A non-destructive tomographic measurement technique for application on nuclear fuel assemblies has beendeveloped at the Uppsala University. Using this technique, the rod-by-rod distribution of selectedradioactive isotopes is determined experimentally. In the present work, the numerical technique to reconstruct the activity distribution inside the fuelassemblies has been analyzed. Three iterative reconstruction algorithms have been investigated, ART(Additive Reconstruction Technique), ML (Maximum Likelihood) and ASIRT (Additive SimultaneousIterative Reconstruction Technique). It was found that the ART algorithm is too sensitive to data points where the gamma-ray intensityis low, while ASIRT handles it in the best manner. Furthermore, ASIRT appears to be the most stablealgorithm and produces the best agreement to theoretical data.

Quantum computers, like classical ones, need to be capable of sending informa-tion between different parts of the computer. Spin chains are one viable method of sending qubits while preserving the superposition and entanglement of a state. We use quantum speed limits, which give bounds on how fast non-relativistic quantum systems can evolve, to study simple Heisenberg chains and their efficacy as quantum channels.

The neutron is a valuable tool in many fields of science due to its properties regarding electric charge and magnetic moment. However, these properties also makes it difficult to detect. Through simulation one can optimize the detector environment to best correspond with the needs for a specific experiment. In this project, a detector environment is simulated, consisting of a neutron and gamma point source and one detector for each type of particle. The simulated results were in good agreement with the underlying theory and the simulation can, therefore, be used in future work.

The potential for characterizing the spin and mass of dark matter at the Large Hadron Collider (LHC) given a 5 sigma confidence level discovery has been studied. The search for dark matter is one of the big open question in particle physics as observations predict 84 percent of the matter in the universe to be dark matter, however no detection has been made. In this study two production channels at the LHC of weakly interacting massive particles with different spin have been studied in the framework of a simplified model. The processes if detected have the signature of a mono-jet and missing transverse energy. The assumption made is that the dark matter is a scalar or a Dirac fermion particle and interacts with the top quark and a Beyond the Standard Model color charged mediator via a loop process. The considered mediators are a scalar top partner and a fermionic top partner. The results from recasting conclude that in a narrow region in the mass-mass-plane for the dark matter and the mediator a search is adequate and given a discovery characterization of the spin and mass is possible.

This feasibility study concerns the implementation and analysis of a Higgs pair decaying to two W bosons, with hadronic final states, and to two τ-leptons, one with a leptonic and one with a hadronic final state, hh→ WhadWhad τlepτhad, in the ATLAS detector at the CERN Large Hadron Collider, by adapting the ``CxAOD framework'' software analysis code of the hh→ bbττ analysis. The purpose is to compare the sensitivity of this final state with other di-Higgs analyses in the ATLAS collaboration. Monte Carlo samples are used for the background and signal simulation. A cut based selection is then performed based on several kinematic variables. Finally, a statistical analysis is performed on the MMC mass distribution of the ττ system. The cross section can be limited at the 95 %  confidence level to σexpected < 89.8 fb, compared to the theoretical σSM = 0.19 fb.

Recently, searches for pair production of Higgs bosons in several final states have been carried out by the ATLAS exeperiment at the Large Hadron Collider (LHC). This study focuses on the search for non-resonant di-Higgs production decaying to a final state with two b-jets and two τ-leptons using 36.1 fb-1 of data recorded by the ATLAS detector. The analysis for this process has already been performed. Boosted decision trees (BDTs) are used in the analysis to improve the separation of the signal from background processes and several variables that provide good discrimination between signal and background are used as inputs to the BDT. This study aims to unfold the BDT of the analysis and optimize a cut-based analysis so that the gain from using the BDT can be estimated. Two variables, related to the invariant masses and angular distances of the Higgs boson decay products, are defined and the optimal cuts are found to be Xmττmbb<1.8 and XΔRττΔRbb <4.0. Then, the upper limits on the SM HH production cross section are set when fitting mHH with the cut-based analysis. An expected limit of 0.78 pb, 23 times the SM prediction is obtained when neglecting systematic uncertainties, compared to the limit of 15 times the SM as recomputed when using the BDT. Comparing the two results, the sensitivity is worsened by 50% when not using the BDT.

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The Qweak experiment will provide the most precise determination of the proton’s weak charge Q^p_W by measuring the parity-violating asymmetry in elastic electron-proton scattering at low momentum transfer Q^2 = 0.0250 (GeV/c)^2. Qpw is related to the weak mixing angle sin^2\theta_W, a fundamental parameter of the standard model. A final measurement of the weak charge at the proposed 4% relative uncertainty is sensitive to certain types of new parity-violating physics beyond the standard model at the TeV mass-scale. Data were taken over a two year period beginning in 2010, using a custom apparatus installed in Hall-C at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. A 180 microamp beam of longitudinally polarized electrons was accelerated to 1.165 GeV and then scattered from unpolarized protons in a liquid hydrogen target. The scattered electrons were then collimated and steered using a magnetic spectrometer onto a set of azimuthally symmetric quartz bar Cherenkov detectors. The performance of this main detector subsystem will be described in detail in this dissertation. A blinded analysis of Run-II, roughly 2/3 of the entire Qweak data set, resulted in an elastic ep asymmetry of -235.6 +/- 8.7 (Stat) +/- 9.3 (Syst.) +/- 39.3 (Blind) ppb. Using this value, the proton’s weak charge was calculated to be QpW = 0.0714 +/- 0.0093. This constitutes a 17% relative measurement, that will reduce to < 9% upon unblinding the asymmetry. This dissertation will provide the details that went into extracting both the parity-violating asymmetry and the weak charge of the proton. / October 2015

A study for the search for vector-like quarks and a new pseudo Nambu-Goldstone boson(pNGB) scalar at the LHC running at $\sqrt{s} = 13$ TeV is presented. A model based on the composite Higgs model in which the Higgs boson emerges as a composite pNGB together with a new pNGB $\eta$ is considered. Simulations are made on the modelshowing a low ratio between signal to background indicating no significance and a low discovery potential. However, further studies are needed as more channels should be investigated. Moreover, a more sophisticated analysis is needed to properly reject the background and for a more thorough statistical analysis. Even so, results still show potential in the search as there do exist distinct differences between signal and SM background.

I replicated the H/A → τ μ τ had analysis of the MSSM extension to the stan-dard model with the ATLAS experiment. I investigated, using a transformedBDT score for τ -lepton identification with a flat distribution, with more work-ing points than the standard loose, medium and tight, the sensitivity of theanalysis. I then compared for each of the working points and found new signalefficiencies that are more sensitive for this analysis than the standard workingpoints. / I replicated the H/A → τ μ τ had analysis of the MSSM extension to the stan-dard model with the ATLAS experiment. I investigated, using a transformedBDT score for τ -lepton identification with a flat distribution, with more work-ing points than the standard loose, medium and tight, the sensitivity of theanalysis. I then compared for each of the working points and found new signalefficiencies that are more sensitive for this analysis than the standard workingpoints.