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.

The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via 204 Pb(48 Ca, xn) 252−x No reactions. Isotopically pure (>99.7%) 204 Pb targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t 1/2 = 3.7 +1.1 −0.8 and 43 +22 −15 µs) were deduced from a total of 158 fission events. Both decays originate from 250 No rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.

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Higher-order anisotropic flows in heavy-ion collisions are affected by nonlinear mode coupling effects. It has been suggested that the associated nonlinear hydrodynamic response coefficients probe the transport properties and are largely insensitive to the spectrum of initial density fluctuations of the medium created in these collisions. To test this suggestion, we explore nonlinear mode coupling effects in event-by-event viscous fluid dynamics, using two different models for the fluctuating initial density profiles, and compare the nonlinear coupling coefficients between the initial eccentricity vectors before hydrodynamic expansion and the final flow vectors after the expansion. For several mode coupling coefficients we find significant sensitivity to the initial fluctuation spectrum. They all exhibit strong sensitivity to the specific shear viscosity at freeze-out, but only weak dependence on the shear viscosity during hydrodynamic evolution.

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in collisions at = 5.02 and 13 TeV and in  + Pb collisions at = 5.02 TeV, and compared to the results obtained for low-multiplicity collisions at = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in  + Pb and low-multiplicity collisions. On the other hand, the results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in , smaller in  + Pb and smallest in collisions. The results show no dependence on the collision energy, nor on the multiplicity.

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The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vectorboson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and lowproduction cross sections. Results are based on 20 fb −1 of proton-proton collision data at ffiffi ffi s p ¼ 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

Isomers have been populated in 246 Cm and 252 No with quantum numbers K π = 8 − , which decay through K π = 2 − rotational bands built on octupole vibrational states. For N = 150 isotones with (even) atomic number Z = 94-102, the K π = 8 − and 2 − states have remarkably stable energies, indicating neutron excitations. An exception is a singular minimum in the 2 − energy at Z = 98, due to the additional role of proton configurations. The nearly constant energies, in isotones spanning an 18% increase in Coulomb energy near the Coulomb limit, provide a test for theory. The two-quasiparticle K π = 8 − energies are described with single-particle energies given by the Woods-Saxon potential and the K π = 2 − vibrational energies by quasiparticle random-phase approximation calculations. Ramifications for self-consistent mean-field theory are discussed.