Displaying theses 110 of 991 total
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K.D. Temmink 
Master programme: Physics  Theoretical Physics  February 28th, 2018  
Institute: ITFA  Research group: Instituut voor Theoretische Fysica  Graduation thesis  Supervisor: dr. Philippe R. Corboz 
On Tensor Network Methods for OneDimensional Open Quantum Systems Many quantum systems of interest cannot be described with detailed mathematical expressions, or even numerically exactly. This is because the number of parameters necessary to describe a quantum system grows exponentially with the system size. Tensor Networks (TN) are powerful numerical methods that allow one to efficiently represent or approximate quantum states and operators. Presently, a wide range of TN methods are available for Isolated Quantum Systems (IQS) that do not have any interaction with their environments. Less has been accomplished for Open Quantum Systems (OQS), systems that do interact with their environments. These interactions typically result in complicated mathematical expressions for the behaviour of a system, if they can be obtained at all. Numerically, these systems are notoriously difficult, because extensions of IQS methods are generally unable to represent these systems in a physically valid manner. In this thesis, we study and compare the most general methods for OQS: a naïve extension of an IQS method, a variational method, and the ‘local purification’ method, which was designed to explicitly ensure the physical validity of simulated systems. We find that this last method performs the best in terms of the combination of accuracy, reliability and computational cost. 

Scientific abstract (pdf 56K) Full text (pdf 6652K) 
E. Malami 
Master programme: Physics  Particle and Astroparticle Physics  November 30th, 2017  
Institute: NIKHEF  Research group: Theoretical Physics at NIKHEF  Graduation thesis  Supervisor: Robert Fleischer 
CP Violation in B0d → π0KS. Are there any Hints of New Physics? We study Charge Conjugation and Parity (CP) Violation in B0→π0KS decay and we search for hints of New Physics. We perform a stateoftheart analysis of the Standard Model picture with predictions, which will be very valuable for Bmeson experiments. The B0d→π0KS decay is dominated by QCD loop diagrams (penguins) but electroweak penguins, where New Physics may enter, also have a signiﬁcant impact on the decay amplitude. Our aim is to propose a new strategy for the optimal determination of the parameters which describe electroweak penguin effects, and apply it to the current data, utilising both neutral and charged B→πK decays. This new method can be fully exploited at the future Belle II experiment, which will hopefully answer the question: Do these decays imply New physics? 

Scientific abstract (pdf 2K) For more info or full text, mail to: robert.fleischer@nikhef.nl 
L.J.T. Verhoeven 
Master programme: Physics  Theoretical Physics  October 9th, 2017  
Institute: NIKHEF  Research group: Theoretical Physics at NIKHEF  Graduation thesis  Supervisor: Prof. dr. Robert Fleischer 

Lepton Flavor Violating Higgs In the Standard Model, the flavor of charged leptons is almost completely conserved. In most beyond the Standard Model theories, however, lepton flavor changing interactions come forward naturally. This makes lepton flavor violation a very promising probe for new physics. So far, no signal of lepton flavor violation has been seen, but measurements on lepton flavor violating tauons give only weak bounds. This makes it interesting to look at many different interactions that involve tauons that change flavor and find out if we can detect a nonzero signal. Many models predict that Higgs interactions can change lepton flavor, so decays in which Higgs bosons play a role are a prime candidate for study. Of these, the Z > tau mu gamma decay is experimentally very interesting, as it has the potential to be measured very accurately. We discuss whether this decay can be useful to find lepton flavor violation. 

Scientific abstract (pdf 140K) Full text (pdf 1372K) 
A.K. Frick 
Master programme: Science for Energy and Sustainability  Physics / Chemistry  October 6th, 2017  
Institute: VU / Physics & Astr.  Research group: Physics of Energy  Graduation thesis  Supervisor: Elizabeth von Hauff 

Elimination of Strong Background autofluorescence on Raman Spectroscopy in mouse brains tissue with low voltage inputs NearInfrared (NIR) Raman spectroscopy is a technique that has reemerged as a potential tool for imaging and analysing biological materials due to new advancements in the field. One of the main areas that has received renewed attention is the reduction of autofluorescence, which has been one of the most troublesome issues within the community. In my Thesis I describe a new, minimally invasive technique for significantly reducing the Raman spectroscopy background fluorescence in mouse brains by electrically contacting them during the measurement. This finding opens up the possibility for a plethora of further investigations on the inner workings of living tissue when connected to a circuit and to what extent this quenching effect can be utilised in practice. The outlook for this new technology is that after some further development of this technique’s extensiveness, with regards to applicable materials and Raman excitation wavelength, biological tissue will be able to be measured with a reduced fluorescence, thus increasing the readability of all hardtoread data. 

Scientific abstract (pdf 81K) For more info or full text, mail to: e.l.von.hauff@vu.nl 
J.C. Veenstra 
Master programme: Physics  Theoretical Physics  September 30th, 2017  
Institute: ITFA  Research group: Condensed Matter Theory  Graduation thesis  Supervisor: J. van Wezel 

Spontaneous Unitarity Breaking in Macroscopic Quantum Systems A system's Hilbert space grows exponentially with the number of particles it contains due to the linearity of quantum mechanics. Classical state space, however, only scales linearly with system size. Naively, this would lead one to conclude that classical states become increasingly rare in the thermodynamic limit. Evidently, the opposite is true and it would seem that a symmetry of the macroscopic Hilbert space is implicitly broken. The macroobjectification problem, as this mismatch between theory and reality is known, has been the subject of debate for over a century. In this thesis we review some existing solutions and develop a dynamical description in terms of spontaneous symmetry breaking. 

Scientific abstract (pdf 2K) Full text (pdf 2910K) 
J.A. Wildeboer 
Master programme: Gravitation and Astroparticle Physics Amsterdam (GRAPPA), track offered by both Physics / Astronomy  September 30th, 2017  
Institute: NIKHEF  Research group: Dark Matter  Graduation thesis  Supervisor: Prof. Dr. Patrick Decowski 

Threshold optimization and Bayesian inference on the XENON1T experiment Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a worldwide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence. The XENON Collaboration now operates the XENON1T experiment. The result from a first science run shows that this detector has a new record low radioactivity level. With a total mass of about 3200kg, XENON1T is at the same time the largest detector of this type ever built. This thesis reports on the optimization of the thresholds inside XENON1T's data processor and examines Bayesian inference as an alternative statistical method to set upper limits on the WIMPnucleon cross section. 

Scientific abstract (pdf 1K) Full text (pdf 7155K) 
V.R. Stalman 
Master programme: Physics  Theoretical Physics  September 27th, 2017  
Institute: ITFA  Research group: String theory  Graduation thesis  Supervisor: Dr. Diego Hofman 
NonRelativistic String Theory in NewtonCartan & Carroll Spacetimes In this work we evaluate the properties of world sheet theories in spacetimes that have nonrelativistic global symmetries supplemented by an infinite dimensional extension of the worldsheet symmetry algebra to possibly find an alternative holography. 

Scientific abstract (pdf 20K) For more info or full text, mail to: D.M.Hofman@uva.nl 
D. Mulder 
Master programme: Physics  Theoretical Physics  August 31st, 2017  
Institute: ITFA  Research group: MathematicalPhysics  Graduation thesis  Supervisor: Bernard Nienhuis 

Percolation and the Ising model in the hyperbolic plane Percolation on the Euclidean plane has a single critical point p_c. At the critical point, the mean cluster size scales with the system size N as a N^43/48. Other quantities have similar scaling behaviour, with different critical exponents. These exponents are related through scaling laws. In the hyperbolic plane, there are two dual critical points, p_c and p_u. In between, the exponent of the mean cluster size continuously varies from 0 to 1. We have tried to find other quantities that scale as a power of N, to establish scaling relations in the hyperbolic plane. The results are not as simile to Euclidean case as expected. Since percolation is the limit case of a class of models called Potts models, we extended our scope to Potts models in general. We have studied the Ising model (which is the 2state Potts model) by using the CTMRGalgorithm for hyperbolic lattices, as presented by Nishino et al. Using a framework given by Nogawa and Hasegawa for percolation, we were able to determine, for the first time, a second critical temperature T_u for the Ising model on hyperbolic lattices. 

Scientific abstract (pdf 3K) Full text (pdf 3505K) 
S.A.M. Lourens 
Bachelor programme: Natuur en Sterrenkunde  August 29th, 2017  
Institute: AMC  Research group: Biomedical Engineering and Physics  Graduation thesis  Supervisor: M.C.G. Aalders 
Fingerprint Detection with Optical Coherence Tomography Damaged or altered fingerprints can not be traced back with conventional fingerprint detection technique, which can be a problem in especially the forensic field. A solution to this problem may be provided by Optical Coherence Tomography, which enables detecting the fingerprint from not only the surface layer, but also the dermis layer of the skin, which is not vulnerable to spoofing and superficial damage to the skin. This research focused on optimizing the process of obtaining these OCT fingerprints step by step. 

Scientific abstract (pdf 3K) For more info or full text, mail to: m.c.aalders@amc.uva.nl 
T. Kaandorp 
Bachelor programme: Natuur en Sterrenkunde  August 28th, 2017  
Institute: AMC  Research group: Department of Radiology  Graduation thesis  Supervisor: A. J. Nederveen 
Accelerated 4D flow MRI, Comparing SENSE, kt PCA and Compressed Sensing To increase patient comfort, it is possible to accelerate a 4d flow mri using techniques such as SENSE, kt PCA or Comrpessed Sensing(CS). The acceleration is based on collecting less data points. However by doing so the image quality decreases. One can therefore wonder what happens with the amplitude of the flow curves created by 4D flow MRI. It turns out that for accelerated SENSE and kt PCA the amplitude of the flow curves decreases by respectfully 33 and 20 percent. The amplitude of the flow curves created with CS reconstruction only decrease with 7.5 percent. Thus making CS a more reliable acceleration method. 

Scientific abstract (pdf 1K) Full text (pdf 2467K) 
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