Abstract: A summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb−1 of proton-proton collision data at centre-of-mass energies of s√=7 and 8 TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented

Abstract: The start of the Large Hadron Collider provides an unprecedent opportunity for the exploration of physics at the TeV scale. It is expected to perform precise tests of the structure of the Standard Model and to hint at the structure of the physical laws at a more fundamental level. The first part of this work describes a tune of the initial- and final- state radiation parameters in the Pythia8 Monte Carlo generator, using ATLAS measurements of ttbar production at 7 TeV. The results are compared to previous tunes to the Z boson transverse momentum at the LHC, and to the LEP event shapes in Z boson hadronic decays, testing of the universality of the parton shower model. The tune of Pythia8 to the ttbar measurements is applied to the next-to-leading or- der generators MadGraph5_aMC@NLO and Powheg, and additional parameters of these generators are tuned to the ttbar data. For the first time in the context of Monte Carlo tuning, the correlation of the experimental uncertainties has been used to constrain the parameters of the Monte Carlo models. In the second section we report results of a model independent search for new phenomena with data recorded by the ATLAS detector at 8 TeV. Event topologies involving isolated electrons, photons, muons, jets, b-jets and missing transverse momentum are investigated. The events are subdivided according to their final states into 697 exclusive analysis channels. For each channel, a search algorithm tests the compatibility of the effective mass distribution in data against the distribution in the Monte Carlo simulated background. No significant deviations between data and the Standard Model expectations have been observed