No description available.

No description available.

No description available.

No description available.

No description available.

In this work, the efficiency for the production of a monoenergetic positronium beam via the charge-exchange reaction of a positron beam in a gaseous target has been determined for molecular hydrogen and molecular nitrogen. In the case of molecular nitrogen, it has been found that the energy range over which a useful intensity of collimated positronium may be produced can be extended to 250eV, 100eV higher than previously achieved. This should enable measurements of the total and partial positronium cross-sections at correspondingly higher energies, where target inelastic effects are expected to be significant A recent measurement of the integrated positronium formation cross-section for xenon found a larger yield of positronium atoms compared to the other noble gases. A shoulder was also seen 10eV above the peak and it was suggested that this might be due to the production of positronium in an excited state. These findings have provided an incentive to investigate the collimated positronium production efficiency from xenon, which has been found to be surprisingly low. The quantum state of the beam atoms has also been found to be dominantly ground state. Possible reasons for these findings are discussed Total cross-sections for positronium-gas scattering have been extracted from the measurements of the positronium beam production efficiency for both molecular nitrogen and xenon. These quantities have also been determined directly by measuring the intensity of the positronium beam transmitted through a gas cell via the Beer-Lambert Law. A good consistency is found between the values obtained using this method and those determined indirectly. Recently, measurements have been made of the absolute integrated cross-section for the fragmentation of positronium in collision with helium atoms, along with the longitudinal energy distributions of the residual positrons in the energy range -Ep/=13-33eV. Measurements of the latter indicate a peak close to half the residual positronium energy, suggesting that they continue to move in a correlated fashion with the emitted electrons. In the present work, these results have been confirmed using a different method, which enables the energy range of investigation to be extended both to higher and lower values. Preliminary results have also been obtained at Epx=60eV for the ejected positrons and for the ejected electrons at Ep/= 33eV.

Rotationally inelastic scattering of symmetric top molecules is examined by crossed molecular beam and velocity map imaging experiments using a newly constructed instrument. The systems studied are collisions of methyl radicals and ammonia molecules with rare gases and diatomic molecules. Resonance-enhanced multi-photon ionization detection provides velocity map images for ND3 molecules in a single n't, level, and for methyl radicals in levels corresponding to a single rotational angular momentum quantum number (n'), but averaged over a subset of the projection quantum number k'. Product level-resolved angular scattering distributions are extracted from measured images, and for CD3, CH3 and ND3 + He systems are compared with full close-coupling quantum-mechanical scattering calculations that used ab initio potential energy surfaces. These calculations were performed by collaborating groups at Radboud University Nijmegen (ND3 + He), and Johns Hopkins University and the University of Maryland (CD3 / CH3 + He). The experimental measurements provide rigorous tests of the accuracy of the potential energy surfaces and computed quantum scattering dynamics, and agreement between experiment and theory is excellent. For the experimental studies of ND3, a hexapole electrostatic lens was used for the 11 initial state-selection of ND3 molecules in their electronic and vibrational ground states in a molecular beam. For selected final quantum states, the effect of collision energy on the differential cross section for the ND3 + He system was explored in the range 230 - 720 cm-I. For methyl radical scattering experiments, this initial state selection was not possible, but colling in a supersonic expansion restricted the radicals to n = 0 and 1 rotational angular momentum levels prior to the collisions of interest. The inelastic scattering of methyl radicals and ammonia molecules with He and H2 (and D2 in the case of methyl radicals) is predominantly forward scattered for small changes in the n quantum number, but the scattering shifts to the sideways and backward directions as I'!.n increases. The comparison of experimental differential cross sections with theoretical calculations also reveals the dependence of scattering on the k' - projection quantum number and on further features of the scattering dynamics such as the impact parameter. The inelastic scattering ofND3 with Ar and methyl radical with Ar and N2 is dominated by forward scattering for all final levels probed. Common features of the scattering dynamics of these two symmetric top molecules, one closed shell and the other an open-shell radical, are identified and discussed.

No description available.

There is strong motivation to study standard model physics using the highest-energy data provided by the Large Hadron Collider. This is aided by the process of defining clusters of hadrons to form ‘jets’. Existing jet-finders are dependent on pre-defined parameters which, to some extent, influence their properties. This thesis introduces a novel algorithm which aims to reconstruct partons outgoing from hard interactions, prior to any splitting, by concentrating solely on the highest momentum transfer scale. In this way parton properties such as fragmentation and structure functions from hadron colliders may be compared directly with results from DIS and e+e− annihilation. This original, standalone tool is named ‘traps’ - the Topological Reconstruction Algorithm for Parton Scatters. The algorithm was developed using Pythia Monte Carlo QCD events, under a pragmatic approach that assumes the model provides a good approximation to reality at both hadronic and partonic level. Various tests were made to gauge the performance of the algorithm against standard jet-finders. The infrared safety and algorithm speed were also assessed. The objective of traps is to have low sensitivity to parameters, and to be fast and robust. A high event acceptance is necessary, as maximum statistics are required where cross-sections are at their lowest. A chapter of this thesis is dedicated to a description of the author’s studies in calibration and monitoring of the timing of the ATLAS Level-1 Calorimeter Trigger system. Pulses from triggered energy are sent via largely η× φ = 0.1 × 0.1 granularity ‘trigger towers’. Synchronous triggering with 1-2 ns precision is required for the system to make an accurate energy estimate.

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