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Browsing by master's degree program "Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma (Particle Physics and Astrophysical Sciences)"

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  • Björn, Vesa (2023)
    Mercury is the smallest planet in the Solar System. The planet has a tenuous atmosphere, which means that it can be modeled as an atmosphereless object. Such Solar System bodies are covered in loose material called regolith, which affects how the object scatters light from the Sun. Photometry is a type of measurement that records the intensity of scattered light as a function of the viewing geometry, which is defined using angles from the surface towards the Sun and towards the observing instrument. Mercury was studied in 2011–2015 by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission of the United States’ National Aeronautics and Space Admin- istration, NASA. The present thesis uses spectrophotometric data, i.e., brightness as a function of wavelength, from the spacecraft’s Mercury Dual Imaging System (MDIS) instrument. Two theo- retical models, the Lommel–Seeliger (LS) and particulate medium (PM) models, are fitted to the observed reflectance using the least-squares method. The PM model is the more complicated of the two, and it includes a shadowing correction that de- pends on three model parameters. The parameters describe the properties of a particulate medium, i.e., regolith. The most important of the parameters is the packing density of the regolith, which is defined as the ratio of the volume of the particles to the total volume. The other two parameters describe the medium’s surface roughness in horizontal and vertical directions. The PM model is fitted to the observed reflectance for various different combinations of the model parameters. Ini- tially, only discrete and predetermined parameter values are used, but the parameter values are extended to arbitrary values using interpolation. Trilinear interpolation is utilized using several methods, followed by Markov chain Monte Carlo (MCMC) sampling for the final results. Most of the methods agree with one another, and fall within the uncertainties of the best solution, which allows to form an argumented conclusion about the best parameter values of the PM model. The best parameter values correspond to a densely-packed regolith with horizontally smooth surface and large height variations. The results of the present study can be used in the BepiColombo mission to Mercury, which is planned to begin its science operations in early 2026.
  • 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.
  • Minkkinen, Tiina (2023)
    Gravitational waves predicted by the theory of general relativity are providing us with an opportunity to study cosmological processes well beyond the horizon for electromagnetic observations. One such process is a first-order phase transition as the universe cools down, which could generate gravitational waves observable in the millihertz range today. The upcoming Laser Interferometer Space Antenna (LISA) is a space-based gravitational wave observatory with peak sensitivity in the millihertz range, so it has the potential to observe a sufficiently strong phase transition signal. The LISA Consortium have developed the LISA simulation pipeline for the purposes of creating mock data, which can then be used to test data analysis methods in preparation for the real LISA data analysis. Our aim is to first test the simulation pipeline by injecting a phase transition signal, with added instrument noises and galactic binary confusion noise, which we expect to be present in the real observations as well. Second, we will attempt to recover the injected signal to see whether it is detectable based on the deviance information criterion (DIC). We will do this for 25 different parameter combinations in order to chart the detectability of signals from different phase transition scenarios. Our results show better detectability for phase transition signals with higher amplitudes and frequencies centered around the mHz range, which is where the expected peak sensitivity of LISA lies. The confusion noise appears to be less of a distraction to our observations than the instrument noises, which set limits in the extremes of the LISA frequency range.
  • Li, Jichao (2024)
    Correlation functions in a superconformal field theory are strictly constrained by conformal symmetry. Notably, one-point functions of conformal operators always vanish. However, when a defect is inserted into the spacetime of the field theory, certain one-point functions become non-zero due to the broken conformal symmetry, highlighting the special properties of the defect. One interesting type of defect is the domain wall, which separates spacetime into two regions with distinct vacua. The domain wall version of $\mathcal{N}=4$ supersymmetric Yang-Mills (SYM) theory has been extensively studied in recent years. In this context, the supersymmetric domain wall preserves integrability, allowing one to evaluate one-point functions in the defect field theory using integrability techniques. As an analogous study of the domain wall version of $\mathcal{N}=4$ SYM theory, this thesis focuses on the ABJM theory with a 1/2-BPS domain wall, meaning that the domain wall preserves half the original supersymmetry. We first review integrability methods, e.g. the Coordinate Bethe ansatz and the Algebraic Bethe ansatz for $\mathfrak{su}(2)$ Heisenberg spin chain. The spectrum of the spin chain can be determined by solving sets of the Bethe equations. Moreover, the Rational $Q$-system is examined, which solves the Bethe equations efficiently and eliminates all nonphysical solutions automatically. On the field theory side, we first review the original ABJM theory and its spectral integrability following J. A. Minahan's work in 2009. There exists an underlying quantum $\mathfrak{su}(4)$ spin chain with alternating even and odd sites, whose Hamiltonian can be identified with the two-loop dilation operator of ABJM theory in the planar limit. This correspondence allows us to find the spectrum of ABJM theory using the Bethe ansatz. We study the $\mathfrak{su}(4)$ alternating spin chain and demonstrate the procedure for constructing eigenstates of ABJM theory. Finally, we study the tree-level one-point functions in the domain wall version of ABJM theory. We derive the classical solutions for the scalar fields that describe a domain wall and explicitly demonstrate how the domain wall preserves half of the supersymmetry. With these classical solutions, we define a domain wall version of ABJM theory. Then, we introduce the so-called Matrix Product State, which is a boundary state in the spin chain's Hilbert space. The domain wall can be identified with an integrable matrix product state, leading to a compact determinant formula for the one-point functions in spin chain language. Consequently, we can evaluate one-point functions explicitly using the Bethe ansatz and boundary integrability.
  • 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.
  • Veltheim, Otto (2022)
    The measurement of quantum states has been a widely studied problem ever since the discovery of quantum mechanics. In general, we can only measure a quantum state once as the measurement itself alters the state and, consequently, we lose information about the original state of the system in the process. Furthermore, this single measurement cannot uncover every detail about the system's state and thus, we get only a limited description of the system. However, there are physical processes, e.g., a quantum circuit, which can be expected to create the same state over and over again. This allows us to measure multiple identical copies of the same system in order to gain a fuller characterization of the state. This process of diagnosing a quantum state through measurements is known as quantum state tomography. However, even if we are able to create identical copies of the same system, it is often preferable to keep the number of needed copies as low as possible. In this thesis, we will propose a method of optimising the measurements in this regard. The full description of the state requires determining multiple different observables of the system. These observables can be measured from the same copy of the system only if they commute with each other. As the commutation relation is not transitive, it is often quite complicated to find the best way to match the observables with each other according to these commutation relations. This can be quite handily illustrated with graphs. Moreover, the best way to divide the observables into commuting sets can then be reduced to a well-known graph theoretical problem called graph colouring. Measuring the observables with acceptable accuracy also requires measuring each observable multiple times. This information can also be included in the graph colouring approach by using a generalisation called multicolouring. Our results show that this multicolouring approach can offer significant improvements in the number of needed copies when compared to some other known methods.
  • Normo, Sanna (2023)
    Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the solar corona. Fast CMEs can drive shock waves which are capable of accelerating charged particles to high energies. These accelerated particles emit electromagnetic radiation, including radio emission. Studying radio emission associated with CME-driven shocks offers a way to remotely investigate shock-accelerated electrons as well the shock itself. Solar radio bursts are transient events where the radio emission of the Sun rises above the background level. A classical division based on their appearance in a dynamic spectrum divides solar radio bursts into five categories: types I-V. Of these five different types, type II and type IV radio bursts are most commonly associated with CMEs. Occasionally, type II radio bursts exhibit a bursty fine structure known as herringbones. These are regarded as signatures of individual electron beams accelerated by CME-driven shocks. This thesis studies the radio emission associated with a CME that erupted on 1 September 2014. White-light imaging of the CME revealed a prominent shock wave. Simultaneously, the dynamic spectrum exhibited spike-like radio emission resembling herringbones. The aim of the study presented in this thesis is to find the source location of this radio emission relative to a three dimensional reconstruction of the shock. The source location of the radio emission can be used to conclude the likely origin of the electrons responsible for it. Additionally, in situ electron flux measurements are investigated in an attempt to connect the remote and in situ detections of energetic electrons. Using interferometric radio observations of the Sun and reconstructing the CME shock in three dimension revealed the location of the radio emission to be at the flank of the CME-driven shock. Such location suggests that the spike-like radio emission observed in the dynamic spectrum originates from shock-accelerated electrons. The location of the radio emission at the flanks of the CME shock was also used to get an estimation of the lateral expansion of the CME. Although the in situ electron flux measurements detected high-energy electrons, their inferred release time at the Sun did not coincide with observed radio emission.
  • Virta, Maxim (2022)
    Strongly coupled matter called quark–gluon plasma (QGP) is formed in heavy-ion collisions at RHIC [1, 2] and the LHC [3, 4]. The expansion of this matter, caused by pressure gradients, is known to be hydrodynamic expansion. The computations show that the expanding QGP has a small shear viscosity to entropy density ratio (η/s), close to the known lower bound 1/4π [5]. In such a medium one expects that jets passing through the medium would create Mach cones. Many experimental trials have been done but no proper evidence of the Mach cone has been found [6, 7]. Mach cones were thought to cause double-bumps in azimuthal correlations. However these were later shown to be the third flow harmonic. In this thesis a new method is proposed for finding the Mach cone with so called event-engineering. The higher order flow harmonics and their linear response are known to be sensitive to the medium properties [8]. Hence a Mach cone produced by high momentum jet would change the system properties and, thus, the observable yields. Different flow observables are then studied by selecting high energy jet events with different momentum ranges. Considered observables for different momenta are then compared to the all events. Found differences in the flow harmonics and their correlations for different jet momenta are reported showing evidence of Mach cone formation in the heavy-ion collisions. The observations for different jet momenta are then quantified with χ 2 -test to see the sensitivities of different observables to momentum selections.
  • Siilin, Kasper (2022)
    I use hydrodynamic cosmological N-body simulations to study the effect that a secondary period of inflation, driven by a spectator field, would have on the Local Group substructures. Simulations of the Local Group have been widely adopted for studying the nonlinear structure formation on small scales. This is essentially because detailed observations of faint dwarf galaxies are mostly limited to within the Local Group and its immediate surroundings. In particular, the ∼ 100 dwarf galaxies, discovered out to a radius of 3 Mpc from the Sun, constitute a sample that has the potential to discriminate between different cosmological models on small scales, when compared to simulations. The two-period inflaton-curvaton inflation model is one such example, since it gives rise to a small-scale cut-off in the ΛCDM primordial power spectrum, compared to the power spectrum of the ΛCDM model with single field power-law inflation. I investigate the substructures that form in a simulated analogue of the Local Group, with initial conditions that incorporate such a modified power spectrum. The most striking deviation, from the standard power-law inflation, is the reduction of the total number of subhalos, with v_max > 10 km/s, by a factor of ∼ 10 for isolated subhalos and by a factor of ∼ 6 for satellites. However, the reduction is mostly in the number of non-star-forming subhalos, and the studied model thus remains a viable candidate, taking into account the uncertainty in the Local Group total mass estimate. The formation of the first galaxies is also delayed, and the central densities of galaxies with v_max < 50 km/s are lowered: their circular velocities at 1 kpc from the centre are decreased and the radii of maximum circular velocity are increased. As for the stellar mass-metallicity and the stellar mass-halo mass relations, or the selection effects from tidal disruption, I find no significant differences between the models.
  • Ruohotie, Julia (2022)
    Small-scale flux ropes (SFRs) are structures with helical magnetic field and they are frequently observed in the solar wind. In addition to the solar wind, SFRs can also be found within larger structures, like interplanetary coronal mass ejections (ICMEs) and their sheath regions that form between the shock and the leading edge of the ICME if the ICME propagates fast enough. ICME-driven sheaths are composed of shocked and compressed solar wind plasma. SFRs can be swept from the upstream solar wind into ICME sheaths when the upstream wind is deflected and compressed into ICME sheaths. Alternatively, SFRs can be formed within ICME sheaths through number of processes. This thesis includes the first comprehensive study of the occurrence of SFRs specifically in ICME-driven sheath regions. SFRs are identified from spacecraft data in both ICME sheaths and the upstream solar wind using the wavelet analysis method. This method calculates normalized reduced magnetic helicity, normalized cross-helicity, and normalized residual energy and uses them to identify SFRs and Alfvén waves. The method is applied to 55 ICME-driven sheath regions observed by Wind spacecraft. The occurrence of SFRs is studied in three different frequency ranges between 10−2 − 10−4 Hz. SFRs are found to be common structures in ICME-driven sheaths and they are more common in ICME sheaths than in the upstream solar wind. This suggest that SFRs are at least to some extent generated within ICME sheaths. The occurrence of SFRs behaves differently in different frequency ranges. The occurrence of SFRs is relatively constant at high frequencies (smallest scale) while the occurrence of low-frequency (largest scale) SFRs increases towards the leading edge of the ICME. This suggests that high- and low-frequency SFRs are generated by different processes. The occurrence of Alfvén waves was found to be somewhat similar in the upstream solar wind and ICME sheaths. However, there was an increase in the occurrence of Alfvén waves near the shock. This indicates that SFRs and Alfvén waves are generated by different processes and shock related processes might be important in the generation of Alfvén waves while the processes near the leading edge of the ICME are important in the generation of larger scale SFRs.
  • Haris-Kiss, Andras Kristof (2022)
    Over the last thirty years more than five thousand exoplanets have been discovered around a wide variety of stellar objects. Most exoplanets have been discovered using the transit method, which relies on observing the periodic brightness changes of stars as their planet transits in front of them. The discovery efficiency of these planets has been strongly enhanced with the advent of space telescopes dedicated to the discovery of planets using the transit method. Planetary signals in the photometric data of active stars can be challenging to find, as the surface features of the stars combined with their rotation might produce signals which are orders of magnitude stronger than those caused by the planetary transit. The question of what statistical methods should be applied to account for the innate variability of stars in order to identify the transits of exoplanets in the lightcurves of active stars is being investigated in this thesis. I test a number of statistical methods in order to combat stellar activity and to identify planetary transit signals. The rotation period of the star is investigated using the Lomb-Scargle and likelihood ratio periodograms. Starspot induced variability is approximated with a number of sinusoids, with periods based on the star's rotation period. Additional stellar activity is filtered out using autoregressive and moving average models. Model fittings are performed with least squares fitting, and using samples generated by the Adaptive Metropolis algorithm. After the lightcurve has been detrended for stellar activity, the likelihoods of planetary transit signals are assessed with a box-fitting algorithm. Models are compared with the Bayesian and Akaike information criteria. Planetary characteristics are then estimated by modeling the shape of the transit lightcurve. These methods are tested and performed on the lightcurve of HD~110082, a highly active young star with one confirmed planetary companion, based on the observations of the TESS space telescope. I find that stellar activity is sufficiently filtered out with a model containing four sinusoid signals. The signal corresponding to the planet is confirmed by the box fitting algorithm, agreeing with results available in scientific literature.
  • Keitaanranta, Atte (2022)
    Results from cosmological zoom-in simulations focusing on the formation and evolution of massive early-type galaxies starting from redshift z=50 until the present time are presented in this thesis. In addition, the dynamics of supermassive black hole (SMBH) binaries found in a subset of the galaxies as a result of galaxy mergers are studied. The first 12 zoom-in simulations were run with GADGET-3, an N-body hydrodynamical tree code on the Puhti supercomputer at the Finnish IT Centre for Science (CSC). The zoom-in regions were chosen from a low resolution large-volume simulation, run with the University of Helsinki computing cluster Kale. The SMBH binary dynamics were simulated with the regularized integrator code KETJU. In total three simulations using KETJU were run on the CSC supercomputer Mahti. The GADGET-3 simulations included both dark matter only runs and runs containing also baryons. The simulations were run at two different resolutions. No significant differences between the two resolutions were found for the dark matter only runs, whereas the runs including a baryonic component showed large differences. The medium resolution runs had too low particle numbers and too large particle masses to correctly resolve star formation and feedback, leading to rotation curves that missed their central peaks. The results from the high resolution simulations agreed with earlier published results. The rotation curves peak in the central regions of the galaxies with the curves becoming almost constant at large radii. The star formation rates peak in the redshift range z~2-3 and at smaller redshifts star formation can momentarily increase due to the occuring of galaxy mergers. At the present day, all of the studied galaxies include mainly old stellar populations, resulting in red colours for all the galaxies. Still, the galaxy formation efficiency parameter of each galaxy is somewhat higher than what is seen in the observations. Finally, all three SMBH binaries for which the dynamics were studied using KETJU led to a coalescence of the two black holes. The orbital decay of SMBH binaries occur in three phases: dynamical friction, three-body interactions and gravitational wave emission. The merger times strongly depend on the eccentricity of the binaries with the semimajor axis of the binary with the highest eccentricity decreasing the fastest. This is expected from theory, with the evolution from subparsec scales to coalescence agreeing very well with the theoretical evolution taking into account the post-Newtonian correction term 2.5PN, which is the lowest order post-Newtonian term responsible for gravitational wave emission.
  • Turkki, Mikael (2024)
    The thesis discusses the observations of linear polarization occurring on the surface of near-Sun asteroid (NSA) (3200) Phaethon. As part of the research, new observational data from the Nordic Optical Telescope (NOT) are analyzed. The data were obtained in 2019, a few weeks after the perihelion passage. The scientific goal of these observations was to explain the partially conflicting results of linear polarization in earlier literature. The conflicting degrees of linear polarization in large phase angles (the Sun - the object - the observer) covered different hemispheres of Phaethon, which may be explained by the differences in surface regolith size distribution. A new pipeline was created to analyze the data. The pipeline was used to calculate the Stokes parameters of Phaethon based on the fluxes in the data frames obtained by using the ALFOSC (Alhambra Faint Object Spectrograph and Camera). The pipeline is directly applicable to other optical linear polarization data observed using ALFOSC. However, it is also applicable to sidereal observations or non-sidereal observations conducted with other instruments with relatively minor modifications. Features of the pipeline include the estimation of the uncertainty and validity of the observations by classical error propagation, as well as the possible dependence of the results based on the diameter of the circular aperture. The results of the analysis are compared to the prior observations of Phaethon presented in the literature. First, the variation of linear polarization is modeled as a function of phase angle using the Lumme-Muinonen Function (LMF) with different initial conditions. The analysis results are in line with previous results, although the data points in large phase angles have relatively large uncertainty estimates. Second, the existence of latitudinal correlation is evaluated, as such analysis is enabled by the sufficiently large total number of observations and a pole solution presented in earlier literature. No clear evidence of latitudinal correlation was found. Third, the sufficiently stable sub-observer latitude during the NOT observations allows the search for a correlation between linear polarization and rotational phase. There are consistent variation features as a function of the rotational phase, but they are classified as hints of detection in terms of statistical uncertainties (<5σ). In addition to analyzing the variation of linear polarization, a statistical summary of the orbital parameters of near-Sun asteroids is presented. To complete the thesis, a credibility analysis of all the reported results is performed.
  • Kosowski, Jacob (2023)
    We investigated the connection between the 3D physical properties of stellar clusters and their measured counterparts from their 2D observed images; primarily focusing on the relationship be- tween the 3D half-mass radius (Rh3D) and the effective radius (Rheff) (also known as the 2D half-light radius) of stellar clusters. We generated an ensemble of 3D models of stellar clusters using the McLuster code. This ensemble is made up of subgroups consisting of different stellar counts, half-mass radius, concentration, maximum mass of the initial mass function, and degree of mass segregation. Each subgroup covered a broad range of their respective property in order to provide a comprehensive overview of the Rh3D to Rheff relationship as a function of these variables. Then, utilizing myosotis, we created synthetic observations of these models and investigated how the Rh3D of the cluster could be inferred from the measured Rheff of the synthetic photometric map. Our analysis reveals that for systems where all stars are of equal mass, independent of their size, the half-mass radius is equal to Rh3D ≈ 1/α Rheff where α ∼ 0.76. We show that the value of α can be inferred by a geometric relationship. We also find that this relationship holds for systems with varying values of concentration. For unsegregated systems of unequal stellar masses, we observe that the value of α oscillates around 0.76, with the amplitude of the oscillations increasing as the maximum mass of the system increases. As Rh3D by construction does not change, the only parameter to cause this variation in α is the Rheff . When we looked at mass segregated systems, we found that the value of Rheff (and similarly α) decreases generally monotonically as a function of the degree of the segregation. The presence of stars of unequal mass is the dominant factor that determines the measurements of Rheff , beyond the geometric effects of projection. The prevalence of this factor is attributed to the non-linear relationship between mass and luminosity that results in a few tens of massive stars greatly influencing the overall luminosity of the cluster, and therefore, its effective radius.
  • Mattero, Max (2024)
    This thesis studies gas-rich galaxy mergers at redshifts of z ∼ 1-2 using numerical simulations, with a particular focus on the effect of feedback from active galactic nuclei (AGNs). In total, 16 galaxy mergers at redshifts z = 1 and z = 2 were modeled using the simulation codes KETJU and GADGET-3. The simulations were performed on the supercomputer Mahti located at the Finnish IT Centre for Science (CSC). AGN feedback can be described as the radiative and mechanical energy released through accretion, which act to heat and disperse the remaining gaseous material surrounding the central supermassive black hole (SMBH). The feedback mechanisms include, for example, photoionization heating due to high-energy photons and winds and jets driven by the AGN. Numerically, AGN feedback was implemented using two models in this thesis: thermal and kinetic AGN feedback, in which the gas particles are either heated or ‘kicked’, respectively. In addition to AGN feedback, the simulations included metal-dependent gas cooling, stochastic star formation, and stellar feedback. The simulated progenitor galaxies were gas-rich spirals consistent with observed galaxies at redshifts z = 1 and z = 2. The virial masses of the progenitors were set to correspond to typical massive galaxies at their redshifts using the Press-Schechter mass function, while the initial masses for the central SMBHs were set using observed MBH-M⋆ and MBH-σ⋆ relations. The gas fractions and metal abundances of the progenitors were calibrated using observational data at their respective redshifts. The KETJU and GADGET-3 simulations produced very similar results for the overall evolution of a given merger configuration. Consistent with earlier studies, the kinetic feedback was observed to be significantly more effective at removing gas from the galaxies than the thermal feedback. The combined effect of AGN and stellar feedback was observed to strongly suppress star formation, with the star formation of one merger being almost completely shut down. The thermal and kinetic feedback models caused noticeable differences in the orbital evolution of the SMBH binaries. Merger timescales were significantly longer for the SMBHs in the KETJU simulations with kinetic feedback. In general, the merger timescales increased with decreasing initial eccentricity for the SMBH binary. The merger remnants were compared to observed MBH-σ⋆, R1/2-M⋆, fgas-M⋆, and mass-metallicity relations. Overall, the remnants were reasonably consistent with the observed relations. Hence, we can conclude that AGN feedback plays a crucial role in galaxy evolution and that both the thermal and kinetic feedback models are able to produce realistic high-redshift galaxies.
  • Koikkalainen, Venla (2023)
    The aim of this study is to inspect fluctuations in the solar wind magnetic field for four different types of solar wind time series. The events considered are fast and slow solar wind, along with magnetic clouds and sheath regions, which are found in coronal mass ejections (CMEs). Time series measurements of these processes are analysed using methods from Information Theory and Complex Network Analysis. The techniques that are used here are the Fisher-Shannon information plane, the Jensen-Shannon complexity-entropy plane, and Horizontal Visibility Graph Analysis. Statistical and information theory measures as well as network analysis have recently been applied to studying time series in an attempt to determine their internal structure. There is promising research into these methods quantifying data as either chaotic, stochastic, or periodic. Knowing whether a process has e.g. a deterministic origin could shed light on the creation of said process. Applying these methods to solar wind, more information could be found about its formation at the Sun. In general, the solar wind data analysed in this thesis was found to be stochastic, which agrees with previous studies. In addition, when analysing magnetic field magnitude B, magnetic clouds appear to have more internal structure in the time series signal than the other types of solar wind data tested. The results obtained here are promising in terms of finding differences in structure within solar wind, and could be investigated further with the use of more solar wind data.
  • Bieleviciute, Auguste (2023)
    The high luminosity upgrade of the Large Hadron Collider (LHC) will result in higher collision rates and current equipment is not up to par with this future era of operations. Identification and reconstruction of hard interactions may be hampered by the spatial overlapping of particle tracks and energy deposits from additional collisions and this often leads to false triggers. In addition, current particle detectors suffer from radiation damage that severely affects the accuracy of our results. The new minimum ionizing particle (MIP) timing detector will be equipped with low gain avalanche detectors which have a comparably small timing resolution that helps with the track reconstruction and their thin design limits the radiation damage over time. In this thesis, we build an experimental set-up in order to study the timing resolution of these detectors closely. In order to find the timing resolution, we take the time difference between the signals from two detectors and put it in a histogram to which we apply a Gaussian fit. The standard deviation of this Gaussian is called the time spread from which we can estimate the timing resolution. We first build, characterize and improve our experimental set-up using reference samples with known timing resolution until our set-up is able to reproduce the reference value. Then we repeat the measurements with irradiated samples in order to study how radiation damage impacts timing. We were able to adjust our setup with reference samples until we measured a timing resolution of 33$\pm$2~ps. We use this result to calculate the timing resolution of an irradiated sample ($8.0 \times 10^{14}$ proton fluence) and we found a timing resolution of 62$\pm$2~ps. This thesis also discusses the analysis of the data and how the data can be re-analyzed to try to improve the final result.