Browsing by study line "Particle Physics and Cosmology"

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.

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.

Particle jets are formed in high energy proton-proton collisions and then measured by particle physics experiments. These jets, initiated by the splitting and hadronization of color charged quarks and gluons, serve as important signatures of the strong force and provide a view to size scales smaller than the size of an atom. So, understanding jets, their behaviour and structure, is a path to understanding one of the four fundamental forces in the known universe. But, it is not only the strong force that is of interest. Studies of Standard Model physics and beyond Standard Model physics require a precise measurement of the energies of final state particles, represented often as jets, to understand our existing theories, to search for new physics hidden among our current experiments and to directly probe for the new physics. As experimentally reconstructed objects the measured jets require calibration. At the CMS experiment the jets are calibrated to the particle level jet energy scale and their resolution is determined to achieve the experimental goals of precision and understanding. During the many-step process of calibration, the position, energy and structure of the jets' are taken into account to provide the most accurate calibration possible. It is also of great importance, whether the jet is initiated by a gluon or a quark, as this affects the jets structure, distribution of energy among its constituents and the number of constituents. These differences cause disparities when calibrating the jets. Understanding of jets at the theory level is also important for simulation, which is utilized heavily during calibration and represents our current theoretical understanding of particle physics. This thesis presents a measurement of the relative response between light quark (up, down and strange) and gluon jets from the data of CMS experiment measured during 2018. The relative response is a measure of calibration between the objects and helps to show where the difference of quark and gluon jets is the largest. The discrimination between light quarks and gluons is performed with machine learning tools, and the relative response is compared at multiple stages of reconstruction to see how different effects affect the response. The dijet sample that is used in this study provides a full view of the phase space in pT and |eta|, with analysis covering both quark and gluon dominated regions of the space. These studies can then be continued with similar investigations of other samples, with the possibility of using the combined results as part of the calibration chain.

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.

Coronal mass ejections (CMEs) are large-scale eruptions of plasma entrained in a magnetic field. They occur in the solar corona, and from there they propagate into interplanetary space along with the solar wind. If a CME travels faster than the surrounding solar wind, a shock wave forms. Shocks driven by CMEs can act as powerful accelerators of charged particles. When charged particles like electrons are accelerated, they emit electromagnetic radiation, especially in the form of radio waves. Much of the radio emission from CMEs comes in the form of solar radio bursts. Traditionally solar radio bursts are classified into five types, called type I–V bursts, based on their characteristics and appearance in a dynamic spectrum. Of these five types of bursts, especially type II radio bursts are believed to be signatures of shock waves in the corona and interplanetary space. There are, however, also radio bursts associated with CMEs and shocks that do not fit the description of any of the five standard types of radio bursts. In this thesis three moving radio bursts associated with a CME that erupted on May 22, 2013 are identified and studied in detail. The characteristics of the bursts do not match those of the usual five types of solar radio bursts. The aim of the work is to ascertain the emission mechanism that causes the observed radio bursts, as well as locate the sites of electron acceleration that are the sources of the emission. The kinematics and the spectral features of the emission are studied in order to find answers to these questions. Analysis of the spectral features of the moving bursts showed that the bursts were emitted via plasma emission. Analysis of the kinematics revealed that the moving radio bursts originated unusually high up in the corona from the northern flank of the CME. The CME studied in this work was preceded by another one which erupted some hours earlier, and the disturbed coronal environment likely caused the radio emission to be emitted from an unusual height. It was found that the bursts likely originated from electrons accelerated at the shock driven by the CME.

Cosmological first-order phase transitions (FOPTs) are a hypothetical scenario occurring in the early universe in which bubbles nucleate and expand, generating gravitational waves (GWs). These transitions interest scientists due to their occurrence in extensions to the Standard Model of particle physics, their potential for providing insight into open questions in particle physics and cosmology, and the possibility of observing their signature with the planned Laser Interferometer Space Antenna (LISA). Modeling GW production from FOPTs is thus a topic of active research. In FOPT models, GW production is split into three sources: collisions between bubble walls Ωenv, overlapping fluid shells Ωsw, and fluid turbulence Ωturb. When modeling the contribution from Ωsw in 1D spherical simulations, a sound shell model is often employed which assumes that fluid shells reach a calculable self-similar state of expansion before overlapping. In this thesis, I determine when this asymptotic expansion state is reached by defining and calculating a relaxation time ts and transition rate βs for 1D expanding fluid shells. I model two scenarios, a thin and a thick-walled perturbed nucleation bubble expanding in a relativistic fluid, in the limit of fast detonations and weak coupling. In each case, respectively, relaxation temperature and transition rate are determined to be: tsTc = 7.422(21) × 103, βs/Tc = 1.3474(38) × 10−4; and tsTc = 9.901(33) × 105, βs/Tc = 1.011(35) × 10−5. When fixing the critical temperature Tc below which bubbles can nucleate, these results predict that when the transition rate β > βs, the GW spectrum produced assuming relaxed fluid shells may be inaccurate. In addition to this main result, I also compare various methods for estimating bubble wall expansion velocity. These results are useful for 3D simulations, in which direct methods for determining wall velocity are unwieldy.