This thesis comprises two parts. In the first part an overview is given of the Standard Model, Supersymmetry and R-hadrons. R-hadrons are predicted within the framework of the Split-SUSY theory, in which the gluino can have a sufficiently long lifetime to hadronise into so-called R-hadrons. These will then propagate through the ATLAS detector as (meta)stable particles. The R-hadrons, which interact strongly, have the property of changing electric charge in interactions as they move through matter. In this thesis a strategy to search for R-hadrons with the ATLAS detcector is developed. The second part describes the ATLAS detector and the developement of the digitizer system of the ATLAS hadronic calorimeter. The digitizer system is the digitial readout system, in which data is read out from the detector to data buffers via an optical link. In the LHC environment, the operation of radiation-hard circuitry is essential. Therefore a quality control program was used to control that the design conforms to the requirements of radiation tolerance. An important part of the work was to test the devices to study their functionality and properties. This work is described in this thesis.

<p>This thesis comprises two parts. In the first part an overview is given of the Standard Model, Supersymmetry and R-hadrons. R-hadrons are predicted within the framework of the Split-SUSY theory, in which the gluino can have a sufficiently long lifetime to hadronise into so-called R-hadrons. These will then propagate through the ATLAS detector as (meta)stable particles. The R-hadrons, which interact strongly, have the property of changing electric charge in interactions as they move through matter. In this thesis a strategy to search for R-hadrons with the ATLAS detcector is developed.</p><p>The second part describes the ATLAS detector and the developement of the digitizer system of the ATLAS hadronic calorimeter. The digitizer system is the digitial readout system, in which data is read out from the detector to data buffers via an optical link. In the LHC environment, the operation of radiation-hard circuitry is essential. Therefore a quality control program was used to control that the design conforms to the requirements of radiation tolerance. An important part of the work was to test the devices to study their functionality and properties. This work is described in this thesis.</p>

Unitary models in two dimensions are classes of low dimensional theories which provide us with a convenient theoretical laboratory for studying various aspects of the theory of elementary particles. In this thesis, purely bosonic U(N) sigma models with the Wess-Zuraino-Witten (WZW) term in two-dimensional Euclidean space and the supersymmetric (Susy) U(N) σ models with and without this term are discussed. Particular attention is paid to the classical solutions of the equations of motion of these models. Due to the integrabihty of these models, we can associate with them a Lax-pair formalism. We observe that solutions of the Lax-pair equations of the U(N) a model provide us with solutions of the U(N) a model with the WZW-term. This is also the case for solutions of the Susy U(N) a model with the WZW-term which can be constructed from solutions of the Lax-pair equations of the Susy U(N) σ model. We present also some explicit solutions of the Susy U(N) a model without the WZW-term. Many properties of the constructed solutions for both the purely bosonic and Susy models are explored. In particular, we calculate the values of the action for some solutions and study the stability properties of these solutions and find that all the constructed solutions of these models correspond to the saddle points of the action. Finally we consider the hnearized fermion equations in the fixed background of a bosonic field. Special attention is paid to the case when the background field is given by a solution of the U(N) σ model with and/or without the WZW-term. Some classes of solutions of this problem are presented and their properties are discussed. We observe that a class of these solutions is related to the components of the energy-momentum tensor of the purely bosonic σ model and prove that some of these solutions are traceless.

Low dimensional models are generally regarded to be a convenient theoretical laboratory for studying various aspects of elementary particle theory. In this thesis, the extended solutions of one particular class of such models, namely the ₵p(^n-1) non-linear a-models in 2 dimensions, are discussed. Special attention is paid to the shape of these extended structures and their dependence on the parameters of the solutions. Time dependence is introduced into the models, and properties of the moving objects in these (2 + l)-dimensional theories are explored. In particular, the Hopf terms of the theories are investigated, and their relation to the spin of the extended solutions is discussed. Also the classical dynamics of these moving objects, and their explanation in terms of the geodesic motions on certain Hermitian and Kāhler manifolds is considered. Finally the embedding of the (₵p(^n-1)) solutions into the 2-dimensional U(n) chiral models is studied, paying particular attention to the stability of these embedded solutions in the larger group space, and to the number of independent negative modes of the fluctuation operator around these solutions.

<p>The Standard Model of particle physics has been very successful in predicting a wide range of phenomena and has so far been confirmed by all existing data to a very high precision.</p><p>The work described in this thesis tests the limits of validity of the Standard Model (SM) in two areas believed to be sensitive to deviations from the theory: the observation of unpredicted particles and CP violation. The studies were performed within the framework of experiments at two hadron colliders, the future ATLAS detector scheduled for operation in 2007 at the Large Hadron Collider in Geneva and the currently running DØ experiment at the Tevatron in Chicago.</p><p>The tau lepton’s distinctive signature is a useful tool in many new physics searches where it is present in the final state. As a first study in ATLAS a Monte Carlo analysis of two-tau final states, which are sensitive to the underlying structure of supersymmetric models, was performed. </p><p>Several extensions of the SM predict the existence of a charged Higgs boson.The major part of this thesis has consisted in using tau leptons to search for the charged Higgs in the context of the Minimal Supersymmetric extension of the SM (MSSM). Results from this thesis show that searches for the <i>H</i>^± → τν_τ decay channel extend the charged Higgs discovery reach for the ATLAS experiment compared to previous studies of other channels: the charged Higgs can be observed for masses up to ~ 600 GeV for tan<i>β</i> > 10. Its mass can be determined with an uncertainty of 1 to 2%, dominated by statistical errors. The tan<i>β</i> parameter can be derived from the absolute rate of this decay to a precision around 6% for 20 < tan <i>β</i> < 50.</p><p>By measuring precisely the unitarity triangle parameter sin2<i>β</i> the SM description of CP violation can be put to a test. As a separate study a Monte Carlo analysis was performed in ATLAS, which shows that the systematic uncertainty is half the attainable statistical uncertainty. </p><p>As part of the effort to search for the charged Higgs in the DØ experiment a trigger algorithm for tau leptons was written, extensively tested and implemented in the experiment. Trigger strategies for events containing taus were designed. These trigger studies will be useful also for many other new physics searches at DØ.</p>

<p>A search for cosmic point sources of neutrinos has been performed using data taken in 1999 with the AMANDA-B10 neutrino telescope. </p><p>This work describes methods for signal and background separation and the statistical analysis of the final data sample. In particular, the multivariate method Support Vector Machines has been applied to achieve good background rejection while at the same time retaining high signal efficiency. </p><p>A grid search covering the complete northern hemisphere revealed no statistical significant excess of events over the expected background from mis-reconstructed cosmic ray induced muons and muons induced by atmospheric neutrinos. Thus, no cosmic point sources of neutrinos have been detected. Upper limits on the neutrino flux for each cell of the grid are presented.</p><p>Twenty potential sources of neutrinos chosen among three classes of astronomical objects (Blazars, Super Nova Remnants and Microquasars) have been preselected. Upper Limits on the flux of cosmic neutrinos from those are presented.</p><p>The presence of systematic uncertainties makes the calculation of confidence limits an intricate problem. A method is presented which makes it possible to include these uncertainties into the frequentist construction of confidence intervals. Statistical properties of the presented method have been studied.</p>

The Standard Model of particle physics has been very successful in predicting a wide range of phenomena and has so far been confirmed by all existing data to a very high precision. The work described in this thesis tests the limits of validity of the Standard Model (SM) in two areas believed to be sensitive to deviations from the theory: the observation of unpredicted particles and CP violation. The studies were performed within the framework of experiments at two hadron colliders, the future ATLAS detector scheduled for operation in 2007 at the Large Hadron Collider in Geneva and the currently running DØ experiment at the Tevatron in Chicago. The tau lepton’s distinctive signature is a useful tool in many new physics searches where it is present in the final state. As a first study in ATLAS a Monte Carlo analysis of two-tau final states, which are sensitive to the underlying structure of supersymmetric models, was performed. Several extensions of the SM predict the existence of a charged Higgs boson.The major part of this thesis has consisted in using tau leptons to search for the charged Higgs in the context of the Minimal Supersymmetric extension of the SM (MSSM). Results from this thesis show that searches for the H± → τντ decay channel extend the charged Higgs discovery reach for the ATLAS experiment compared to previous studies of other channels: the charged Higgs can be observed for masses up to ~ 600 GeV for tanβ &gt; 10. Its mass can be determined with an uncertainty of 1 to 2%, dominated by statistical errors. The tanβ parameter can be derived from the absolute rate of this decay to a precision around 6% for 20 &lt; tan β &lt; 50. By measuring precisely the unitarity triangle parameter sin2β the SM description of CP violation can be put to a test. As a separate study a Monte Carlo analysis was performed in ATLAS, which shows that the systematic uncertainty is half the attainable statistical uncertainty. As part of the effort to search for the charged Higgs in the DØ experiment a trigger algorithm for tau leptons was written, extensively tested and implemented in the experiment. Trigger strategies for events containing taus were designed. These trigger studies will be useful also for many other new physics searches at DØ.

A search for cosmic point sources of neutrinos has been performed using data taken in 1999 with the AMANDA-B10 neutrino telescope. This work describes methods for signal and background separation and the statistical analysis of the final data sample. In particular, the multivariate method Support Vector Machines has been applied to achieve good background rejection while at the same time retaining high signal efficiency. A grid search covering the complete northern hemisphere revealed no statistical significant excess of events over the expected background from mis-reconstructed cosmic ray induced muons and muons induced by atmospheric neutrinos. Thus, no cosmic point sources of neutrinos have been detected. Upper limits on the neutrino flux for each cell of the grid are presented. Twenty potential sources of neutrinos chosen among three classes of astronomical objects (Blazars, Super Nova Remnants and Microquasars) have been preselected. Upper Limits on the flux of cosmic neutrinos from those are presented. The presence of systematic uncertainties makes the calculation of confidence limits an intricate problem. A method is presented which makes it possible to include these uncertainties into the frequentist construction of confidence intervals. Statistical properties of the presented method have been studied.

<p>The annihilation of weakly interacting massive particles (WIMPs), accumulated in gravitational potentials (e.g., the core of the Earth, the Sun or the Galactic halo) would lead to neutrino production. This thesis investigates the possibility of searching for WIMPs in the form of the lightest supersymmetric particle (neutralino) trapped in the Sun using the AMANDA-II neutrino telescope. AMANDA-II is a large Cherenkov detector located deep in the ice at the geographical South Pole. The presented work is based on data taken during the year 2001. An analysis optimized to search for the neutralino-induced flux from the Sun has been developed. The observation of no excess with respect to the expected atmospheric neutrino background has been interpreted as an upper limit on the neutralino annihilation rate in the Sun and on the neutralino-induced muon flux in the vicinity of the detector.</p>

<p>The nature of the dark matter in the Universe is one of the greatest mysteries in modern astronomy. The neutralino is a nonbaryonic dark matter candidate in minimal supersymmetric extensions to the standard model of particle physics. If the dark matter halo of our galaxy is made up of neutralinos some would become gravitationally trapped inside massive bodies like the Earth. Their pair-wise annihilation produces neutrinos that can be detected by neutrino experiments looking in the direction of the centre of the Earth.</p><p>The AMANDA neutrino telescope, currently the largest in the world, consists of an array of light detectors buried deep in the Antarctic glacier at the geographical South Pole. The extremely transparent ice acts as a Cherenkov medium for muons passing the array and using the timing information of detected photons it is possible to reconstruct the muon direction.</p><p>A search has been performed for nearly vertically upgoing neutrino induced muons with AMANDA-B10 data taken over the three year period 1997-99. No excess above the atmospheric neutrino background expectation was found. Upper limits at the 90 % confidence level has been set on the annihilation rate of neutralinos at the centre of the Earth and on the muon flux induced by neutrinos created by the annihilation products.</p>