A measurement of the Drell-Yan differential cross section at low invariant mass is presented in the di-muon channel. A 1.64 pb−1 dataset of √s = 7 TeV proton-proton collision data collected by the ATLAS experiment at the LHC is used. The measurement is made in an invariant mass range of 26 < M < 66 GeV where M is the invariant mass of the muon pair. A review of the relevant theoretical physics and the ATLAS detector is made. The analysis is described with particular attention paid to the determination of the isolation efficiency corrections for the Monte Carlo and the estimate of the multijet background. The fiducial differential cross section is calculated with a statistical uncertainty that varies between 0.8% and 1.2%. The systematic uncertainty is seen to vary between 2.4% and 4.1%. A cross section extrapolated to the full phase space is also presented. This is dominated by theoretical uncertainties from the variation of the factorisation and renormalisation scales. The obtained fiducial differential mass cross section is compared to theoretical predictions at NLO and NNLO in perturbative QCD. It is shown that a move beyond NLO is needed to describe the distribution well due to the restrictions of using a fixed order theoretical prediction. A combination with the electron channel measurement is also briefly discussed as well as comparisons to a di-muon measurement in an extended invariant mass range. This allows similar, but stronger conclusions to be drawn. A discussion is made of a PDF fit that uses the measurement presented here. The fit demonstrates the impact of the measurement on the PDFs and further supports the conclusion that a move to NNLO in pQCD is needed to describe the data.

This dissertation documents a study of very rare B-meson decays at the LHCb experiment, using data taken during the first experiment run of the Large Hadron Collider (LHC) and during the second experiment run until September 2016. The LHCb experiment was designed to test the Standard Model of particle physics and to search for New Physics effects that go beyond the scope of the Standard Model through the decay of b hadrons produced in high energy proton-proton collisions at the LHC. The measurements described in this dissertation are made using data samples of proton-proton collisions with integrated luminosities of 1.0, 2.0 and 1.4fb⁻¹, collected at centre-of-mass energies of 7, 8 and 13 TeV, respectively. All results are consistent with the predictions of the Standard Model.

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Tese de mestrado. Engenharia Electrotécnica e de Computadores (Área de especialização de Informática e Automação). Faculdade de Engenharia. Universidade do Porto. 2006

Estágio realizado no CERN, Suiça e orientado pelo Eng.º Ronny Billen e Chris Roderick / Tese de mestrado integrado. Engenharia Informátca e Computação. Faculdade de Engenharia. Universidade do Porto. 2008

This thesis describes the research I did during my Master's study. I investigated the stau-neutralino coannihilation region of the Mirage Mediation Model at the Large Hadron Collider (LHC). By constructing five kinematic observables at the LHC, the masses of supersymmetric particles (sparticles) were determined. The Mirage Mediation Model parameters were determined from the sparticles' masses. This is the first time to establish the Mirage Mediation Model at the LHC. All these techniques can be applied to other coannihilation regions of the Mirage Mediation Model and other supersymmetry (SUSY) models.

The ATLAS Experiment is one of the four large detectors located at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. In summer 2008, ATLAS is expected to start collecting data from proton-proton collisions at 14 TeV centre-of-mass energy. In the centre of the detector, the reconstruction of charged particle tracks is performed by silicon and drift tube based sub-detectors. In order to achieve the ATLAS physics goals the resolutions of the measured track parameters should not be degraded by more than 20% due to misalignment. Thus, the relative positions of the silicon detector elements have to be known to an accuracy of about 10 micrometers in the coordinate with the best measurement precision. This requirement can be achieved by track based alignment algorithms combined with measurements from hardware based alignment techniques. A robust track based alignment method based on track residual and overlap residual optimisation has been developed and implemented into the ATLAS offline software framework. The alignment algorithm has been used to align a test beam setup and also part of the final ATLAS detector using cosmic ray muons. Several simulation studies showed that the algorithm will be able to align the full detector with collision data. In addition to detector misalignments, limitations in the knowledge of the proton structure are going to affect physics discoveries at the LHC. Therefore, parton distribution uncertainties in high-mass Drell-Yan processes have been determined. This study includes the analysis of the forward-backward asymmetry. It has been performed on the level of next-to-leading order in both, Monte Carlo simulation using k-factors and parton distribution functions. This analysis is crucial for new physics searches with the ATLAS detector.

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