This paper describes the ATLAS experiment as installed in i ts experimental cavern at point 1 at CERN. It also presents a brief overview of the expec ted performance of the detector.

http://absimage.aps.org/image/SES08/MWS_SES08-2008-020010.pdf

The ATLAS Experiment at the CERN Large Hadron Collider

We present a preliminary set of updated NLO parton distributions. For the first time we have a quantitative extraction of the strange quark and antiquark distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections. Additional jet data from HERA and the Tevatron improve our gluon extraction. Lepton asymmetry data and neutrino structure functions improve the flavour separation, particularly constraining the down quark valence distribution.

pdf/parton-distributions-for-the-lhc-3jcnh64voi.pdf

Parton distributions for the LHC

Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a W or a Z boson or a pair of top quarks, and of the six decay modes H → ZZ,WW , γγ, ττ, bb, and μμ. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton-proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb−1 at √ s = 7 TeV and 20 fb−1 at √ s = 8 TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09±0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the H → ττ decay of 5.4 and 5.5 standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.

Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC pp collision data at √s = 7 and 8 TeV

Introduction.- Neutrinos and Weak Interactions in the Standard Model.- Particle Oscillations.- Neutrino Oscillation.- Experiments.- Present Status.- Future Possibilities of Neutrino Oscillation Experiments.- Appendix.

Neutrino oscillations : a practical guide to basics and applications

The Standard Model of particle physics describes the phenomena in the infinitesimal with great success. However, it suffers from several inadequacies : it can not explain the neutrino masses, it has no candidate for the dark matter and it has no solution for the gauge hierarchy problem, which all require new physics and beyond the Standard Model theories. Such a theory is Supersymmetry (SUSY) and occupies a primer place in the LHC physics program. Here two proton beams are colliding at energies up to 14 TeV and gigantic detectors were built to reconstruct the collision events. For the presented studies only the data recorded with the ATLAS detector is analyzed. More details on the theoretical framework, the LHC collider and the ATLAS experiment are given in the first part of this thesis.
At hadron colliders, the lepton identification and reconstruction are crucial for precise SM cross sections and coupling measurements or for new physics searches. Therefore, the second part of my thesis is dedicated to performance studies : in-situ measurements of electron identification and reconstruction efficiencies. Both the methodology employed to obtain the results and the methods applied to estimate the background are detailed. The search for new physics is presented in the third part of the thesis. For these studies the channel with two same-sign leptons and missing transverse energy is considered. The results are obtained with $L$ = 20 fb$^{-1}$ of data recorded at $\sqrt s$ = 8 TeV and with the very first data at $\sqrt s$ = 13 TeV. At the end of Run-1 no significant excess in data over the Standard Model prediction is observed and stringent limits on the supersymmetric particle masses are set. With this analysis, the gluino mass is constrained to be above 1 TeV and the sbottom mass should be above 600\GeV when using simplified assumptions. These results provide new and very strong constraints on natural SUSY models.
With the two same-sign leptons and jets final state it is also possible to measure directly the top quark Yukawa coupling using the \ttbar\ + $H$ channel. This coupling is the largest in the Standard Model and it connects two of the heaviest particles predicted by the Standard Model, the top quark and the Higgs boson. At $\sqrt s$ = 8 TeV the results are dominated by the systematic uncertainties and only upper limits can be obtained. In the fourth part of this thesis, a complete optimization is performed to reach the highest sensitivity for this signal in the two same-sign leptons channel at $\sqrt s$ = 13 TeV with 100 fb$^{-1}$ of data, corresponding to the end of Run-2. This prospect study is based on a simplified but realistic framework for systematics and the 8\TeV Monte Carlo samples are PDF re-weighted. Two signal regions are proposed and a precision of 60$\%$ on the signal strength is reached. When combined with other measurements performed in \ttbar\ + $H$ ($\rightarrow \gamma\gamma$) and \ttbar\ + $H$ ($\rightarrow b\bar b$) channels the top Yukawa coupling can be measured with a precision below 30$\%$.

Search for new physics in events with same sign leptons and missing energy with ATLAS at LHC

The Large Hadron Collider (LHC) at CERN restarted in spring 2015 for three years (Run2) at an unexplored center-of-mass energy of 13 TeV; An ideal place to search for physics beyond the Standard Model such as supersymmetry (SUSY) and extra dimensions. A precise measurement of electron reconstruction efficiency in ATLAS, one of the two general purpose experiments of the LHC, is presented in the first part of this thesis with Z→ee data sample using 3.2 fb−1 of data recorded in 2015. This allows to extract scale factors between data and simulation that are used by all ATLAS physics analyses involving electrons. The results show the high ability of the ATLAS detector to reconstruct electrons from one hand and the good understanding of its performance on the other hand. The second part of the thesis is dedicated to a search for the Higgs boson production in association with a top quark pair (tt¯H), which could allow a first direct measurement of the top quark Yukawa coupling and could reveal new physics. The signature with two same-charge light leptons (electron or muon) without a hadronically decaying tau lepton final state, targeting the decays H→ WW⋆, is examined using the first 10% of the total expected Run2 dataset. Events with fake (non-prompt) leptons represent the main reducible background of this signature. The estimation of this background largely drives the signal sensitivity. An improved method to estimate it has been developped and is discussed in details in this thesis. Driving the total error, fake leptons background is found to be 1.5 to 3.6 times higher than in simulation and represent between 32 and 48% of the total background. The best-fit value of the ratio of observed and Standard Model cross sections of tt¯H production process, combining with other multilepton channels, is 2.5± 0.7 (stat) +1.1−0.9 (syst), and an upper limit on this ratio of 4.9 (2.3 expected) is found at 95% confidence level.

Search for Higgs boson production in association with a top quark pair in two same-sign lepton final states with the ATLAS detector at LHC

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Search for Higgs boson production in association with a top quark pair in two same-sign lepton final states with the ATLAS detector at LHC