also discussed in this thesis, with focus on the resulting moduli spaces. Certain quantum corrections to type IIA string theory compactified on a rigid Calabi-Yau threefold are analyzed.Manifolds with special holonomy constitute target spaces of the topological subsectors in string and M-theory. The low energy effective action of these theories consists of a classical contribution from a form theory of gravity, which receives quantum corrections from branes wrapping supersymmetric cycles in the target space. In particular the dynamics of the M2- and M5-branes are discussed in the context of a topological version of M-theory.
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| 7. |
- Matonoha, Oliver
(författare)
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Production of strangeness in partonic interactions at the LHC
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strong interaction is responsible for nearly all observable baryonic matter in the Universe. Quantum Chromodynamics, which describes interactions between quarks and gluons, however, cannot be solved analytically in the non-perturbative regime, involving low momentum transfers. In this regime, interesting phenomena occur, such as the formation of colour-neutral hadrons from constituent quarks and in extreme conditions, the transition of hadronic matter to a plasma of deconfined quarks and gluons. This quark-gluon plasma (QGP) is believed to have comprised the Universe in the first several microseconds after the Big Bang and can be recreated in laboratory conditions, such as in collisions of ultra-relativistic heavy nuclei at the Large Hadron Collider (LHC).The QGP exhibits certain signatures whose strength varies with the multiplicity of particles produced in the collisions, which is directly linked to the number of colliding nucleons in the collision and the energy density in the initial state. In the last decade, contrary to expectations, it has been discovered that pp collisions and pA collisions also exhibit QGP-like behaviour, including an increase in the production of strange particles and an increase in the ratio of neutral strange hadrons, Lambda to K0s, at intermediate transverse momentum p_T. However, in pp collisions, it is challenging to link particle multiplicity to the initial state.This dissertation aims to investigate the origin of QGP-like behaviour in pp collisions by analysing the production of K0s and Lambda particles and their dependence on event shape and sub-structure. Specifically, measurements are performed using the ALICE detector at the LHC for pp collisions at $\sqrt{s}=13$ TeV. For the first time ever, observables quantifying the event shape geometry, the transverse spherocity S_O, and the magnitude of the underlying event activity, the R_T, R_T,min, and R_T,max, are employed to investigate their effect on the production of K0s and Lambda particles. These observables allow for a more differentiated understanding of the collision dynamics and help access the number of colliding quarks and gluons (partons). The results of this study will contribute to the understanding of the QGP-like behaviour in pp collisions and help further the understanding of the strong interaction at low momentum transfers.
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| 8. |
- Palmkvist, Jakob, 1980
(författare)
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Exceptional Lie algebras and M-theory
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we study algebraic structures in M-theory, in particular the exceptional Lie algebras arising in dimensional reduction of its low energy limit, eleven-dimensional supergravity. We focus on e8 and its infinite-dimensional extensions e9 and e10. We review the dynamical equivalence, up to truncations on both sides, between eleven-dimensional supergravity and a geodesic sigma model based on the coset E10/K(E10), where K(E10) is the maximal compact subgroup. The description of e10 as a graded Lie algebra is crucial for this equivalence. We study generalized Jordan triple systems, which are closely related to graded Lie algebras, and which may also play a role in the description of M2-branes using three-dimensional superconformal theories. The introductory part is followed by five research papers.In Paper I we show that the spinor and vector-spinor representations of k(e10) in the fermionic extension of the original E10 coset model lead, upon restriction to k(e9), to the R-symmetry transformations in eleven-dimensional supergravity reduced to two dimensions. Paper II provides an explicit expression for the primitive E8 invariant tensor with eight symmetric indices, which is expected to appear in M-theory corrections in the reduction to three dimensions. In Paper III we show that e8, e9 and e10 can be constructed in a unified way from a Jordan algebra, via generalized Jordan triple systems. Also Paper IV deals with generalized Jordan triple systems, but in the context of superconformal M2-branes. We show that the recently proposed theories with six or eight supersymmetries can be expressed in terms of a graded Lie algebra. In Paper V we return to the bosonic E10 coset model, and apply it to gauged maximal supergravity in three dimensions.
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| 9. |
- Bélanger-Champagne, Camille, 1982-
(författare)
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Measurements of Angular Correlations in Minimum Bias Events and Preparatory Studies for Charged Higgs Boson Searches at the Tevatron and the LHC
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Studies of minimum bias events at colliders probe the behavior of QCD in the non-perturbative regime. The phenomenology of events in this regime is described by empirical models that take many parameters, which all need to be tuned to the observed data. Measurements based on angular correlations between the highest transverse momentum charged particle track and the other charged particle tracks in collision events can, because of their robustness against experimental and detector effects, be a component of the tuning inputs for the models. We measure such observables in a variety of pseudorapidity ranges and at many center-of-mass energies at DØ and ATLAS. We observe that such observables are poorly described by current models and tunes that are used to produce simulated event samples, making them valuable information for the tuning process. The Matrix Element method is a powerful analysis tool to extract precise measurements from data samples of limited statistics. We have investigated the potential of the Matrix Element method to measure the mass of the charged Higgs in the exclusive decay H±→τ±ν→e±+3ν when produced in top quark decays at the Tevatron, with emphasis on the construction of transfer functions in the τ decay chain. We concluded that the τ decay chain can be successfully parametrized via a transfer function and that the method has the potential to provide an accurate charged Higgs mass measurement in this channel. Triggering on τ leptons is a key component for many beyond the Standard Model searches at ATLAS, such as the search for the charged Higgs boson. Events containing Z bosons can be used to measure the efficiency of the ATLAS τ hadronic-decay trigger. We have used a tag-and-probe method on simulated Z boson decays to 2 τ leptons where one decays to a μ while the other decays hadronically. The μ is used as the tag and the τ side is probed. We demonstrated that the efficiency of the τ hadronic-decay trigger can be accurately measured with this method using the first 100 pb-1 of ATLAS data.
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| 10. |
- Bergeås Kuutmann, Elin, 1980-
(författare)
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Calibration of the ATLAS calorimeters and discovery potential for massive top quark resonances at the LHC
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- ATLAS is a multi-purpose detector which has recently started to take data at the LHC at CERN. This thesis describes the tests and calibrations of the central calorimeters of ATLAS and outlines a search for heavy top quark pair resonances.The calorimeter tests were performed before the ATLAS detector was assembled at the LHC, in such a way that particle beams of known energy were targeted at the calorimeter modules. In one of the studies presented here, modules of the hadronic barrel calorimeter, TileCal, were exposed to beams of pions of energies between 3 and 9 GeV. It is shown that muons from pion decays in the beam can be separated from the pions, and that the simulation of the detector correctly describes the muon behaviour. In the second calorimeter study, a scheme for local hadronic calibration is developed and applied to single pion test beam data in a wide range of energies, measured by the combination of the electromagnetic barrel calorimeter and the TileCal hadronic calorimeter. The calibration method is shown to provide a calorimeter linearity within 3%, and also to give a reasonable agreement between simulations and data. The physics analysis of this thesis is the proposed search for heavy top quark resonances, and it is shown that a narrow uncoloured top pair resonance, a Z', could be excluded (or discovered) at 95% CL for cross sections of 4.0±1.6 pb (in the case of M=1.0 TeV/c2) or 2.0±0.3 pb (M=2.0 TeV/c2), including systematical uncertainties in the model, assuming √s = 10 TeV and an integrated luminosity of 200 pb-1. It is also shown that an important systematical uncertainty is the jet energy scale, which further underlines the importance of hadronic calibration.
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