The dark matter paradigm has been a great source of speculation in both the 20th and 21st Centuries. Since its proposed existence in 1933, the mounting evidence has led to this theoretical particle becoming one of the greatest mysteries of modern physics. However, despite its dominant presence in the Universe, little is known about its nature and how it behaves. In this thesis I critically analyse one particular property of dark matter: the self-coupling. The self-interacting dark matter paradigm hypothesises that dark matter is not collisionless as assumed in most cosmological simulations, and in-fact has some probability that it will scatter off itself. Such a self-coupling will resolve many discrepancies that exist between observations and theory, particularly on small, non-linear scales. Moreover, any detection of a self-interaction cross-section will place considerable limitations on the acceptable particle physics models of dark matter and hence has grown to become an important question. In this thesis I develop and implement a method to constrain the self-interaction cross-section of dark matter that exploits continually accreting and merging groups of galaxies as they fall into galaxy clusters. Utilising the ubiquitous nature of accreting substructure, I measure the offsets between dark matter and baryonic gas as they become separated due to their differing interaction properties. Studying this effect over a sample of events, I will be able to make the first ever statistical estimate of the cross-section of dark matter, while averaging over many different unknown merging scenarios. I begin my thesis by deriving an analytical description of sub-halo in-fall, allowing me to constrain dark matter self-interaction models directly from observations. In this study, I find that current archival data should be able to detect a difference in the dynamical behaviour of dark matter and standard model particles at 6σ, and measure the total interaction cross-section σDM/m with 68% confidence limits of ±1 cm2g-1. Having constructed a new method to derive constraints on the cross-section of dark matter I carry out a study into the potential systematics that may affect a measurement. I determine the accuracy of weak gravitational lensing, which is the distortion of light due to intervening mass, as a tool to estimate the positions of substructure in galaxy clusters. I find that the public Lenstool software can measure the position of individual 1:5 x 1013Mʘ peaks with ~ 0:3" systematic bias, as long as they are at least ~ 30" from the cluster centre. Finally, I develop a pipeline that can analyse a sample of inhomogeneous observations from The Hubble Space Telescope and the Chandra X-ray Observatory. By measuring the positions of dark matter, gas and galaxies for 68 individual merging events, from a total of 28 galaxy clusters, I detect a 7:4σ offset between gas and an unobserved dark mass. I make the first ever measurement of cross-section of dark matter from a sample of clusters finding σDM < 0:50cm2/g [95% CL], the best constraints to date. In addition to this I find that the brightest group galaxy in-fact tends to lead the dark matter halo during merging events. Although evidence for the existence of interacting dark matter, I conclude that the astrophysics of the BCG is complicated, and that this apparent directional bias should be considered in all galaxy cluster analyses. Moreover, I show that this technique is easily extendable for future surveys that have larger samples of galaxy clusters, with constraints of σDM < 0:001cm2/g potentially attainable.

Abstract The reactivity and bonding of allyl groups (C3H5) on a Ag(111) surface have been investigated under ultrahigh vacuum conditions by temperature-programmed reaction/desorption (TPR/D) and reflection-adsorption infrared spectroscopy (RAIRS). The atomically clean surface was achieved by Ar+ sputtering and verified by AES. The surface crystallinity was assured by LEED. Surface -bound allyl groups were generated by dissociative adsorption of allyl halides. Our study shows that the C-X (X= I or Cl) bond can be ruptured below 200K to render adsorbed allyl species. Upon further heating, three gas-phase products were detected at ~280 K, 295 K and 320 K in the TPR/D spectra, which are attributed to 1,5-hexadiene, allene, and propene, respectively. These results suggest that allyl undergoes

The Standard Model scalar potential contains a minimum at the Electroweak scale, responsible for the masses of the weak gauge bosons through the Higgs mechanism. However, if the Electroweak minimum is only a local minimum, and there exists a global minimum at a higher energy in the Higgs potential, then in a su ciently old universe we would expect the vacuum expectation value to be at the global minimum. The absence of a global minimum at higher energy is related to the condition that the Higgs self coupling is greater than or equal to zero for all energies. For any model that fails this, we expect new physics to enter before the energy at which the coupling becomes negative. We developed tools to automate the derivation of beta functions for renormalisable gauge theories, and used these to carry out evolution of the renormalisation group equations for the Standard Model and three extensions to the Standard Model | the Standard Model with a fourth generation, the Standard Model with right-handed neutrinos and a Left-Right Symmetric Model. We conclude that of these four models, the Standard Model is the only one in which all the couplings remain perturbative, and in which the Electroweak minimum is a global minimum.

Ein fundamentaler Baustein des Standardmodels, des heute am weitesten akzeptierten Models der Elementarteilchenphysik, ist die Selbstkopplung der elektroschwachen Eichbosonen gamma, Z und W. Waehrend andere Vorhersagen des Standardmodels mit hoher Praezision getestet wurden, ist ueber die Staerke der Selbstkopplung der Bosonen wenig bekannt. Erste indirekte Hinweise ueber solche Kopplungen wurden aus praezisen Messungen der Fermionpaarproduktion auf dem Z-Pol gewonnen. Diese Messungen sind sensitiv auf Strahlungskorrekturen. In dieser Analyse werden zum ersten Mal alle verfuegbaren elektroschwachen Praezisionsdaten, die unter anderen bei LEP 1, am SLAC und am TEVATRON gewonnen wurden, benutzt, um in einer globalen Anpassung die Kopplungsstaerken der elektroschwachen Eichbosonen zu ermitteln. Praezise direkte Messungen der Kopplungsstaerke wurden durch die Erhoehung der Schwerpunktsenergie am LEP-Beschleuniger im Jahre 1996 moeglich, die die Paarproduktion von W-Bosonen, e+e- -> W+W-, erlaubte. Zusaetzlich zu diesem Kanal wurde auch noch die Kopplungsabhaengigkeit des Wirkungsquerschnitts der einfach-resonanten W-Produktion, e+e- -> W e nu, und der Photonproduktion, e+e- -> nu nu gamma, benutzt, um die Selbstkopplung der Bosonen zu bestimmen. Zur Analyse wurden Daten, die einer Gesamtluminositaet von 77 pb^-1 entsprechen und bei Schwerpunktsenergien von 161, 172 und 183 GeV in den Jahren 1996 und 1997 mit dem L3 Detektor aufgezeichnet wurden, benutzt. Die Vorhersagen des Standardmodels sind in guter Uebereinstimmung mit allen Messungen. Insbesondere mit der Messung von g_1^Z konnte zum ersten Mal die Existenz des ZWW Vertex experimentell nachgewiesen werden. Zusaetzlich fordert das Standardmodel die Erhaltung der C- und P-Paritaet am ZWW Vertex. Diese Vorhersage wurde durch die Messung getestet und es wurde gute Uebereinstimmung mit der Standardmodelvorhersage gefunden. Die Messung der Kopplungsstaerken in drei unterschiedlichen Kanaelen entspricht der Messung in unterschiedliche Regionen von Impulsuebertraegen. Die Messungen zeigen keine Abhaengigkeit, so dass sowohl das magnetische Dipolmoment als auch das elektrische Quadrupolmoment des W-Bosons aus den Kopplungen hervorgehen. Diese statischen Eigenschaften des W-Bosons geben Informationen ueber dessen Groesse und geometrische Struktur. So folgt aus der Messung das der Radius der W-Bosons kleiner als 10^-18 m ist. Zusaetzlich zu diesen Informationen ueber das W-Boson, konnte der Parameterbereich einer Erweiterung der Standardmodels durch ein sequentielles Z'-Boson eingeschraenkt werden. Ein Model von Klein das die Vereinigung von Kraeften und Materie beschreibt wurde mit 10 Standardabweichungen ausgeschlossen. Zusammenfassung als PostScript-Datei / The couplings between the bosons of the electroweak interaction, gamma, Z and W, is one of the fundamental building blocks of the Standard Model, which was not yet tested with high precision. Indirect hints for the existence of boson self-coupling have been obtained by analysing Z pole data with respect to radiative corrections. This analysis uses for the first time all available electroweak precision data obtained at LEP 1, SLC, TEVATRON and at low energy experiments. The coupling strength between the electroweak gauge bosons is obtained by a global fit to all these data. A precise direct measurement of triple gauge boson couplings became possible in 1996 at LEP 2, where W bosons could be produced in pairs, e+e- -> W+W-. In addition single-resonant W production, e+e- -> W e nu, and single photon production, e+e- -> nu nu gamma, are evaluated with respect to boson self-couplings. In total a luminosity of 77 pb^-1 was collected with the L3 detector at 161, 172 and 183 GeV centre-of-mass energy in the years 1996 and 1997. The Standard Model expectations show good agreement with this measurement. The measurement is the first proof of the existence of a ZWW vertex. The LEP 2 data were further used to limit violation of parity and C-parity at the ZWW vertex. The results of the measurement for the three different channels, corresponding to three different regions of momentum transfer showed no dependence. Thus the magnetic dipole moment and electric quadrupole moment are derived. These two static properties of the W give information on the size and the geometrical form of the W, such that the W radius could be limited to 10^-18 m. In addition the coupling constants were used to limit the phase space of the extension of the Standard Model with a sequential Z' boson in terms of mixing angle and Z' mass. The unified matter theory by Klein is ruled out with more than ten standard deviations. abstract in PostScript

The degree by which a species can adapt to the demands of its changing environment defines how well it can exploit the resources of new ecological niches. Since the nervous system is the seat of an organism's behavior, studying adaptation starts from there. The nervous system adapts through neuronal plasticity, which may be considered as the brain's reaction to environmental perturbations. In a natural setting, these perturbations are always changing. As such, a full understanding of how the brain functions requires studying neuronal plasticity under temporally varying stimulation conditions, i.e., studying the role of plasticity in carrying out spatiotemporal computations. It is only then that we can fully benefit from the full potential of neural information processing to build powerful brain-inspired adaptive technologies. Here, we focus on homeostatic plasticity, where certain properties of the neural machinery are regulated so that they remain within a functionally and metabolically desirable range. Our main goal is to illustrate how homeostatic plasticity interacting with associative mechanisms is functionally relevant for spatiotemporal computations. The thesis consists of three studies that share two features: (1) homeostatic and synaptic plasticity act on a dynamical system such as a recurrent neural network. (2) The dynamical system is nonautonomous, that is, it is subject to temporally varying stimulation. In the first study, we develop a rigorous theory of spatiotemporal representations and computations, and the role of plasticity. Within the developed theory, we show that homeostatic plasticity increases the capacity of the network to encode spatiotemporal patterns, and that synaptic plasticity associates these patterns to network states. The second study applies the insights from the first study to the single node delay-coupled reservoir computing architecture, or DCR. The DCR's activity is sampled at several computational units. We derive a homeostatic plasticity rule acting on these units. We analytically show that the rule balances between the two necessary processes for spatiotemporal computations identified in the first study. As a result, we show that the computational power of the DCR significantly increases. The third study considers minimal neural control of robots. We show that recurrent neural control with homeostatic synaptic dynamics endows the robots with memory. We show through demonstrations that this memory is necessary for generating behaviors like obstacle-avoidance of a wheel-driven robot and stable hexapod locomotion.