Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Studies of MicroMegas Chamber for the New Small Wheel using Cosmic Muons

Micromesh Gaseous (MicroMegas) Detectors will be implemented in the ATLAS detector in the framework of the New Small Wheel Upgrade during the long shut down II in 2019/20. These detectors are used for position measurement and have a high spatial resolution of 100$\mu$m. In parallel to the ongoing constructions of the later modules, additional performance studies with small MicroMegas prototypes of a size of 10 $\times$ 10 cm$^2$ are performed. The studies include a cosmic muon test stand in combination with a scalable readout system, such that the influences of variations in the pressure of the operation gas or changes in the humidity at the lower ppm level to the detector performance are investigated. These parameters will impact the later design of detector slow control system at the New Small Wheel in ATLAS

Studies of MicroMeGas Chambers for the New Small Wheel using Cosmic Muons

Micromesh Gaseous Detectors (MicroMeGas) will be implemented in the ATLAS detector in the framework of the New Small Wheel Upgrade during the long shut down II in 2019/20. These detectors are used for position measurement and have a high spatial resolution of 100 μm. In parallel to the ongoing constructions of the later modules, additional performance studies with small MicroMeGas prototypes of a size of 10 × 10 cm$^2$ are performed. These studies include a cosmic muon test setup in combination with a scalable readout system, such that the influences of variations in the pressure of the operation gas or changes in the humidity at the lower ppm level to the detector performance are investigated. These parameters will impact the later design of the slow control system at the New Small Wheel in ATLAS

Performance Study of an aSi Flat Panel Detector for Fast Neutron Imaging of Nuclear Waste

Introduction: Radioactive waste must be characterized to check its conformance for intermediate storage and final disposal according to national regulations. For the determination of radiotoxic and chemotoxic contents of radioactive waste packages non-destructive analytical techniques are preferentially used. Fast neutron imaging is a promising technique to assay large and dense items providing, in complementarity to photon imaging, additional information on the presence of structures in radioactive waste packages. Therefore the feasibility of a compact Neutron Imaging System for Radioactive waste Analysis (NISRA) using 14 MeV neutrons is studied in a cooperation framework of Forschungszentrum Jülich GmbH, RWTH Aachen University and Siemens AG. However due to the low neutron emission of neutron generators in comparison to research reactors the challenging task resides in the development of an imaging detector with a high efficiency, a low sensitivity to gamma radiation and a resolution sufficient for the purpose.Setup: The setup is composed of a commercial D-T neutron generator (Genie16GT, Sodern) with a surrounding shielding made of polyethylene, which acts as a collimator and an amorphous silicon flat panel detector (aSi, 40 x 40 cm², XRD-1642, PerkinElmer). Neutron detection is achieved using a general propose plastic scintillator (EJ-260, Eljen Technology) linked to the detector. The thermal noise of the photodiodes is reduced by employing an entrance window made of aluminium. Optimal gain and integration time for data acquisition are set by measuring the response of the detector to the radiation of a 500 MBq 241Am-source.Performance Tests and Results: Detector performance was studied by recording neutron radiography images of materials with various, but well known, chemical compositions, densities and dimensions (Al, C, Fe, Pb, W, concrete, polyethylene, 5 x 8 x 10 cm3). To simulate gamma-ray emitting waste radiographies in presence of a gamma-ray sources (60Co, 137Cs, 241Am) were performed. A homemade algorithm was developed to determine a value which is related to the neutron absorption of the sample with the analysis of the raw detector data. As an example Figure 1 a) shows the radiography of a polyethylene cylinder (d=10.5 cm, h=14.5 cm) and a lead brick (5 x 8 x 10 cm3). The detector was placed 42 cm away from the neutron source. Distance between detector and the samples was 0.5 cm. At the sample position the fast neutron flux was estimated to 9*10^3 n cm-2 s-1 for a neutron emission of 10^8 n s-1. The acquisition time was 15 minutes. Figure 1 b) shows the average profile from the detector data (ADU=analog-to-digital units) of the image shown in Figure 1 a) for predefined zones with and without the samples.First neutron radiographies were successfully recorded despite the low detector efficiency and low neutron emission. Analysis of the data shows a correlation between the measured signal and determined neutron absorption as shown in Figure 2. Thus discrimination between different materials of same thicknesses may be achieved. The measurements and results will be presented and discussed in details

Fast Neutron Imaging with an aSi Detector for Nuclear Waste Assay

Introduction: Radioactive waste has to undergo a process of quality checking in order to check its conformance with national regulations prior to its transport, intermediate storage and final disposal. Within the quality checking of radioactive waste packages non-destructive assays are required to characterize their radiotoxic and chemotoxic contents. In a cooperation framework Forschungszentrum Jülich GmbH, RWTH Aachen University and Siemens AG are studying the feasibility of a compact Neutron Imaging System for Radioactive waste Analysis (NISRA) using 14 MeV neutrons. Fast neutron imaging is a promising technique to assay large and dense items providing in complementarity to photon imaging additional information on the presence of structures in radioactive waste packages. However due to the low neutron emission of neutron generators the challenging task resides in the development of an imaging detector with a high efficiency, a low sensitivity to gamma radiation and a resolution sufficient for the purpose.Setup: The setup of the experiment is shown in Figure 1. The 14 MeV neutrons are produced by a D-T neutron generator. Neutron detection is achieved using a 40 x 40 cm² amorphous silicon (aSi) flat panel detector linked to a plastic scintillator. The detector thermal noise was reduced by employing an entrance windows made of aluminium. The optimal gain and integration time were obtained by measuring the response of the detector to the radiation of a 241Am source.Performance Tests: First test measurements were carried out with different materials made of Al, C, Fe, Pb, W, concrete and polyethylene (50 x 80 x 100 mm³). Each sample was irradiated with PE as a reference. Data analysis was performed with a homemade algorithm which allows determining a value related to the neutron absorption.Results: First neutron radiographies were successfully recorded despite the low detector efficiency and low neutron intensity. In addition the correlation between the absorption and measured signal attenuation was determined and is shown in figure 2. The measurements and results will be presented and discussed

Searches for the ZγZ\gamma decay mode of the Higgs boson and for new high-mass resonances in pppp collisions at s=13\sqrt{s} = 13 TeV with the ATLAS detector

International audienceThis article presents searches for the Zγ decay of the Higgs boson and for narrow high-mass resonances decaying to Zγ, exploiting Z boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb$^{−1}$ of pp collisions at $\sqrt{s}=13$ recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected — assuming Standard Model pp → H → Zγ production and decay) upper limit on the production cross section times the branching ratio for pp → H → Zγ is 6.6. (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level

Search for direct top squark pair production in final states with two leptons in s=13\sqrt{s} = 13 TeV pppp collisions with the ATLAS detector

International audienceThe results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using $36.1~\hbox {fb}^{-1}$ of integrated luminosity from proton–proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark $\tilde{t}$ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay $\tilde{t} \rightarrow b \tilde{\chi }_{1}^{\pm }$ into a b-quark and the lightest chargino with $\tilde{\chi }_{1}^{\pm } \rightarrow W \tilde{\chi }_{1}^{0}$ , the decay $\tilde{t} \rightarrow t \tilde{\chi }_{1}^{0}$ into an on-shell top quark and the lightest neutralino, the three-body decay $\tilde{t} \rightarrow b W \tilde{\chi }_{1}^{0}$ and the four-body decay $\tilde{t} \rightarrow b \ell \nu \tilde{\chi }_{1}^{0}$ . No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the $\tilde{t}$ and $\tilde{\chi }_{1}^{0}$ masses. The results exclude at 95% confidence level $\tilde{t}$ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches