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Browsing by study line "Physics"

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  • Pirttikoski, Antti (2021)
    LHC is the highest energy particle collider ever built and it is employed to study elementary particles by colliding protons together. One intriguing study subject at LHC is the stability of the electroweak vacuum in our universe. The current prediction suggests that the vacuum is in the metastable state. The stability of the vacuum is dependent on the mass of the top quark, and it is possible that more precise measurement of the mass could shift the prediction to the border of the metastable and stable states. In order to measure the mass of the top quark more precisely, we need to measure the bottom (b) quarks decaying from it at high precision, as top quark decays predominantly into a W boson and a b quark. Due to the phenomenon called hadronisation, we can not measure the quarks directly, but rather as sprays of collimated particles called jets. The jets originating from b quarks (b jet) can be identified by b-tagging. The precise measurement and calibration of the b jet energy is crucial for top quark mass measurement. This thesis studies the b jets and their energy calibration at the CMS, which is one of the general purpose detectors along the LHC. Especially the b jet energy scale (bJES) is under the investigation and the various phenomena affecting to it. For example, large fraction of b jets contain neutrinos, which cannot be measured directly. This increases uncertainties related to the energy measurement. Also there are problems how precisely the formation and evolution of the b jets can be modelled by Monte Carlo event generators, such as Pythia8, which was utilized in this thesis. The aim of this thesis is to evaluate how big effect on the bJES is caused by the various different phenomena, which presumably weaken the precision of the b jet measurements. The studied phenomena are the semileptonic branching ratios of b hadrons, branching ratios of b hadron to c hadron decays, b hadron production fraction and parameterization of the b quark fragmentation function. The combined effect of all four different rescaling features mentioned above, suggests that bJES is known at 0.2% level. A small shift of -0.1% in the Missing transverse energy Projection Fraction (MPF) response scale is detected at low pt values, which vanishes as the pt increases. This improves remarkably 0.4-0.5% JES accuracy achieved during at CMS during Run 1 of the LHC. However, there are still many ways we can improve the performance presented here. Definitely there is a need for further studies of the rescaling methods before results could be utilized in the corrections of bJES to do precision measurement of the top quark mass.
  • Korhonen, Keijo (2022)
    The variational quantum eigensolver (VQE) is one of the most promising proposals for a hybrid quantum-classical algorithm made to take advantage of near-term quantum computers. With the VQE it is possible to find ground state properties of various of molecules, a task which many classical algorithms have been developed for, but either become too inaccurate or too resource-intensive especially for so called strongly correlated problems. The advantage of the VQE comes in the ability of a quantum computer to represent a complex system with fewer so-called qubits than a classical computer would with bits, thus making the simulation of large molecules possible. One of the major bottlenecks for the VQE to become viable for simulating large molecules however, is the scaling of the number of measurements necessary to estimate expectation values of operators. Numerous solutions have been proposed including the use of adaptive informationally complete positive operator-valued measures (IC-POVMs) by García-Pérez et al. (2021). Adaptive IC-POVMs have shown to improve the precision of estimations of expectation values on quantum computers with better scaling in the number of measurements compared to existing methods. The use of these adaptive IC-POVMs in a VQE allows for more precise energy estimations and additional expectation value estimations of separate operators without any further overhead on the quantum computer. We show that this approach improves upon existing measurement schemes and adds a layer of flexibility, as IC-POVMs represent a form of generalized measurements. In addition to a naive implementation of using IC-POVMs as part of the energy estimations in the VQE, we propose techniques to reduce the number of measurements by adapting the number of measurements necessary for a given energy estimation or through the estimation of the operator variance for a Hamiltonian. We present results for simulations using the former technique, showing that we are able to reduce the number of measurements while retaining the improvement in the measurement precision obtained from IC-POVMs.
  • Lankinen, Juhana (2020)
    Due to the unique properties of foams, they can be found in many different applications in a wide variety of fields. The study of foams is also useful for the many properties they share with other phenomena, like impurities in cooling metals, where the impurities coarsen similarly to bubbles in foams. For these and other reasons foams have been studied extensively for over a hundred years and continue being an interesting area of study today due to new insights in both experimental and theoretical work and new applications waiting to be used and realized in different industries. The most impactful early work in the study of the properties of foams was done in the late 1800s by Plateau. His work was extended in the early to mid-1900s by Lifshitz, Slyozov, Wagner and von Neumann and by many more authors in recent years. The early work was mostly experimental or theoretical in the sense of performing mathematical calculations on paper, while the modern methods of study have kept the experimental part -- with more refined methods of measurement of course -- but shifted towards the implementation of the theory as simulations instead of solving problems on paper. In the early 90s Durian proposed a new method for simulating the mechanics of wet foams, based on repulsive spring-like forces between neighboring bubbles. This model was later extended to allow for the coarsening of the foam, and a slightly changed version of this model has been implemented in the code presented in this thesis. As foams consist of a very large number of bubbles, it is important to be able to simulate sufficiently large systems to realistically study the physics of foams. Very large systems have traditionally been too slow to simulate on the individual bubble level in the past, but thanks to the popularity of computer games and the continuous demand for better graphics in games, the graphics processing units have become very powerful and can nowadays be used to do highly parallel general computing. In this thesis, a modified version of Durian's wet foam model that runs on the GPU is presented. The code has been implemented in modern C++ using Nvidia's CUDA on the GPU. Using this program first a typical two-dimensional foam is simulated with 100000 bubbles. It is found that the simulation code replicates the expected behaviour for this kind of foam. After this, a more detailed analysis is done of a novel phenomenon of the separation of liquid and gas phases in low gas fraction foams that arises only with sufficiently large system sizes. It is found that the phase separation causes the foam to evolve as would a foam of higher gas fraction until the phases have mixed back together. It is hypothesized that the reason causing the phase separation is related to uneven energy distribution in the foam, which itself is related to jamming and uneven distribution of the sizes of the bubbles in the foam.
  • Keller, Levi (2019)
    The spin-orbit-coupled insulator Sr 3 NiIrO 6 is a strongly correlated transition metal compound, where an interplay of geometric frustration and spin anisotropy gives rise to novel magnetic phases. Resonant inelastic x-ray scattering (RIXS) is a powerful probe of the low-lying quasi-particle excitations that underpin these emergent properties. In this work, we partition the active space into approximately non-interacting parts in order to introduce a tight-binding single-particle model Hamiltonian describing the distorted IrO6 octahedra in Sr3NiIrO6. We then use this model to calculate its RIXS spectrum at the Ir L3-edge in the sub-electronvolt range. The results of this calculation are compared with experiments performed at the European Synchrotron Radiation Facility, and with a multiplet crystal field model calculation. We find that this one electron model largely agrees with the full-multiplet model and describes the d-d excitations observed in experiment. The addition of an exchange field term explains the low-lying temperature-dependent magnetic feature, disambiguating the sign of the crystal-field term, and suggesting that the feature is well localized at low temperatures, and is best described as an orbitally- entangled local spin-flip excitation. However, the correspondence at room temperature diminishes, suggesting that dispersive description is necessary to model this regime. The drastic reduction in active space entailed by this model facilitates the creation of extended non-collinear Heisenberg-like models, which can be calculated at a lower computational cost than full multiplet extended models.
  • Kupiainen, Tomi (2020)
    In this work we consider the method of unitarily inequivalent representations in the context of Majorana neutrinos and a simple seesaw model. In addition, the field theoretical framework of neutrino physics, namely that of QFT and the SM, is reviewed. The oscillating neutrino states are expressed via suitable quantum operators acting on the physical vacuum of the theory, which provides further insight to the phenomenological flavor state ansatz made in the standard formulation of neutrino oscillations. We confirm that this method agrees with known results in the ultrarelativistic approximation while extending them to the non-relativistic region.
  • Mäkelä, Mikko (2020)
    Ultrasonic transducers convert electric energy into mechanical energy at ultrasonic frequencies. High-power ultrasound is widely used in the industry and in laboratories e.g. in cleaning, sonochemistry and welding solutions. To be effective in these cases, a piezoelectric transducer must deliver maximal power to the medium. Most of these systems rely on having the power delivery maximized during long driving sequences where stable performance is critical. Power ultrasonic transducers are typically narrowband, featuring high Q-value, that are finely tuned to a specific resonance frequency. The resonance frequency can vary during driving due to temperature, mechanical loading and nonlinear effects. When the transducers resonance frequency changes, drastic changes in its impedance (resonance to anti-resonance) can lead quickly to damage or failure of the driving electronics or the transducers themselves. In this work we developed a multi-channel high-power ultrasonic system with a software-based resonance frequency tracking and driving frequency control. The implementation features a feedback loop to maximize power delivery during long driving sequences in an ultrasonic cleaning vessel. The achieved total real power increased from 6.5 kW to almost 10 kW in peak with our feedback loop. The feedback loop also protected the electronics and transducers from breaking due to heating and varying impedance.
  • Adio, Luqmon (2019)
    Particle Induced X-ray Emission (PIXE) was originally introduced as an ion-beam analytical technique in Lund in the 1970s and has since then been part of the available techniques in many laboratories around the world. The external beam PIXE set-up is used in probing the annual tree rings. The goal is to see the effects of volcanic eruption activities from the perspectives of tree plants here in Finland. In the theory part, I tried to include the description of how volcanoes are formed and created with a bit of volcanic activity history, the growth metabolism mechanism in tree plants and characteristics x-ray productions. The two tree sample used for this experiment were gotten from two different regions of Finland. The first tree is a Pine tree from Parikkala(a small place near Savolinna) in the south-eastern part of Finland and the second tree is a Spruce tree from Pielavesi (place near Kuopio) in the central part of Finland. These samples were carefully prepared for ionisation. The collected spectra data were analysed in a software called PyMCA. PyMCA has been developed by the Software Group of the European Synchrotron Radiation Facility (ESRF). PyMCA is a ready to use and in many aspects state-of-the-art, set of applications implementing most of the needs of X-ray fluorescence data analysis. PyMCA is use to interpret X-ray fluorescence spectra from a diverse array of samples
  • Kulmala, Kimmo (2019)
    Perinteinen luento-opetus on todella vanha traditio ja laajalti käytössä edelleen, vaikka sen huonoista puolista, mm. passivoivasta vaikutuksesta, on runsaasti tutkimusnäyttöä. Oppiakseen opiskelijan on tärkeää aktiivisesti osallistua eri opetustilanteissa ja pohtia mitä hän opiskellessaan tekee. Tätä ristiriitaa on pyritty ratkomaan monin keinoin, esimerkiksi lisäämällä luennoille opiskelijaa aktivoivia elementtejä tai lisäämällä kurssin luennoille ennakkoaktiviteetteja. Tässä työssä tutkin tapaustutkimuksen avulla miten verkkotehtävät toimivat luentoon valmistavina aktiviteetteina, ja minkälaista tulisi väärästä vastauksesta annetun palautteen olla. Toteutin kurssin verkkotehtävät itse käyttäen STACK-järjestelmää. Tutkimus toteutettiin neljänä viikkona (viikot 3–6) syksyllä 2018 Helsingin yliopiston fysiikan peruskurssilla Vuorovaikutukset ja kappaleet. Kurssi kesti 7 viikkoa ja käsitteli mekaniikan perusteita. Kurssin aikana kerättiin kvantitatiivista ja kvalitatiivista tietoa tehtävien toiminnasta viikottaisten kyselyjen ja tehtäväalustan tarjoaman metadatan avulla. Tutkimuksen mukaan opiskelijat kokivat verkkotehtävät mielekkäiksi luentoon valmistaviksi aktiviteeteiksi ja tehtävistä saadun palautteen hyödylliseksi. Joidenkin viikkojen jotkut tehtävät koettiin kuitenkin liian haastaviksi tehtäväksi itsenäisesti. Tämä ja tehtäviin käytetty melko pitkä aika antoivat aihetta ainakin joidenkin tehtävien helpottamiselle. Tutkimuksesta selvisi myös, että tehtävät yksin tehneet kokivat tehtävät ja niistä saadun palautteen hyödyllisemmäksi kuin ryhmässä tehneet. Ero oli myös tilastollisesti merkittävä, joka on tehtävien luonteen kannalta kannustava tulos. Sen sijaan esimerkiksi koulutusohjelmalla ei ollut merkittävää vaikutusta siihen kuinka opiskelijat tehtävät tai palautteen kokivat.
  • Lavikainen, Emmi-Lotta (2019)
    Rehevöityminen on yksi merkittävimmistä vesistöjen ekologista tilaa heikentävistä tekijöistä. Järvivesissä fosfori on useimmiten levien kasvua rajoittava tekijä, jolloin fosforin määrän kasvu vastaanottavassa vesistössä lisää kasvillisuuden perustuotantoa ja kiihdyttää näin ollen vesistön rehevöitymistä. Fosforia päätyy vesistöihin valumavesien mukana liuenneessa muodossa ja eroosion ansiosta hiukkasmaisessa muodossa sitoutuneena esimerkiksi erodoituneessa maa-aineksessa oleviin raudan oksideihin. Erodoitunut maa-aines laskeutuu vesistöjen pohjalle, päätyy osaksi pohjasedimenttiä ja lopulta kohtaa hapettomat olosuhteet. Pohjasedimentteihin varastoituneet ravinteet voivat hapettomissa olosuhteissa vapautua takaisin vesistön tuottavaan kerrokseen ja näin ollen kiihdyttää rehevöitymistä. Hiukkasmaisen fosforin vapautuminen liuenneeseen, leville käyttökelpoiseen muotoon tunnetaan huonosti, joten erodoituneen maa-aineksen mukana kulkeutuvan fosforin merkitys vesistöjen rehevöitymisessä ei ole täysin selvä. Tässä työssä tutkitaan maa-aineksen matkaa pelloilta järvien pohjasedimentteihin käyttäen raudan K-reunan röntgenabsorptiospektroskopiaa lähireuna-alueella. Keskipisteessä on raudan kemiallisen tilan muutos siirryttäessä hapellisista olosuhteista hapettomiin olosuhteisiin sekä mahdollisesti järvien sedimenteissä muodostuva raudan fosfaattimineraali vivianiitti, jolla voi olla merkittävä rooli fosforin pidättämisessä sedimenttiin. Lisäksi tutustutaan röntgenabsorptiospektroskopian teoriaan sekä tutkimuksessa käytettävän spektrometrin toimintaan. Erilaisten maa-ainesten päätymistä järvien sedimentteihin simuloitiin valmistamalla järviveden ja maa-ainesten seoksia, joista osaan lisättiin myös orgaanista hiiltä. Seoksista valmistettiin näytteet röntgenabsorptiomittauksia varten vuorokauden aerobisen sekä neljän kuukauden anaerobisen inkuboinnin jälkeen. Mittaustuloksista havaitaan selvästi raudan pelkistyminen anaerobisissa olosuhteissa, ja että pelkistymistä edesauttaa orgaanisen hiilen läsnäolo. Vertailemalla järvivesi-maanäytteiden spektrejä mitattuun vivianiittimineraalista valmistetun näytteen spektriin, havaitaan että raudan pelkistyminen anaerobisessa ympäristössä suosii Fe(II)-mineraalien muodostumista. Mitattuja spektrejä verrattiin vivianiitin spektriin ja vertailun perusteella vivianiittia muodostui neljän kuukauden anaerobisen inkuboinnin aikana hiilettömissä näytteissä 12 - 22 % ja hiilellisissä näytteissä 31 - 50 % verrattuna hapelliseen alkutilanteeseen. Prosenttiluvut ovat todennäköisesti yliarvioita vivianiitin muodostumiselle, mutta tuloksista voidaan kuitenkin päätellä, että raudan voimakkaampi pelkistyminen edesauttaa vivianiitin tai muiden Fe(II)-mineraalien muodostumista ja että vivianiitin muodostumisella saattaa olla vaikutus fosforin pidättämisessä sedimenttiin.