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Browsing by study line "Elementarpartikelfysik och kosmologi"

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  • Stirling, Nico Toivo (2023)
    In this thesis a computation of the non-perturbative Lorentzian graviton propagator, which has appeared in the literature, is outlined. Firstly, the necessary ingredients for the computation are introduced and discussed. This includes; General Relativity (GR), its path integral quantisation around a Minkowski space background, and the definition of the graviton propagator along with its relation to the one-particle-irreducible (1PI) graviton 2-point function. A brief discussion on the perturbative non-renormalizability of the theory is followed by the introduction of the functional renormalization group (fRG) equation from which a fRG equation for the scalar coefficient function of the transverse-traceless (TT) 1PI graviton 2-point function is derived. After these ingredients have been introduced we proceed to outline the computation in question, skipping the details of its most involved steps. The computation starts by defining the spectral function and the Källén-Lehmann spectral representation of propagators. The non-perturbative TT 1PI graviton 2-point function, the propagators and the spectral functions, are parameterized and the fRG flow equation for the TT 1PI graviton 2-point function is used together with certain renormalization conditions to define renormalization group (RG) flow equations for these parameters. The solution of the flow of the parameters is displayed and is used to construct the graviton spectral function and the graviton propagator, which are both displayed graphically. Finally, a discussion of the features of the spectral function and propagator are given, and these results are briefly discussed in the context of the asymptotic safety program for quantum gravity and some of its open issues.
  • Nincă, Ilona Ştefana (2020)
    Cadmium Telluride (CdTe) has a high quantum efficiency and a bandgap of 1.44 eV. As a consequence, it is being used to efficiently detect gamma rays. The aim of this thesis is to explore the properties of the CdTe pixelated detector and the procedures conducted in order to fine-tune the electronic readout system. A fully functional CdTe detector would be useful in medical imaging techniques such as Boron Neutron Capture Therapy (BNCT). BNCT requires a detector with a good energy resolution, a good timing resolution and a good stopping power. Although the CdTe crystal is a promising material, its growing process is difficult due to the fact that different types of defects appear inside the crystal. The quality assurance process has to be thorough in order for suitable crystals to be found. An aluminum oxide layer (Al2O3) was passivated onto the surface of the crystal. The contacts for both sides were created using Titanium Tungsten (TiW) and gold (Au) sputtering deposition, followed by an electroless nickel growth. I tested the CdTe pixelated detector with different radioactive sources such as Am-241, Ba-133, Co-57, Cs-137 and X-ray quality series in order to study the sensitivity of the device and its capacity to detect gamma and X-rays.
  • Arvo, Jukka (2023)
    Tutkielmani käsittelee kullan pysyvän isotoopin Au-197 tuottamista neutroniaktivaatiolla luonnollisesta elohopeanäytteestä. Kokeen kannalta pääasiallinen reaktiomenetelmä oli Hg-196 neutronikaappaus. Kyseinen transmutaatio suoritettiin myös kokeellisesti. Elohopeaa sisältävänä näytteenä käytettiin Ardentin valmistamia Futura Standard -hammasamalgaamikapseleita. Turvallisuussyistä kapselit olivat koejärjestelyssä alkuperäisessä purkissaan. Kapseleita oli kaikkiaan 50 kappaletta, ja jokaisessa oli 400 mg elohopeaa. Yhteensä näytteessä oli siis 20 grammaa elohopeaa. Näytettä säteilytettiin STUKin tiloissa 14 vuorokauden ajan kolmen AmBe-neutronilähteen avulla. Valmistajan ilmoittamat neutronituotot käytetyille lähteille ovat 2.0E+7 n/s, 2.1E+6 n/s ja 6.7E+5 n/s. Lähteiden ilmoitetut aktiivisuudet ovat vastaavasti 333 GBq, 37.0 GBq ja 11.1 GBq. Neutronien hidastamiseen käytettiin HDPE-tankoa. Säteilytyksen jälkeen näytettä mitattiin STUKin gammaspektrometrian laboratorion B6 p-HPGe BE5030 -germaniumilmaisimella, ja kullan synty voitiin todentaa spektristä löytyvien karakterististen gamma- ja röntgenpiikkien avulla. Koejärjestely onnistui, ja työni osoittaa, että kullan pysyvän isotoopin Au-197 valmistaminen luonnollisesta elohopeasta havaittavissa määrin on mahdollista toteuttaa melko yksinkertaisella koejärjestelyllä käyttäen varsin pienitehoisia neutronilähteitä. Varsinaisen kokeen lisäksi käsittelen työssäni myös kullanteon historiaa sekä aiheeseen liittyvää teoriaa.
  • Blomqvist, Sofia (2024)
    The matter in neutron stars exist under extreme conditions, and the cores of these stars harbour densities unreachable in any laboratory setting. Therefore, this unique environment provides an exceptional opportunity to investigate high-density matter, described by the theory of Quantum Chromodynamics (QCD). This thesis centers on the exploration of twin stars, hypothetical compact objects that extend beyond the neutron star sequence. Originating from a first-order phase transition between hadronic matter and quark matter, our focus is on understanding the constraints on these phase transitions and their effect on the observable properties of twin stars. In our investigation of twin stars, we construct a large ensemble of possible equations of state featuring a strong first-order phase transition. We approximate the low- and high-density regions with polytropic form and connect them to chiral effective field theory results at nuclear densities and extrapolated perturbative QCD at high densities. The resulting equations of state are then subjected to astrophysical constraints obtained from high-mass pulsars and gravitational wave detections to verify their compatibility with observations. Within our simple study, we identify two distinct types of twin stars, each providing a clear signature in macroscopic observables. These solutions originate from separate regions in the parameter space, with both regions being relatively small. Twin stars in our approach generally obtain small maximum masses, while the part of the sequence corresponding to neutron stars extends to large radii, indicating that these solutions only marginally pass the astrophysical constraints. Finally, we find that all twin stars obtain sizable cores of quark matter.
  • Al-Adulrazzaq, Aula (2023)
    Dark matter direct detection experiments still have found no evidence of the dark matter WIMPs. The search has therefore been expanded for lighter dark matter candidates. Light dark matter is nearly invisible to current detectors through the elastic nuclear recoils. This thesis is meant to provide understanding on the inelastic atomic scatterings, which are one good way to detect dark matter particles with mχ ∼ GeV. In this thesis we consider spin-independent scatterings. Inelastic scatterings are based on the fact that in an atom, electrons do not follow the motion of the recoil nucleus immediately, but instead it takes time. This results in a small probability of observable ionization or excitation of the atom. This is known as the Migdal effect. We will first study the theoretical framework of dark matter-nucleus scatterings, showing how to get the event rate and how it is factorized into the astrophysical, the particle physics and the target response part. Then we will move to the inelastic processes, Migdal and Bremsstrahlung effects, deriving their event rates. In the first, we try to detect ionized electrons. The latter one, the Bremsstrahlung, is a similar process to the Migdal, but there we try to detect photons emitted from the de-excitations of atoms excited in the inelastic recoils. We will also look into the Migdal in semiconductors. Because of the smaller gap for electron excitations in crystals, we find that the rate for the Migdal effect is much higher in semiconductors than in atomic targets, thus allowing the search for even lighter dark matter particles. The rate can be expressed in terms of the energy loss function of the target material.
  • Nurminen, Niilo Waltteri (2021)
    Phase transitions in the early Universe and in condensed matter physics are active fields of research. During these transitions, objects such as topological solitons and defects are produced by the breaking of symmetry. Studying such objects more thoroughly could shed light on some of the modern problems in cosmology such as baryogenesis and explain many aspects in materials research. One example of such topological solitons are the (1+1) dimensional kinks and their respective higher dimensional domain walls. The dynamics of kink collisions are complicated and very sensitive to initial conditions. Making accurate predictions within such a system has proven to be difficult, and research has been conducted since the 70s. Especially difficult is predicting the location of resonance windows and giving a proper theoretical explanation for such a structure. Deeper understanding of these objects is interesting in its own right but can also bring insight in predicting their possibly generated cosmological signatures. In this thesis we have summarized the common field theoretic tools and methods for the analytic treatment of kinks. Homotopy theory and its applications are also covered in the context of classifying topological solitons and defects. We present our numerical simulation scheme and results on kink-antikink and kink-impurity collisions in the $\phi^4$ model. Kink-antikink pair production from a wobbling kink is also studied, in which case we found that the separation velocity of the produced kink-antikink pair is directly correlated with the excitation amplitude of the wobbling kink. Direct annihilation of the produced pair was also observed. We modify the $\phi^4$ model by adding a small linear term $\delta \phi^3$, which modifies the kinks into accelerating bubble walls. The collision dynamics and pair production of these objects are explored with the same simulation methods. We observe multiple new effects in kink-antikink collisions, such as potentially perpetual bouncing and faster bion formation in comparison to the $\phi^4$ model. We also showed that the $\delta$ term defines the preferred vacuum by inevitably annihilating any kink-antikink pair. During pair production we noticed a momentum transfer between the produced bion and the original kink and that direct annihilation seems unlikely in such processes. For wobbling kink - impurity collisions we found an asymmetric spectral wall. Future research prospects and potential expansions for our analysis are also discussed.
  • Mozejko, Arik (2023)
    Dark matter (DM) is introduced and explored in a holistic perspective. Topics include observational evidence, various DM properties, potential candidates, and the tenets of indirect versus direct DM detection. Then an emphasis is placed on understanding the cryogenic detection of weakly interacting massive particles, with explicit connection to phonon-based detection of DM. The importance of improving methods of DM direct detection are emphasised, with specifically the usage of molecular dynamics simulations as an avenue of studying defect creation in cryogenic detector materials. Previous investigations into this area are reviewed and expanded upon through novel experimentation into how defect properties vary when changing thermal motion of the crystal lattice. This experimentation is conducted via the usage of molecular dynamics simulations on sapphire (Al2O3) as a DM direct detection material, and it is found that while atomic velocity does not impact the overall emergent defect structure, it does have an impact on the energy lost in these defects. Changing the temperature of the lattice produces the expected results, generating greater variance in both defect band structure as well as average energy loss.
  • Gonzalez Ateca, Marcos (2020)
    The distribution of matter in space is not homogeneous. Large structures such as galaxy groups, clusters or big empty spaces called voids can be observed at large scales in the Universe. The large scale structure of the Universe will depend on both the cosmological parameters and the dynamics of galaxy formation and evolution. One of the main observables that allow us to quantify this structure is the two-point correlation function, with which we can trace different galaxy properties such as luminosity, stellar mass and also, it enables us to track its evolution with redshift. In galaxy surveys, we do not obtain the location of galaxies in real space. We obtain our data in what it is called redshift space. This redshift space can be defined as a distortion of the real space generated by the redshift introduced by the peculiar velocities of galaxies and from the Hubble expansion of the Universe. Therefore, the distribution of galaxies in redshift space will look different from the one obtained in real space. These differences between both spaces are small but not negligible, and they depend strictly on the cosmology. In this work, we will assume a ΛCDM cosmology. Therefore, in order to find the different 1-dimensional or 2-dimensional correlations functions, we will use the most updated version of the code provided by the Euclid consortium, which belongs officially to the ESA Euclid mission. Moreover, we will also need different galaxy catalogues. These catalogues have already been simulated and they are called Minerva mocks, which are a set of 300 different cosmological mocks produced with N-body simulations. Finally, as there is a well-defined relation between real and redshift space, one could also assume that there is a relation between the two-point correlation functions in both real and redshift space. In this project, we will prove that the real-space one-dimensional two-point correlation function, which is the physically meaningful one, can be derived from the two-dimensional two-point correlation function in redshift space following a geometrical procedure independent of approximations. This method, in theory, should work for all distance scales.
  • Gibson, Natalie (2023)
    The search for a profound connection between gravity and quantum mechanics has been a longstanding goal in theoretical physics. One such connection is known as the holographic principle, which suggests that the dynamics within a given region of spacetime can be fully described on its boundary surface. This concept led to the realization that string theory provides a lower-dimensional description that encapsulates essential aspects of spacetime. While the "AdS/CFT correspondence" exemplifies the success of this holographic theory, it was discovered soon after that the Universe has a positive cosmological constant, Λ. This immediately sparked interest in a potential correspondence centered around de Sitter (dS) space, which is also characterized by a positive cosmological constant. This thesis comprehensively explores the de Sitter/Conformal Field Theory (dS/CFT) correspondence from various perspectives, along with the unique challenges posed by the distinct nature of dS space. The original dS/CFT duality proposes that a two-dimensional Conformal Field Theory resides on the boundary of three-dimensional asymptotic dS space. However, the definition and interpretation of physical observables within the dS/CFT framework remain open questions. Therefore, the discussions in this thesis not only cover the original dS/CFT conjecture, but also extend into more recent advancements in the field. These advancements include a higher-spin dS/CFT duality, the relationship between string theory and dS space, and the intriguing proposal of an "elliptical" dS space. While the dS/CFT correspondence is still far from being well-defined, there have been extensive efforts devoted to shedding light on its intricate framework and exploring its potential applications. As the Universe may be evolving towards an approximately de Sitter phase, understanding the dS/CFT correspondence offers a unique opportunity for gaining fresh insights into the link between gravity and quantum field theory.
  • Annala, Jaakko (2020)
    We study how higher-order gravity affects Higgs inflation in the Palatini formulation. We first review the metric and Palatini formulations in comparative manner and discuss their differences. Next cosmic inflation driven by a scalar field and inflationary observables are discussed. After this we review the Higgs inflation and compute the inflationary observables both in the metric and Palatini formulations. We then consider adding higher-order terms of the curvature to the action. We derive the equations of motion for the most general action quadratic in the curvature that does not violate parity in both the metric and Palatini formulations. Finally we present a new result. We analyse Higgs inflation in the Palatini formulation with higher-order curvature terms. We consider a simplified scenario where only terms constructed from the symmetric part of the Ricci tensor are added to the action. This implies that there are no new gravitational degrees of freedom, which makes the analysis easier. As a new result we found out that the scalar perturbation spectrum is unchanged, but the tensor perturbation spectrum is suppressed by the higher-order curvature couplings.
  • Pankkonen, Joona (2020)
    The Standard Model is one of the accurate theories that we have. It has demonstrated its success by predictions and discoveries of new particles such as the existence of gauge bosons W and Z and heaviest quarks charm, bottom and top. After discovery of the Higgs boson in 2012 Standard Model became complete in sense that all elementary particles contained in it had been observed. In this thesis I will cover the particle content and interactions of the Standard Model. Then I explain Higgs mechanism in detail. The main feature in Higgs mechanism is spontaneous symmetry breaking which is the key element for this mechanism to work. The Higgs mechanism gives rise to mass of the particles, especially gauge bosons. Higgs boson was found at the Large Hadron Collider by CMS and ATLAS experiments. In the experiments, protons were collided with high energies (8-13 TeV). This leads to production of the Higgs boson by different production channels like gluon fusion (ggF), vector boson fusion (VBF) or the Higgsstrahlung. Since the lifetime of the Higgs boson is very short, it cannot be measured directly. In the CMS experiment Higgs boson was detected via channel H → ZZ → 4l and via H → γγ. In this thesis I examine the correspondence of the Standard Model to LHC data by using signal strengths of the production and decay channels by parametrizing the interactions of fermionic and bosonic production and decay channels. Data analysis carried by least squares method gave confidence level contours that describe how well the predictions of the Standard Model correspond to LHC data
  • Berlea, Vlad Dumitru (2020)
    The nature of dark matter (DM) is one of the outstanding problems of modern physics. The existence of dark matter implies physics beyond the Standard Model (SM), as the SM doesn’t contain any viable DM candidates. Dark matter manifests itself through various cosmological and astrophysical observations of the rotational speeds of galaxies, structure formation, measurements of the Cosmic Microwave Background (CMB) and gravitational lensing of galaxy clusters. An attractive explanation of the observed dark matter density is provided by the WIMP (Weakly Interacting Massive Particle) paradigm. In the following thesis I explore this idea within the well motivated Higgs portal framework. In particular, I explore three options for dark matter composition: a scalar field and U(1) and SU(2) hidden gauge Fields. I find that the WIMP paradigm is still consistent with the data. Even though it finds itself under pressure from direct detection experiments, it is not yet in crisis. Simple and well motivated WIMP models can fit the observed DM density without violating the collider and direct DM detection constraints.
  • Garcia Sturba, Sebastian (2023)
    Quantum field theory is often presented without clearly defined mathematical structures, especially in the case of field operators. We discuss axiomatic quantum field theory, where quantum fields and states are defined rigorously using distribution theory, alongside their assumed properties in the form of the Wightman axioms. We present the two key results that come from this construction, namely CPT symmetry and the spin-statistics connection. We then consider the construction of quantum fields in curved spacetime so as to discuss their behaviour in regions of large curvature, such as near black holes. This requires us to redefine fields and states in terms of *-algebras. We then present the GNS reconstruction theorem which allows us to get back the original definitions of these objects in Minkowski spacetime.
  • Raivio, Riku (2023)
    In high energy physics the microscopic nature of our universe is studied. A common experimental way is to collide particles such as protons with almost the speed of light and study the fragments that fly away from the interaction point. The results are compared to existing theories and help to modify or create knowledge of our universe. However, our senses are not capable of directly measuring those tiny and fast fragments from the collisions. Therefore, we need dedicated devices called particle detectors. Several types of radiation detectors are known to us. Most of those utilize the fact that particles in such experiments are ionizing when traversing matter. The detectors we are interested in contain gas as the main interacting material for the ionizing radiation, called gaseous detectors. One of these is the Gas Electron Multiplier (GEM), which uses electric fields to make electrons drift through the holes of one or several foils producing a signal that can be detected. Gain and ion backflow are two properties that can be used to evaluate the effectiveness of the detector. The effect of the hole diameters of the foil on the gain and on the ion backflow is insufficiently known, however. In this thesis such measurements have been performed. For this purpose a GEM detector operating in proportional region was constructed. The detector contained a special foil with four quadrants in such a way that the diameters of the holes were different in each quadrant. The functionality of the detector was verified by measuring the leakage current, by using a multichannel analyzer, and by calculating the sum of all currents. The detector constructed passed all the tests. The diameters of the holes in the foil quadrants were estimated by calculating the amount of pixels from the foil image taken. The outer hole diameters were around 50 µm and inner hole diameters 40 µm in the quadrant with smallest holes. The corresponding diameters in the quadrant with largest holes were around 70 µm and 66 µm. The measurements were started by searching the proper values for the drift and the induction fields and for the GEM voltage range. A drift field of 2 kV/cm, an induction field of 7 kV/cm, and a voltage range from 400 V to 500 V were chosen. The results from the actual measurements were similar on both sides of the foil. The effective gain increased steadily along with the voltage being around 10-fold at 500 V compared to that at 400 V . The ion backflow, on the other hand, stayed constant or even slightly decreased. The results measured from the four quadrants differed clearly from each other. In the quadrant with the smallest holes the effective gain was about twice as high as in the quadrant with the largest holes. Respectively the ion backflow was about 30 % higher in the quadrant with small holes than in the quadrant with large holes.
  • Otero Sanchez, Hector (2023)
    Matter-antimatter asymmetry is one of the problems that the Standard Model of particle physics faces. All the processes and interactions described by it cannot explain why in the universe the density of matter is greater than the density of antimatter. Baryogenesis is the name given to the mechanisms that can explain this asymmetry. The necessary conditions for a process to generate the asymmetry are the Sakharov conditions. The process must violate the baryon number conservation, must violate charge and charge-parity symmetries (C and CP violation) and must happen out of equilibrium which is related with the charge-parity-time (CPT) violation. Possible processes that can violate the baryon number are proton decay and neutron oscillations. None of them have been observed experimentally. In some theories that allow proton decay, the half-life is some orders of magnitude greater than the age of the universe which implies that high energy scales are needed for testing this decay. However, neutron oscillations have less restrictive bounds. Two options for these oscillations are the neutron-antineutron oscillations and the neutron-mirror neutron oscillations. In the first one, a neutron transforms into an antineutron over time, while in the second one, a neutron transforms into a sterile neutron (only interacts with our universe through gravity). The focus of this work will be neutron oscillations. Some experiments have helped to set bounds on the neutron-antineutron oscillation period and nowadays more advanced experiments based on the improvements of technology are being developed. These new experiments will be able to set new bounds or discover physics beyond the Standard Model. In the theoretical frame, some modifications can be implemented into the Dirac Lagrangian that produce a baryon number violation of two units; this corresponds to a neutron-antineutron oscillation. Once the Lagrangian is formulated the properties of the oscillations are studied. In particular, the probability of the oscillation and the symmetry properties of both the Lagrangian and the oscillation can be computed to check if the Sakharov conditions are satisfied. To do this diagonalization techniques, chiral notation and the charge and charge-parity conjugation operators will be used. The discovering of a process that violates baryon number conservation would be very important for the Standard Model. It could imply the existence of new physics and it could potentially solve matter- antimatter asymmetry.
  • Hällfors, Jaakko (2023)
    Topological defects are some of the more common phenomena of many extensions of the standard model of particle physics. In some sense, defects are a consequence of an unresolvable misalignment between different regions of the system, much like cracks in ice or kinks in an antiquated telephone cord. In our context, they present themselves as localised inhomogeneities of the fundamental fields, emerging at the boundaries of the misaligned regions at the cost of, potentially massive, trapped energy. Should the cosmological variety exist in nature, they are hypothesised to emerge from some currently unknown cosmological phase transition, leaving their characteristic mark on the evolution of the nascent universe. As of date, so called cosmic strings are perhaps the most promising type of cosmic defect, at least with respect to their observational prospects. Cosmic strings, as the name suggest, are linelike topological defects; exceedingly thin, yet highly energetic. Given the advent of gravitational wave astronomy, a substantial amount of research is devoted to detailed and expensive real-time computer simulations of various cosmic string models in hopes of extracting their effects on the gravitational wave background. In this thesis we discuss the Abelian-Higgs model, a toy model of a gauge theory of a complex scalar field and a real vector field. Through a choice of a symmetry-breaking scalar potential, this model permits line defects, so called local strings. We discuss some generalities of classical field theory as well as those of the interesting mathematical theory of topological defects. We apply these to our model and present the necessary numerical methods for writing our own cosmic string simulation. We use the newly written simulation to reproduce a number of contemporary results on the scaling properties of the string networks and present some preliminary results from a less investigated region of the model parameter space, attempting to compare the effects of different types of string-string interactions. Furthermore, preliminary results are presented on the thermodynamic evolution of the system and the effects a common computational trick, comoving string width, are discussed with respect to the evolution of the equation of state.
  • Molander, Andreas (2020)
    The Standard Model (SM) is the best established theory describing the observed matter and its interactions through all the fundamental forces except gravity. The SM is however not complete. For example, it does not explain the large difference between the electroweak scale and the Planck scale, which is known as the hierarchy problem, nor does it explain dark matter. Therefore there is a need for more comprehensive theories beyond the SM. Supersymmetry (SUSY) extends the SM with predictions of a partner particle (sparticle) for each currently known elementary particle. A few of its benefits are that it gives an explanation to the hierarchy problem and predicts the existence of a good particle candidate for dark matter. However, there is no experimental evidence for SUSY so far. The search for SUSY particles is currently on-going at the experiments using the Large Hadron Collider (LHC) at CERN. So far, the searches have been focusing on strongly interacting supersymmetric particles, still without findings. One of the parameter ranges still to be covered, is the compressed mass scenario in the lower mass end for weakly interacting sparticles, where the masses of the lightest and second lightest supersymmetric particle do not differ much in mass. If they exist, low mass SUSY particles could be created in the LHC from two fusing photons emitted by forward-scattered protons. In such two-photon (central exclusive) processes, both protons might remain on-shell and continue their path down the beamline. Central exclusive processes are rather rare, so to advance the study of these events, new tagging techniques are required to record as many of these events as possible. We are interested in the kinematic range with a mass difference of less than 60 GeV between the slepton and the neutralino, which are the supersymmetric partners of the lepton and the neutral bosons. The CMS detector in the LHC has two event filtering (trigger) systems; the low level (L1) trigger and the high level trigger (HLT). A study has been conducted on how a specific HLT could increase the number of recorded events for the previously mentioned process, without significantly increasing the total HLT rate. To select more events, the transverse momentum threshold value of the produced leptons ought to be lowered. The forward-scattered protons will be detected by the Precision Proton Spectrometer (PPS). This thesis shows that requiring proton tracks in the PPS tracking detectors and tuning the multiplicity cut of these, will compensate for the lowering of the transverse momentum threshold, keeping the overall HLT rate sensible, while still enabling more interesting physics to be recorded.
  • Stendahl, Alex (2020)
    The Standard model of particle physics has been very successful in describing particles and their interactions. In 2012 the last missing piece, the Higgs boson, was discovered at the Large Hadron Collider. However even for all its success the Standard model fails to explain some phenomena of nature. Two of these unexplained phenomena are dark matter and the metastability of the electroweak vacuum. In this thesis we study one of the simplest extensions of the Standard model; the complex singlet scalar extension. In this framework the CP-even component of the singlet mixes with the Standard model like Higgs boson through the portal operator to form new mass eigenstates. The CP-odd component is a pseudo-Goldstone boson which could be a viable dark matter candidate. We analyse parameter space of the model with respect to constraints from particle physics experiments and cosmological observations. The time evolution of dark matter number density is derived to study the process of dark matter freeze-out. The relic density of the Dark Matter candidate is then calculated with the micrOmegas tool. These calculations are then compared to the measured values of dark matter relic density. Moreover, the electroweak vacuum can be stabilised due the contribution of the singlet scalar to the Standard Model Higgs potential. We derive the β-functions of the couplings in order to study the renormalisation group evolution of the parameters of the model. With the contribution of the portal coupling to the β-function of the Higgs coupling we are able to stabilise the electroweak vacuum up to the Planck scale. The two-loop β-functions are calculated using the SARAH tool.
  • Halkoaho, Johannes (2022)
    The primordial perturbations created by inflation in the early Universe are known to be able to produce significant amount of primordial black holes and gravitational waves with large amplitudes in some inflationary models. Primordial black holes are produced by primordial scalar perturbations and gravitational waves are partly primordial tensor perturbations and partly produced by scalar perturbations. In this thesis we review some of the current literature on the subject and discuss a few inflationary models that are capable of producing primordial scalar perturbations large enough to create a significant amount of primordial black holes. The main focus is on ultra-slow roll inflation with a concrete example potential illustrating the dynamics of the scenario followed by a briefer treatment of some of the alternative models. We start by explaining the necessary background theory for the understanding of the subject at hand. Then we move on to the inflationary models covered in this thesis. After that we explain the production of the primordial black holes and gravitational waves from scalar perturbations. Then we consider primordial black holes as a dark matter candidate and go through the most significant known restrictions on the existence of primordial black holes with different masses. We discuss some of the possible future constraints for the remaining possible mass window for which primordial black holes could explain all of dark matter. We then briefly discuss two planned space-based gravitational wave detectors that may be able to detect gravitational waves created by inflation.
  • Rantanen, Milla-Maarit (2020)
    Semiconductor radiation detectors are devices used to detect electromagnetic and particle radiation. The signal formation is based on the transportation of charges between the valence band and conduction band. The interaction between the detector material and the radiation generates free electrons and holes that move in opposite directions in the electric field applied between the electrodes. The movement of charges induces a current in the external electrical circuit, which can be used for particle identification, measurement of energy or momentum, timing, or tracking. There are several different detector materials and designs and, new options are continuously developed. Diamond is a detector material that has received a great amount of interest in many fields. This is due to its many unique properties. Many of them arise from the diamond crystal structure and the strength of the bond between the carbon atoms. The tight and rigid structure makes diamond a strong and durable material, which allows operation of diamond detectors in harsh radiation environments. This, combined with the fast signal formation and short response time makes diamond detector an excellent choice for high energy physics applications. The diamond structure leads also to a wide band gap. Thanks to the wide band bap, diamond detectors have low leakage current and they can be operated even in high temperatures without protection from surrounding light. Especially electrical properties of semiconductors strongly depend on the concentration of impurities and crystal defects. Determination of electrical properties can therefore be used to study the crystal quality of the material. The electrical properties of the material determine the safe operational region of the device and knowledge of the leakage current and the charge carrier transportation mechanism are required for optimized operation of detectors. Characterization of electrical properties is therefore an important part of semiconductor device fabrication. Electrical characterization should be done at different stages of the fabrication in order to detect problems at an early stage and to get an idea of what could have caused them. This work describes the quality assurance process of single crystal CVD (chemical vapour deposition) diamond detectors for the PPS-detectors for the CMS-experiment. The quality assurance process includes visual inspection of the diamond surfaces and dimensions by optical and cross polarized light microscopy, and electrical characterization by measurement of leakage current and CCE (charge collection efficiency). The CCE measurement setup was improved with a stage controller, which allows automatic measurement of CCE in several positions on the diamond detector. The operation of the new setup and the reproducibility of the results were studied by repeated measurements of a reference diamond. The setup could successfully be used to measure CCE over the whole diamond surface. However, the measurement uncertainty is quite large. Further work is needed to reduce the measurement uncertainty and to determine the correlation between observed defects and the measured electrical properties.