The efficient identification of photons is a crucial aspect in the search for the Higgs boson at ATLAS. With the high luminosity and collision energies provided by the Large Hadron Collider, rejection of backgrounds to photons is of key importance. It is often not feasible to fully simulate background processes that require large numbers of events, due to processing time and disk space constraints. The standard fast simulation program, ATLFAST-I, is able to simulate events ∼1000 times faster than the full simulation program but does not always provide enough detailed information to make accurate background estimates. To bridge the gap, a set of photon reconstruction efficiency parameterisations, for converted and unconverted photons, have been derived from full simulation events and subsequently applied to ATLFAST-I photons. Photon reconstruction efficiencies for isolated photons from fully simulated and ATLFAST-I, plus parameterisations, events are seen to agree within statistical error. A study into a newly proposed Two Higgs Doublet Model channel, gg → H → hh → γγγγ, where the light Higgs (h) boson is fermiophobic, has been investigated. The channel is of particular interest as it exploits the large production cross-section of a heavy Higgs (H) boson via gluon-fusion at the LHC in conjunction with the enhanced branching ratio of a light fermiophobic Higgs (h) boson to a pair of photons. This channel is characterised by a distinct signature of four high pT photons in the final state. Samples of signal events have been generated across the (mh,mH) parameter space along with the dominant backgrounds. An event selection has been developed with the search performed at generator-level. In addition, the search was also performed with simulated ATLFAST-I events utilising the above photon reconstruction efficiency parameterisations. For both analyses, the expected upper limit on the cross-section at 95% confidence level is determined and exclusion regions of the (mh,mH) parameter space are defined for integrated luminosities of 1 f b−1 and 10 f b−1 in seven fermiophobic model benchmarks.

The search potential of a Standard Model Higgs boson in the Vector Boson Fusion production mechanism with Higgs boson decaying to two leptons and two neutrinos via decay to two Z bosons with the ATLAS detector is investigated. The ATLAS detector is a general purpose detector in operation at CERN measuring proton-proton collisions produced by the Large Hadron Collider. This channel has been shown to have high sensitivity at large Higgs mass, where large amounts of missing energy in the signal provide good discrimination over expected backgrounds. This work takes a first look at whether the sensitivity of this channel may be improved using the remnants of the vector boson fusion process to pro- vide extra discrimination, particularly at lower mass where sensitivity of the main analysis is reduced because of lower missing energy. Simulated data samples at centre of mass energy 7 Te V are used to derive signal significances over the mass range between 200-600 Ge V / c2. Because of varying signal properties with mass, a low and a high mass event selection were developed and optimized. A comparison between simulated and real data (collected in 2010) is made of variables used in the analysis and the effect of pileup levels corresponding to those in the 2010 data is investigated. Possible methods to estimate some of the main backgrounds to this search are described and discussed. The impact • of important theoretical and detector related systematics are taken into account. Final results are presented in the form of 95 % Confidence Level exclusion limits on the signal cross section relative to the SM prediction as a function of Higgs boson mass, based on an integrated luminosity of 33.4 pb -1 of data collected during 2010.

The Grid has been recognised as the next-generation distributed computing paradigm by seamlessly integrating heterogeneous resources across administrative domains as a single virtual system. There are an increasing number of scientific and business projects that employ Grid computing technologies for large-scale resource sharing and collaborations. Early adoptions of Grid computing technologies have custom middleware implemented to bridge gaps between heterogeneous computing backbones. These custom solutions form the basis to the emerging Open Grid Service Architecture (OGSA), which aims at addressing common concerns of Grid systems by defining a set of interoperable and reusable Grid services. One of common concerns as defined in OGSA is the Grid accounting service. The main objective of the Grid accounting service is to ensure resources to be shared within a Grid environment in an accountable manner by metering and logging accurate resource usage information. This thesis discusses the origins and fundamentals of Grid computing and accounting service in the context of OGSA profile. A prototype was developed and evaluated based on OGSA accounting-related standards enabling sharing accounting data in a multi-Grid environment, the World-wide Large Hadron Collider Grid (WLCG). Based on this prototype and lessons learned, a generic middleware solution was also implemented as a toolkit that eases migration of existing accounting system to be standard compatible.

LHCb is an experiment designed to make precision measurements of Charge- Parity violation in the B meson system. We report a measurement of the B± crosssection and production asymmetry, using B± → J/u K± decays collected at the LHCb detector in 2010 and 2011. Using 27.6 pb-1 of pp collisions at a centre-of-mass energy 7 TeV, we obtain a B± cross-section of [41.6 ± 0.6 (stat.) ± 3.0 (sys.) ± 4.2 (lumi.)] μb in the rapidity region 2 to 4.5. Using 371.1 pb-1 of pp collisions at a centre-of-mass energy 7 TeV, we obtain a B± production asymmetry of [-2.09 ± 1.20 ± 0.8 (CP) ]% in the same rapidity region. The Ring Imaging Cherenkov system of LHCb uses Hybrid photon detectors (HPDs) for single photon detection. This thesis summarises the use of ion feedback measurements as indicators of HPD vacuum quality.

The Large Hadron Collider started operation this year, 2008. LHCb is a precision heavy-flavour experiment at this collider. The precision of LHCb is greatly aided by the LHCb Ring Imaging Cherenkov system for the separation and identification of charged hadrons. This system uses pixel Hybrid Photon Detectors, an innovative new technology for single photon imaging. The simulation and testing of these photon detectors are reported and discussed. The photodetectors were measured to have reached or exceeded the specifications in key areas. In particular, the detector quantum efficiencies far exceed expectations, by a relative 27 %. The precision of LHCb will be used to examine CP-violation and rare decays of B-mesons. A key part of the physics programme will be a measurement of the CP-violating flavour specific asymmetry in neutral B-meson mixing. This asymmetry is expected to be very small in the Standard Model, of order 10-4, however it is very sensitive to new physics, which can increase the asymmetry dramatically. We present an improved event selection and a novel method to control systematics. This will enable us to make a world-leading measurement of this parameter in one nominal year of data taking (2 fb-1).

Searches for the supersymmetric partner of the top quark (stop) are motivated by natural supersymmetry, where the stop has to be light to cancel the large radiative corrections to the Higgs boson mass. This thesis presents three different searches for the stop at √s = 8 TeV and √s = 13 TeV using data from the ATLAS experiment at CERN’s Large Hadron Collider. The thesis also includes a study of the primary vertex reconstruction performance in data and simulation at √s = 7 TeV using tt and Z events. All stop searches presented are carried out in final states with a single lepton, four or more jets and large missing transverse energy. A search for direct stop pair production is conducted with 20.3 fb−1 of data at a center-of-mass energy of √s = 8 TeV. Several stop decay scenarios are considered, including those to a top quark and the lightest neutralino and to a bottom quark and the lightest chargino. The sensitivity of the analysis is also studied in the context of various phenomenological MSSM models in which more complex decay scenarios can be present. Two different analyses are carried out at √s = 13 TeV. The first one is a search for both gluino-mediated and direct stop pair production with 3.2 fb−1 of data while the second one is a search for direct stop pair production with 13.2 fb−1 of data in the decay scenario to a bottom quark and the lightest chargino. The results of the analyses show no significant excess over the Standard Model predictions in the observed data. Consequently, exclusion limits are set at 95% CL on the masses of the stop and the lightest neutralino.

Doctor of Philosophy / Department of Physics / Yurii Y. Maravin / This thesis presents the first study of a rare production of Z boson in association with two photons (Zγγ), where the Z boson decays into a pair of muons or electrons, by proton-proton collisions at the Large Hadron Collider (LHC). This study uses full data samples that have been collected with the Compact Muon Solenoid (CMS) detector in 2012 with a center of mass energy of 8TeV, corresponding to an integrated luminosity of 19.7 fb⁻¹. The Zγγ production cross section is measured within a fiducial region defined by two leptons with two photons where transverse momentum over 15 GeV and distance between gamma and lepton above 0.4. Using the obtained samples, the Zγγ cross section is measured to be: 12.6 ±1.6 (stat.) ± 1.7 (syst.) ± 0.3 (lumi.) fb. where stat., syst., and lumi. denote the statistical uncertainty, systematic uncertainty, and the uncertainty in integrated luminosity, respectively. This result is in an excellent agreement with the theoretical prediction of 13.0 ± 1.5 fb.

Many interesting physical processes produce non-interacting particles that could only be measured using the missing transverse momentum. The increase of the proton beam intensity in the Large Hadron Collider (LHC) provides sensitivity to rare physics processes while inevitably increasing the number of simultaneous proton collisions in each event. The missing transverse momentum (MET) is a variable of great interest, defined as the negative sum of the transverse momentum of all visible particles. The precision of the MET determination deteriorates as the complexity of the recorded data escalates. Given the current complexity of data analysis, a new algorithm is developed to effectively determine the MET. Several well-understood physics processes were used to test the effectiveness of the newly designed algorithm. The performance of the new algorithm is also compared to that of the standard algorithm used in the ATLAS experiment. / Graduate

A dissertation submitted in fulfillment of the requirements for the degree of Master of Physics in the School of Physics November 1, 2017. / The ATLAS detector at the Large Hadron Collider (LHC) at CERN is undergoing upgrades to its instrumentation, as well as the hardware and software that comprise its Trigger and Data Acquisition (TDAQ) system. The increased energy will yield larger cross sections for interesting physics processes, but will also lead to increased artifacts in on-line reconstruction in the trigger, as well as increased trigger rates, beyond the current system’s capabilities. To meet these demands it is likely that the massive parallelism of General-Purpose Programming with Graphic Processing Units (GPGPU) will be utilised. This dissertation addresses the problem of integrating GPGPU into the existing Trigger and TDAQ platforms; detailing and analysing GPGPU performance in the context of performing in a high-throughput, on-line environment like ATLAS. Preliminary tests show low to moderate speed-up with GPU relative to CPU, indicating that to achieve a more significant performance increase it may be necessary to alter the current platform beyond pairing suitable GPUs to CPUs in an optimum ratio. Possible solutions are proposed and recommendations for future work are given. / LG2018

Following the announcement of the discovery of a new particle on the 4th of July 2012 at the ATLAS and CMS experiments at the LHC, many efforts were needed for the understanding of its properties and to discern whether it is the Standard Model Higgs boson. The research presented in this thesis is based on the H → ZZ(*) → 4l decay channel. Three main contributions are discussed: the Standard Model Higgs boson mass measurement, the search for a heavy Higgs boson, and lastly, the implementation of a kinematic likelihood fitter as a new approach to improve the invariant mass resolution of the final states. The Standard Model Higgs boson mass measurement is presented. The measured mass is 124:51± 0:52(stat)± 0:06(syst) GeV for the combined data taken during 2011 and 2012 (4:6 fb-¹ at 7 TeV and 20:7 fb-¹ at 8 TeV). Contributing to the mass measurement, a tool was developed to validate the model used by generating several pseudo datasets from Monte Carlo samples and fitting them with the profile likelihood. The results show that the model is correct and only small deviations are seen in the parameters of interest, mH, and the signal strength, μ. Studies in the asymptotic limit show that these deviations are a symptom of low statistics in some of the final states. The search for a heavy Higgs boson is presented as well. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the profile likelihood gives 95% confidence level upper limits on the production cross-section of a heavy Higgs times the branching ratio to Z boson pairs in the mass range from 140 GeV to 1 TeV. Contributing to this search, a pseudo dataset, called Asimov dataset, is created from the Monte Carlo samples to test the profile likelihood fits and validate the model used. The results show that fit the model is correct. In addition, the limits are also interpreted in the context of Type I and Type II Two Higgs Doublet Models (2HDM). Finally, a Kinematic Likelihood Fitter (KLFitter) is studied and used to constrain the Z boson mass as an alternative to the standard tool used for the 2011 and 2012 measurement. This affects the distribution of the invariant mass, m4l, from which the Higgs boson mass is inferred. Small improvements are seen in the invariant mass resolution when higher hypothetical Higgs boson masses are considered.