Browsing by study line "Physics"

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.

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.

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.

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.

Scanning probe microscopy (SPM), which includes atomic force microscopy (AFM), is an affordable and powerful tool for investigating surfaces. However, to ensure accuracy, a metrologically informed approach is required. In this thesis, a commercial Jupiter XR AFM was used for calibration experiments testing its reliability and accuracy. AFMs can further be improved by the use of active probes with added capabilities that allow for faster scanning speeds and other improvements. Efforts have been made to enable their broader use. At VTT MIKES, the commercial Jupiter XR AFM is being modified to use active probes capable of self-sensing and self-actuation. A simple AFM was first built as a test setup to better understand the probes and to split their integration into the Jupiter AFM into a step-by-step process where the mechanical, electronic, and software changes necessary could be separated into distinct parts. In this thesis, I describe experiments done to test the reliability of the commercial AFM, and the process of constructing and using the test setup AFM, after which I present some results obtained using them, and discuss the experiments and the integration process. Included is also an overview of metrology and some of the physics and other theory relevant to AFM, and a summary of the principles of AFM and its role in microscopy. This work is part of the MetExSPM project, which seeks to develop traceable high speed scanning probe microscopy, for example by achieving higher scanning speeds and larger scanning areas, while maintaining good resolution and metrologically traceable high accuracy. This will greatly increase the utility of SPMs, especially for industrial applications.

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.