The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for largescale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the AT-LAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 3.2 fb −1 at ffiffi ffi s p ¼ 13 TeV collected in 2015 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons. Several signal regions are considered with increasing missing-transverse-momentum requirements between E miss T > 250 GeV and E miss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with large extra spatial dimensions, pair production of weakly interacting dark-matter candidates, and the production of supersymmetric particles in several compressed scenarios.

The luminosity determination for the ATLAS detector at the LHC during pp collisions at  8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of is obtained for the of pp collision data delivered to ATLAS at  8 TeV in 2012.

A search for heavy Majorana neutrinos in events containing a pair of high-p T leptons of the same charge and high-p T jets is presented. The search uses 20.3 fb −1 of pp collision data collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of √ s = 8 TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons W R and Z . Conclusions 20The ATLAS collaboration 28 IntroductionThe discovery of mixing between generations of neutrinos [1] has established that at least two of the neutrinos have small non-zero masses. A unique feature of neutrinos compared to other fermions in the Standard Model (SM) is that neutrinos could be their own antiparticles, so-called Majorana fermions. If this is realised in nature, then the unusually low mass scale of the light neutrinos could be generated by a seesaw mechanism [2][3][4][5][6][7] which -1 -JHEP07(2015)162 Figure 1. The tree-level diagram for the production of a heavy Majorana neutrino (N ) in the mTISM model. Lepton flavour is denoted by α and β. Lepton flavour is assumed to be conserved, such that α = β. The W boson produced from the N decay is on-shell and, in this case, decays hadronically.would imply the existence of yet unobserved heavy Majorana neutrino states. The nature of Majorana neutrinos would explicitly allow for lepton number violation.In this paper, a search is presented for heavy Majorana neutrinos using the ATLAS detector at the Large Hadron Collider (LHC). The data sample was collected in 2012 during √ s = 8 TeV pp collisions and corresponds to an integrated luminosity of 20.3 fb −1 . Heavy Majorana neutrinos with masses above 50 GeV are considered. In this regime, the production and subsequent decay of heavy Majorana neutrinos could lead to a final state containing exactly two charged leptons, where the leptons may have the same or opposite charge in equal fractions of the heavy neutrino decays. Only lepton pairs of the same charge (same-sign) are considered as there is a smaller expected SM background for pairs of same-sign leptons than for pairs of leptons of opposite charge (opposite-sign). The search includes the ee and µµ final states.The search is guided by two theoretical models. In the first model, the SM is extended in the simplest way to include right-handed neutrinos [8], such that light neutrino masses are generated by a Type-I seesaw mechanism or by radiative corrections [9]. In this minimal Type-I seesaw mechanism...

The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.