Search for Light Dijet Resonances using Trigger Jets with the ATLAS Experiment at the Large Hadron Collider

Searches for resonances in the dijet invariant mass spectrum provide a model-independent approach to search for physics beyond the Standard Model at the Large Hadron Collider. In the sub-TeV mass range the sensitivity of dijet searches is statistically limited due to the bandwidth capacity of the data acquisition system. This limitation can be circumvented by only recording the calorimeter jets that are reconstructed within the trigger system, omitting full detector readout. This approach is applied to the analysis of proton-proton collision data recorded with the ATLAS detector at a center of mass energy of 13 TeV with an integrated luminosity of 29.7 fb^-1. The jets are subjected to a dedicated calibration procedure using calorimeter information only. The search targets dijet resonances in the mass range between 400 and 2000 GeV. Using a data-driven background estimate, no significant excesses are found

Imaging of carbon nanomembranes with helium ion microscopy

Beyer A, Vieker H, Klett R, Meyer Zu Theenhausen H, Angelova P, Gölzhäuser A. Imaging of carbon nanomembranes with helium ion microscopy. Beilstein Journal of Nanotechnology. 2015;6:1712-1720.Carbon nanomembranes (CNMs) prepared from aromatic self-assembled monolayers constitute a recently developed class of 2D materials. They are made by a combination of self-assembly, radiation-induced cross-linking and the detachment of the cross-linked SAM from its substrate. CNMs can be deposited on arbitrary substrates, including holey and perforated ones, as well as on metallic (transmission electron microscopy) grids. Therewith, freestanding membranes with a thickness of 1 nm and macroscopic lateral dimensions can be prepared. Although free-standing CNMs cannot be imaged by light microscopy, charged particle techniques can visualize them. However, CNMs are electrically insulating, which makes them sensitive to charging. We demonstrate that the helium ion microscope (HIM) is a good candidate for imaging freestanding CNMs due to its efficient charge compensation tool. Scanning with a beam of helium ions while recording the emitted secondary electrons generates the HIM images. The advantages of HIM are high resolution, high surface sensitivity and large depth of field. The effects of sample charging, imaging of multilayer CNMs as well as imaging artefacts are discussed

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