Browsing by study line "Particle Physics and Cosmology"
Now showing items 120 of 33

(2023)In this thesis a computation of the nonperturbative 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 oneparticleirreducible (1PI) graviton 2point function. A brief discussion on the perturbative nonrenormalizability 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 transversetraceless (TT) 1PI graviton 2point 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 KällénLehmann spectral representation of propagators. The nonperturbative TT 1PI graviton 2point function, the propagators and the spectral functions, are parameterized and the fRG flow equation for the TT 1PI graviton 2point 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.

(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 finetune 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 Am241, Ba133, Co57, Cs137 and Xray quality series in order to study the sensitivity of the device and its capacity to detect gamma and Xrays.

(2023)Tutkielmani käsittelee kullan pysyvän isotoopin Au197 tuottamista neutroniaktivaatiolla luonnollisesta elohopeanäytteestä. Kokeen kannalta pääasiallinen reaktiomenetelmä oli Hg196 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 AmBeneutronilähteen 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 HDPEtankoa. Säteilytyksen jälkeen näytettä mitattiin STUKin gammaspektrometrian laboratorion B6 pHPGe BE5030 germaniumilmaisimella, ja kullan synty voitiin todentaa spektristä löytyvien karakterististen gamma ja röntgenpiikkien avulla. Koejärjestely onnistui, ja työni osoittaa, että kullan pysyvän isotoopin Au197 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.

(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 highdensity 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 firstorder 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 firstorder phase transition. We approximate the low and highdensity 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 highmass 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.

(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 spinindependent 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 matternucleus 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 deexcitations 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.

(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 kinkantikink and kinkimpurity collisions in the $\phi^4$ model. Kinkantikink pair production from a wobbling kink is also studied, in which case we found that the separation velocity of the produced kinkantikink 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 kinkantikink 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 kinkantikink 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.

(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 phononbased 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.

(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 twopoint 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 1dimensional or 2dimensional 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 Nbody simulations. Finally, as there is a welldefined relation between real and redshift space, one could also assume that there is a relation between the twopoint correlation functions in both real and redshift space. In this project, we will prove that the realspace onedimensional twopoint correlation function, which is the physically meaningful one, can be derived from the twodimensional twopoint correlation function in redshift space following a geometrical procedure independent of approximations. This method, in theory, should work for all distance scales.

(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 lowerdimensional 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 twodimensional Conformal Field Theory resides on the boundary of threedimensional 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 higherspin 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 welldefined, 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.

(2024)Gravitational waves from cosmological phase transitions are a promising probe of the early universe. Many theories beyond the Standard Model predict the early universe to have undergone a cosmological firstorder phase transition at the electroweak scale. This transition would have produced gravitational waves potentially detectable with the future spacebased detector Laser Interferometer Space Antenna (LISA). We study the gravitational wave power spectrum generated by sound waves, which are a dominant source of gravitational waves from firstorder phase transitions. We compare two methods for calculating the sound wave power spectrum: a simulationmotivated broken powerlaw fit of the shape of the spectrum, and a wider theoretical framework called the Sound Shell Model, which includes hydrodynamic calculations of the phase transition. We present an implementation of the Sound Shell Model into the PTPlot tool, which is currently based on the broken powerlaw fit. With PTPlot, we calculate the signaltonoise ratios of LISA for the sound wave power spectrum of each method. The signaltonoise ratio allows us to estimate the detectability of gravitational wave signals with LISA. We analyse how the detectability of certain particle physics models changes between the two different methods. Our results show that the Sound Shell Model has a potentially significant impact on the signaltonoise ratio predictions, but it does not uniformly improve or worsen the detectability of the gravitational wave signals compared to the broken power law. The code implementation is overall successful and lays the foundation for an updated release of PTPlot and future work within this topic.

(2020)We study how higherorder 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 higherorder 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 higherorder 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 higherorder curvature couplings.

(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 (813 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

(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.

(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 spinstatistics 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.

(2024)In this thesis, the metric junction conditions are investigated for null and nonnull hypersurfaces. Along with the junction conditions we also investigate the thin shell formalism, which arises when the second junction condition is violated. The second junction condition can be violated since violating it leads to only a deltafunction singularity in the Riemann tensor. This singularity is allowed since it has a physical explanation via the Einstein equation. The deltafunction singularity in the Riemann tensor corresponds to an infinitesimally thin layer of matter at the hypersurface. We also present an example calculation for thin shell formalism by calculating junction conditions for general spherically symmetric spacetimes being joined by stationary spherically symmetric hypersurface. At the end, we also mention the practical usage of the junction conditions.

(2024)Color confinement, the inability for free quarks to exist at normal temperatures and densities, is one of the most important properties of Quantum Chromodynamics, the quantum field theory (QFT) of the strong interaction. A simple representation of confinement can be obtained by considering a static (i.e. infinitely massive) quarkantiquark pair. The potential of the pair contains a term linear with respect to the separation. Therefore, separating the pair would require infinite energy meaning that the quarks are confined. The static potential can be related to the expectation value of a QFT operator called the Wilson loop. In the nonperturbative large coupling regime lattice field theory can be applied to estimate expectation values of observables. The idea of lattice field theory is to replace continuous spacetime with a lattice, where the fields are defined on the sites and links. The discretization of spacetime allows evaluating path integrals numerically using Monte Carlo methods. An alternate way of computing the Wilson loop expectation value is provided by holographic duality. It is an equivalence between a QFT in fourdimensional flat spacetime and a higherdimensional theory of gravity in curved spacetime. The duality allows evaluation of hard, nonperturbative QFT calculations with easy classical computations on the gravity side. From the lattice data of the static potential, we can construct a holographic model that could produce those results in a process called bulk reconstruction. The constructed holographic model can then be used to compute other QFT quantities such as entanglement entropy. These quantities allow us to study confinement among other things. In this thesis, lattice field theory measurements of the static potential at different temperatures are presented. Then using a machine learning method, the corresponding holographic metrics are constructed from the lattice results. The lattice simulations have been done as a learning exercise and for the bulk reconstruction method, this thesis is a proof of concept, where no further computations are done using the constructed metrics. The lattice results are in good agreement with previous ones and the bulk reconstruction method seems to work as intended. In future works the method should be applied to a bigger dataset and other quantities should be computed with the constructed metrics.

(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 10fold 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.

(2024)Vector boson scattering (VBS) is a process that occurs when vector bosons, the spin1 fundamental particles of the Standard Model (SM), are radiated by quarks accelerated to high energies in a collider environment. These bosons may interact with each other before decaying due to their instability. This process is of interest because of its strong sensitivity to physics Beyond the Standard Model (BSM): occurring in the highenergy regime at the cutting edge of modern technology, VBS is one of few processes able to probe the coupling of the gauge fields to each other, as well as to the Higgs field, both of which are sensitive to new physics which may spoil the delicate cancellations required to maintain unitarity. In particular, the polarization states of the massive vector bosons—the W and Z, carriers of the weak interaction—are acquired as a direct consequence of electroweak symmetry breaking (EWSB). EWSB has been at the frontier of known physics since the Higgs boson was confirmed to exist in 2012. VBS has three possible decay channels: fully leptonic, semileptonic, and allhadronic. We focus on the allhadronic channel, in which each boson begets two quarks, to take advantage of its higher branching ratio: nearly half of all VBS events decay through this channel. With the Large Hadron Collider (LHC) in a period of long shutdown as it is upgraded to reach higher luminosities, one of the major steps in VBS analyses is to separate the BSMsensitive electroweak VBS from chromodynamicallyinduced (QCD) diboson production, in which vector bosons do not interact; furthermore, a deeper understanding of EWSB requires the various polarization states to be distinguished from each other. To these ends, simulation is a critical component of analyses, as VBS is among the rarest processes at the LHC and overwhelmed by backgrounds with larger cross sections. We investigate the viability of employing the MadGraph5_aMC@NLO suite to generate VBS events, successfully producing electroweak, QCD, and mixed samples of all possible combinations of weak bosons: the samesign WW, oppositesign WW, WZ, and ZZ channels. Furthermore, we create working samples of these VBS channels with enforced polarization: both bosons longitudinally polarized, both transversely polarized, and one of each. We provide histograms that report differences in the angular distributions of the produced events, showing how to distinguish the modes of production and polarizations based on kinematic topology.

(2023)Matterantimatter 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 chargeparity symmetries (C and CP violation) and must happen out of equilibrium which is related with the chargeparitytime (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 halflife 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 neutronantineutron oscillations and the neutronmirror 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 neutronantineutron 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 neutronantineutron 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 chargeparity 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.

(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 realtime computer simulations of various cosmic string models in hopes of extracting their effects on the gravitational wave background. In this thesis we discuss the AbelianHiggs model, a toy model of a gauge theory of a complex scalar field and a real vector field. Through a choice of a symmetrybreaking 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 stringstring 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.
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