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Displaying theses 61-70 of 1078 total
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M. Meijers
Master programme: Physics - Physics of Life and Health July 4th, 2018
Institute: AMOLF Research group: AMOLF - Biochemical Networks Group Graduation thesis Supervisor: Pieter Rein ten Wolde
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The behaviour of information flow near criticality
Experiments in recent years have highlighted that different natural systems, ranging from neural networks in the brain to flocking behaviour of starling, have a common way to organise themselves. This very special organisation, called criticality, puts strict restrictions on the system and is usually observed in systems that change their phase (e.g. from order to disorder). While experiments indicate criticality in natural systems, it is unclear why nature would want to organise this way. I have investigated the possibility that systems do this in order to optimise the information transmission through the system. The behaviour of information transmission near criticality is unknown. I use the canonical Ising system to investigate the behaviour of information transmission near criticality. I find that two effects from criticality are important for information transmission: 1. A possitive effect from the stronger correlations in the system. 2. A negative effect because of slow dynamics near criticality. These two competing effects cause that the information transmission is optimal close to, but not at criticality. The exact distance from the critical point depends on the distance over which you transmit information and the total size of the system. This is a striking result with potential wide application.
picture that illustrates the research done
Scientific abstract (pdf 1K)   For more info or full text, mail to: tenwolde@amolf.nl

L.M. Stapper
Bachelor programme: Natuur- en Sterrenkunde July 4th, 2018
Institute: API Research group: Planet formation group Graduation thesis Supervisor: Jayne Birkby
Peek-a-boo: is Proxima b hiding in the flares? Determining the impact of flares from Proxima Centauri on the detection of Proxima b
A relatively new technique used to characterize exoplanets is High Resolution Spectroscopy (HRS). With this technique, the signal coming from the exoplanet is buried inside the light coming from the star. The light of the planet is Doppler shifted more than the star’s light, this is used to identify the planet. However, this technique is highly dependent on the state of the star. It could happen that a star produces a flare which could obscure the planets signal. To find out what the impact is of flares on HRS, data coming from Proxima Centauri is used, our nearest neighbour at a distance of 4.2 lightyears from the Solar System. Problem is, Proxima Centauri is ~10^7 times as bright as its planet, Proxima b. To suppress the light of the star, some data analysis steps were applied to high resolution spectroscopy datasets of Proxima Centauri. These steps were done on both data without a flare and data with a simulated flare. After these steps, it turned out that a flare makes it considerably more difficult to retrieve the planet.
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Scientific abstract (pdf 2K)   Full text (pdf 9921K)

J. Kager
Bachelor programme: Natuur- en Sterrenkunde July 4th, 2018
Institute: ITFA Research group: Condensed Matter Theory Graduation thesis Supervisor: dr. J. van Wezel
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Impurity interactions, critical behavior and resonance phenomena in classical φ^4 theory
Fold a sheet of A4 paper several times. Hold the sheet in the middle of the short sides, push the edges upward on one side, and downward on the other. Now, you should see a kink form in the center of the sheet, where the transition from folded upwards to folded downwards occurs. The kink can be moved by changing how much the sides are folded. Such kinks show up across condensed matter physics, and if they appear in a material, they never come alone. Therefore, we may wonder what happens when kinks collide. It turns out that the kinks rebound if they move faster than some “critical velocity”. On the other hand, if the kinks move slower than that, they usually form a bound state. However, velocity ranges, called bounce windows, also exist where the kinks briefly form a bound state before rebounding. In this work, the influence of impurities in the material on kink collision dynamics was studied. How do impurities affect the critical velocity and bounce windows? We used a model called φ^4 theory to carry out numerical simulations of kink collisions. In doing so, we hope to gain understanding in the role of kinks in condensed matter.
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Scientific abstract (pdf 1K)   Full text (pdf 641K)

S.J.J. de Lange
Bachelor programme: Natuur- en Sterrenkunde July 4th, 2018
Institute: ITFA Research group: Instituut voor Theoretische Fysica Graduation thesis Supervisor: Jasper van Wezel
Properties of kinks and kink-impurity interactions in phi^4 theory
In a system with two stable states, a kink is a local transition from one state to the other. For example: An atom in the lattice of a ferromagnet wants to be spin-up if its neighbours are spin-up as well. This results in areas within the crystal wherein all spins point up, or all spins point down. The transition between these areas are called kinks. Kinks can freely move through the medium and they can be affected by impurities in the medium (for example, in a crystal, this may be an atom that doesn’t belong there). A kink may be attracted or repelled by such an impurity. A variety of aspects of these interactions are investigated in this research. Impurities can also have impurity modes. Such a mode is an oscillating motion at the site of the impurity. They oscillate with a certain frequency and amplitude, and they contain energy, which can be exchanged with kinks. Energy exchanges between the impurity mode and the kink may determine whether a kink escapes from the attractive power of the impurity.
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Scientific abstract (pdf 0K)   Full text (pdf 712K)

F.A. van der Ploeg
Bachelor programme: Natuur- en Sterrenkunde July 3rd, 2018
Institute: ITFA Research group: Condensed Matter Theory Graduation thesis Supervisor: P. Corboz
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Simulation of the two-dimensional Ising model using the Corner Transfer Matrix Renormalisation Group method on the square, honeycomb and triangular lattice
I have simulated the two-dimensional Ising model on the square, honeycomb and triangular lattices using the Corner Transfer Matrix Renormalisation Group method (CTMRG).
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 12975K)

D.R. van Arneman
Bachelor programme: Natuur- en Sterrenkunde July 3rd, 2018
Institute: ITFA Research group: GRAPPA Graduation thesis Supervisor: Christoph Weniger
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Determining the dark matter mass density profile of dwarf spheroidal galaxies using Jeans analysis
Dark matter (DM) is one of the biggest unanswered questions in modern physics and astronomy. What exactly is DM? Does it even exist? These are just some of the questions surrounding the nature of DM that physicists have been trying to answer. In this study we investigate the DM in dwarf spheroidal galaxies. If DM were to consist of the proposed ‘neutralino’ particles, it should exhibit behavior of ‘known’ particles. An example of such behavior is annihilation. If two neutralino particles were to annihilate with each other, they would emit an observable gamma ray signal. A quantity that is related to this signal is called the J-factor. This J-factor is also directly related to the mass density of the DM within a galaxy. By researching the J-factor, we could eventually gain more insight into the DM phenomenon. The goal of this study is to reproduce the results of an earlier study using a different analysis method. In our research we determine the DM mass density of specific dwarf galaxies using numerical calculations. With this density we can compute the J-factor. When compared to the results of the previous study, we conclude that our method can properly reproduce the earlier results.
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Scientific abstract (docx 12K)   Full text (pdf 1076K)

M.R. Pothast
Master programme: Physics - Particle and Astroparticle Physics July 1st, 2018
Institute: ITFA Research group: GRAPPA Graduation thesis Supervisor: Christoph Weniger
Gamma Rays as Probes of Cosmic-Ray Diffusion Throughout the Galaxy
The Earth is constantly being bombarded by charged elementary particles from outer space called cosmic rays. The bulk of these particles originate from the Milky Way where they are thought to be injected by exploding stars (supernovae). Because cosmic rays are charged they follow random magnetic fields and we cannot trace these particles back to where they came from. The movement of cosmic rays through the magnetic fields in the Galaxy can be described by a process we call diffusion. In this thesis I robustly studied the recent results from the Fermi-LAT gamma-ray telescope that indicate that the inner part of the Galaxy contains relatively more high energy cosmic rays. This observation contradicts the standard treatment of cosmic-ray diffusion and results from this thesis will help in determining a better model for cosmic-ray diffusion throughout the Galaxy.
picture that illustrates the research done
Scientific abstract (pdf 45K)   Full text (pdf 2178K)

E. Tourou
Master programme: Physics - Physics of Life and Health June 29th, 2018
Institute: VU / Other Research group: VUmc Department of Radiation Oncology Graduation thesis Supervisor: G.J. Streekstra
Knowledge-based radiotherapy treatment planning for stage III lung cancer patients
Radiotherapy treatment for lung cancer requires the creation of a treatment plan that provides a homogeneous dose to target volumes whilst optimally sparing the organs-at-risk (OAR). The treatment of large-volume lung cancer is carried out mostly using two techniques, the full-RapidArc (f-RA) or the hybrid-RapidArc (h-RA). The choice between the two methods is critical and depends on the individual characteristics of the patient, while it often happens that the treatment planners have to make both plans in order to choose for the optimal treatment technique for the patient. However, manual treatment planning is a labor-intensive and time-consuming process which, in many cases, does not yield consistent or optimal plans. Knowledge-based planning is a promising technique which utilizes a large number of prior treatment plans to construct a model that can predict the possible dosimetry and create treatment plans for future patients based on their own anatomical characteristics. The present study investigates the possibility of utilizing RapidPlan (Varian Medical Systems, Palo Alto, USA), a knowledge-based treatment solution, as a tool for selecting f-RA or h-RA technique for individual lung cancer patients, without the requirement of creating actual treatment plans.
picture that illustrates the research done
Scientific abstract (pdf 2K)   Full text (pdf 2478K)

F. Faura Iglesias
Bachelor programme: Natuur- en Sterrenkunde June 29th, 2018
Institute: NIKHEF Research group: ATLAS Graduation thesis Supervisor: Hella Snoek
Method to determine the width of the Higgs Boson of simulations of gg -> H -> ZZ and gg -> ZZ decay modes
The Standard Model of particle physics predicts the lifetime to be 1.56 · 10−22s, however the ATLAS and CMS detectors are not able to measure the lifetime due to their resolution. It is therefore necessary to find an alternative method to determine the lifetime of the Higgs boson. This report describes the test of a method to determine the lifetime. The method consists of the analysis on five simulations of processes that create an Higgs boson. Every simulation has a predetermined lifetime. The result of the analysis is a parameter that is a function of the lifetime. This function could then be used on a dataset that contained an unknown lifetime. This dataset also contained the above mentioned parameter, therefore creating the possibility to find the unknown lifetime in the data set. There are however uncertainties in determining the lifetime that suggest that the used method is not complete. It is therefore a necessitiy to conduct more research and clear the uncertainties.
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Scientific abstract (pdf 1K)   Full text (pdf 1776K)

K.M. Weerman
Bachelor programme: Natuur- en Sterrenkunde June 29th, 2018
Institute: NIKHEF Research group: ATLAS Graduation thesis Supervisor: mw. dr. H. Snoek
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The effect of the Higgs-coupling constant gHZZ on the invariant mass distribution of gg → (H →) ZZ → 4l
The coupling strength of the Higgs-boson to the Standard Model particles gives us insight on the number of elementary particles in nature. If one of the coupling constants deviates from the values calculated in the Standard Model, it would indicate that there are more particles than we know of. We study the coupling of the Higgs-boson to two Z-bosons, gHZZ, with the gluon-gluon induced ZZ-production process gg→(H→)ZZ→4l. The value of the coupling constant is dependent on the width of the resonance and can be obtained through the invariant mass distribution of the four leptons. However, the resolution of the ATLAS-detector makes it impossible to measure the width so in this research we define a parameter dependent on the coupling constant. This parameter is the ratio between the integral of the resonance and the integral of the off-peak section in the invariant mass distribution. A relation between the ratio and the strength of the coupling constant between 1gSM and 10 gSM is found to be a second degree polynomial. However, the fit does not completely follow the data points for gHZZ higher than 4 gSM so further research should be executed on a higher range.
picture that illustrates the research done
Scientific abstract (pdf 53K)   Full text (pdf 1062K)

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