Browsing by master's degree program "Magisterprogrammet i elementarpartikelfysik och astrofysikaliska vetenskaper"
Now showing items 21-40 of 47
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(2022)One of the most noticeable effects of solar–terrestrial physics is the aurora which regularly appears in the polar regions. This polar light is the result of the excitation of atmospheric species by charged particles originating from the solar wind and magnetosphere that enter the Earth’s atmosphere, which are called precipitating particles. We present the first results on auroral proton precipitation into the ionosphere using a global 3-dimensional simulation of near-Earth space plasma with the Vlasiator hybrid-Vlasov model, driven with a southward interplanetary magnetic field and steady solar wind parameters. The hybrid-Vlasov approach describes ions through their velocity distribution function in phase space (3-dimensional ordinary space and 3-dimensional velocity space), while electrons are represented by a massless charge-neutralizing fluid. Vlasiator is a global model describing the whole region of near-Earth space including the Earth’s magnetosphere (whole dayside and part of the magnetotail), the magnetosheath, as well as the foreshock region and some solar wind. The precipitating proton differential number fluxes for this run are determined from the proton phase-space density contained within the bounce loss-cone, which is set at a constant angle of 10 degrees everywhere. To determine the precipitation of particles at ionospheric altitudes (in this case a height of 110 km above the Earth’s surface), we trace magnetic field lines from the ionosphere to the inner boundary of the Vlasiator domain using the Tsyganenko model. With this, we obtain a magnetic local time–geomagnetic latitude map of differential number flux of precipitating protons in 9 energy bins between 0.5 and 50 keV. From the differential number flux, proton integral energy fluxes and mean energies can be obtained. The integral energy fluxes in the Vlasiator run are then compared to data of the Precipitation Electron/Proton Spectrometer (SSJ) instrument of the Defense Meteorological Satellite Program (DMSP) for several satellite overpasses during events with similar solar wind conditions as in the Vlasiator run. The SSJ instrument bins proton energies between 0.03 and 30 keV. Typical values of the total integral energy flux are between 5 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the cusp and between 1 · 10^6 and 3 · 10^7 keV cm−2 s−1 sr−1 in the evening sector for both Vlasiator and DMSP, although DMSP fluxes can locally be up to an order of magnitude higher. Additionally, global precipitation patterns in Vlasiator are compared to Ovation Prime, which is an empirical model based on data from DMSP which can be used to forecast precipitation of auroral electrons and protons. Although Ovation Prime shows a much wider cusp region compared to Vlasiator, both show similar maximum integral energy fluxes around 1 to 2 · 10^7 keV cm−2 s−1 sr−1 in the cusp region, and between 3 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the nightside oval.
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(2023)A stream of charged particles known as the solar wind constantly flows with supersonic speed in our solar system. As the supersonic solar wind encounters Earth's magnetic field, a bow shock forms where the solar wind is compressed, heated and slowed down. Not all ions of the solar wind pass through the shock but rather a portion are reflected back upstream. What happens to the reflected ions depends on the magnetic field geometry of the shock. In the case where the angle between the upstream magnetic field and the shock normal vector is small, the reflected ions follow the magnetic field lines upstream and form a foreshock region. In this case the shock is called quasi-parallel. In the case of a quasi-perpendicular shock, where the angle is large, the reflected ions gyrate back to the shock, accelerated by the convection electric field. Upon returning to the shock, the ions have more energy and either pass through the shock or are reflected again, repeating the process. Ion reflection is important for accelerating ions in shocks. In this work we study the properties and ion reflection of the quasi-perpendicular bow shock in Vlasiator simulations. Vlasiator is a plasma simulation which models the interaction between solar wind and the Earth's magnetic field. The code simulates the dynamics of plasma using a hybrid-Vlasov model, where ions are represented as velocity distribution functions (VDF) and electrons as magnetohydrodynamic fluid. Two Vlasiator runs are used in this work. The ion reflection is studied by analysing VDFs at various points in the quasi-perpendicular shock. The analysis is performed with reflections in multiple different frames. A virtual spacecraft is placed in the simulation to study shock properties and ion dynamics, such as the shock potential and ion reflection efficiency. These are compared to spacecraft observations and other simulations to test how well Vlasiator models the quasi-perpendicular bow shock. We find that the ion reflection follows a model for specular reflection well in all tested frames, especially in the plane perpendicular to the magnetic field. In addition, the study was extended to model second specular reflections which were also observed. We conclude that the ions in Vlasiator simulations are nearly specularly reflected. The properties of the quasi-perpendicular bow shock are found to be in quantitative agreement with spacecraft observations. Ion reflection efficiency is found to match observations well. Shock potential investigations revealed that spacecraft observations may have large uncertainties compared to the real shock potential.
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(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.
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(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.
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(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.
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(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.
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Optimization of the CMS high level trigger for low mass supersymmetry searches with forward protons (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.
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(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.
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(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.
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(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.
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(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.
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(2021)Active galactic nuclei (AGN) are one of the most powerful sources of the luminous Universe. Radio-loud AGN exhibit prominent relativistic outflows known as jets, whose synchrotron radiation can be detected in the radio domain. The launching, evolution and variable nature of these sources is still not fully understood. We study 3C 84, because its proximity, brightness and the intermittent nature of its jet makes it a good target to investigate these open questions of the AGN phenomena. 3C 84 (optical counterpart: NGC 1275) is a Fanaroff-Riley type I radio galaxy, located in the Perseus cluster at z = 0.0176. Due to its close proximity, 3C 84 has been a favourable target for observations throughout the entire electromagnetic spectrum, especially for ones in the radio domain. Its most recent activity started 2003, when a new component emerged from the core in the form of a restarted parsec-scale jet. This provided a rare opportunity to study the formation and evolution of a jet (see Nagai et al. 2010, 2014, 2017 and Suzuki et al. 2012). The highest resolution results were obtained by Giovannini et al. (2018), who imaged the source with the Global VLBI Network together with the Space Radio Telescope, RadioAstron. This enabled them to capture the limb-brightened structure of the restarted jet and measure its collimation profile from ~350 gravitational radii. In this work I present the 22 GHz RadioAstron observations carried out 3 years later, in a similar configuration, but with a significantly different sampling of the space baselines than the ones presented in Giovannini et al. (2018). The calibration was carried out in the Astronomical Image Processing System (AIPS), whereas imaging was done in Difmap (Shepherd 1997). The aim of this thesis work was to obtain a high-resolution image of the source, measure the collimation profile of the restarted jet, and compare the results with those of Giovannini et al. (2018) and verify the observed source structures and measured jet properties, if possible. Comparing the images of the two epochs (angular resolution of the 2016 observations is 0.217x0.072 mas at Pa=-49.6°), they both show a similar structure, with the radio core, a diffuse emission region (C2), and the hotspot (C3) at the end of the restarted jet. Edge-brightening is confirmed in the jet and the counter-jet. However, the jet has advanced ~1 mas, corresponding to the velocity of 0.55c. C3 has moved from the center of the feature to the jet head, indicating an interaction between the jet and the clumpy external medium (Kino et al. , 2018 and Nagai et al., 2017). The base of the jet has also changed between the observation, approximately by ~20°. In the light that in the 1990s the jet pointed towards C2, then swinged westwards when the jet emerged (Suzuki et al., 2012 and Giovannini et al., 2018), and on the 2016 image has moved towards its initial position. This suggest a precessing jet, observed and modeled by Dominik et al. (2021) and Britzen et al. (2019). Measuring the brightness temperature of the core and the hotspot shows a signifacant drop of 70% and 50% since the 2013 measurements, respectively, due to emission of jet material and the expansion of the jet. Jet width measurements between 1200 and 19000 gravitational radii reveal a less cylindrical collimation profile, with r ~ z0.31 – where z is the de-projected distance from the core and r is the width of the jet. The evolution of the restarted jet’s profile from quasi-cylindrical (Giovannini et al. 2018) to less cylindrical implies that the cocoon surrounding the jet (Savolainen, 2018) cannot confine the jet material as it moves further from the core. The measured collimation profile corresponds to a slowly decreasing density, and more steeply decreasing pressure gradient in the external medium. Since the closest jet width measurement is only at 1200 gravitational radii from the core (here the jet width is 750 gravitational radii), it cannot confirm the wide jet base measured by Giovannini et al. (2018) at 350 gravitational radii. Based on this result, we arrive at the same conclusion as Giovannini et al. (2018), that the jet is either launched from the accretion disk, or it is ergosphere-launched, but undergoes a quick lateral expansion below 1000 gravitational radii.
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(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.
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(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.
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(2020)Core galaxies are bright elliptical galaxies that contain a shallow central surface brightness profile. They are expected to form in mergers of massive gas-poor elliptical galaxies that contain supermas- sive black holes (SMBHs) in their respective centres. During the merger process, these black holes form a coalescing binary, which causes the ejection of stars from the centre of the galaxy merger in complex three-body interactions, resulting in the creation of a low-luminosity core. I have studied whether core galaxies can form according to the formation model described above. I analysed the results of seven galaxy merger simulations done using KETJU, a simulation code specifically made for studying the dynamics of supermassive black holes in galaxies. KETJU is a regularised tree-code, combining both the GADGET-3 tree-code and an AR-CHAIN integrator. This allows for the simultaneous simulation of both general galactic dynamics and accurate particle motion near black holes, respectively. All seven simulations consisted of a merger of two identical galaxies. Six of the simulations had galaxies with equal mass central SMBHs, where the mass of the black holes changed from one simulation to another, and ranged from 8.5 × 10 8 M to 8.5 × 10 9 M . For the sake of comparison, the galaxies in the seventh simulation did not contain SMBHs. The other properties of the merged galaxies were determined in such a way, that the resulting merger remnants would be as similar as possible to the well studied core galaxy NGC 1600. Naturally, these properties were identical across all of the simulation runs. By calculating the surface brightness profiles of the merger remnants in the simulation results, I found out that only simulations that contained SMBHs produced remnants with cores. Furthermore, I identified a clear positive correlation between the size of the core and the mass of the coalescing binary SMBH. Both of these results corroborate the theory, that the cores are formed by interacting SMBH binaries. This interpretation of the results was further enforced by the fact that, according to their velocity anisotropy profiles, stellar orbits near the centre of the remnants were tangentially dominated, implying that stellar particles on more radial orbits had been ejected from the system. I also generated 2D maps of the stellar line-of-sight velocity distributions in the simulated merger remnants. These maps showed kinematic properties similar to observed core galaxies, such as "kinematically distinct cores". Finally, I compared both photometric and kinematic properties of the simulated merger remnant containing the largest SMBH binary to the observed properties of NGC 1600. I found that the simulation and the observations agree well with each other. Since the properties of the simulated merger remnants follow theoretical expectations and is in general good agreement with the obser- vations, I conclude that the formation of the cores in bright elliptical galaxies is likely caused by coalescing binary black holes in dry mergers of elliptical galaxies.
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(2020)Core galaxies are bright elliptical galaxies that contain a shallow central surface brightness profile. They are expected to form in mergers of massive gas-poor elliptical galaxies that contain supermas- sive black holes (SMBHs) in their respective centres. During the merger process, these black holes form a coalescing binary, which causes the ejection of stars from the centre of the galaxy merger in complex three-body interactions, resulting in the creation of a low-luminosity core. I have studied whether core galaxies can form according to the formation model described above. I analysed the results of seven galaxy merger simulations done using KETJU, a simulation code specifically made for studying the dynamics of supermassive black holes in galaxies. KETJU is a regularised tree-code, combining both the GADGET-3 tree-code and an AR-CHAIN integrator. This allows for the simultaneous simulation of both general galactic dynamics and accurate particle motion near black holes, respectively. All seven simulations consisted of a merger of two identical galaxies. Six of the simulations had galaxies with equal mass central SMBHs, where the mass of the black holes changed from one simulation to another, and ranged from 8.5 × 10 8 M to 8.5 × 10 9 M . For the sake of comparison, the galaxies in the seventh simulation did not contain SMBHs. The other properties of the merged galaxies were determined in such a way, that the resulting merger remnants would be as similar as possible to the well studied core galaxy NGC 1600. Naturally, these properties were identical across all of the simulation runs. By calculating the surface brightness profiles of the merger remnants in the simulation results, I found out that only simulations that contained SMBHs produced remnants with cores. Furthermore, I identified a clear positive correlation between the size of the core and the mass of the coalescing binary SMBH. Both of these results corroborate the theory, that the cores are formed by interacting SMBH binaries. This interpretation of the results was further enforced by the fact that, according to their velocity anisotropy profiles, stellar orbits near the centre of the remnants were tangentially dominated, implying that stellar particles on more radial orbits had been ejected from the system. I also generated 2D maps of the stellar line-of-sight velocity distributions in the simulated merger remnants. These maps showed kinematic properties similar to observed core galaxies, such as "kinematically distinct cores". Finally, I compared both photometric and kinematic properties of the simulated merger remnant containing the largest SMBH binary to the observed properties of NGC 1600. I found that the simulation and the observations agree well with each other. Since the properties of the simulated merger remnants follow theoretical expectations and is in general good agreement with the obser- vations, I conclude that the formation of the cores in bright elliptical galaxies is likely caused by coalescing binary black holes in dry mergers of elliptical galaxies.
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(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.
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(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.
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(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.
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(2020)The magnetic field of Earth interacts with the supersonic solar wind that emanates from the outer part of the Sun’s atmosphere. The interaction results in the formation of Earth’s magnetosphere with a bow shock and a foreshock upstream of it. Together, they form a complex system that hosts a large number of different phenomena, ranging from aurorae visible with the naked eye from Earth’s surface to magnetic waves and transient structures only observable by spacecraft with in-situ measurements. In addition to spacecraft measurements, numerical simulations performed with computers have become increasingly important in space research with the constantly growing amount of available computing power. The topic of this thesis, two types of transient structures found upstream of the bow shock in the foreshock, cavitons and spontaneous hot flow anomalies (SHFAs), are examples of phenomena that have been discovered and studied with the combination of numerical simulations and spacecraft observations. These transient types are related, as cavitons can evolve into SHFAs. In this thesis, cavitons and SHFAs are studied with the global hybrid-Vlasov simulation Vlasiator. The transients are studied statistically in a global simulation for the first time, granting the largest statistical sample up to date. The approach taken in this study is to track individual transients in time, for which purpose a tracking algorithm was developed as a part of this thesis. With this method, the first detailed investigation of the evolution of cavitons and SHFAs is conducted. The statistical results obtained in this work indicate that cavitons and SHFAs form in a uniform region near the bow shock. There is a distinct distance to the shock within which cavitons can become SHFAs, and it is found that SHFAs can either form independently, or evolve from cavitons. The properties of the transients are found to have some dependence on the transients’ location relative to the bow shock. The propagation velocity of the transients is measured, and is found to agree with prior spacecraft observations.
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