Mono-X searches for simplified models of dark matter

© 2016 Dr. Amelia BrennanThe identity of dark matter remains one of the big open questions in particle physics; while much is known about its distribution throughout the Universe, very little is understood about its particle nature. In particular, a small but non-zero coupling to the Standard Model (SM) sector has not yet been ruled out. WIMP-type dark matter (DM), with weak-scale mass and couplings, may therefore be produced in proton collisions with the Large Hadron Collider (LHC), and detected by the ATLAS experiment. Several collider searches are presented, which utilise the mono-X+ MET (missing transverse energy) topology, wherein DM (the presence of which is inferred through the observation of missing transverse energy) is produced in association with some object X. The mono-jet process has the largest cross section, however mono-boson analyses, the focus of this thesis, have other advantages. The mono-Z(l+l−) channel benefits from the straightforward identification of charged leptons within the detector and removal of the multi-jet background, while the mono-W/Z(jj) channel is able to utilise the growing collection of electroweak boson identification techniques which exploit the two-prong substructure of a large-radius jet. This thesis describes two ATLAS analyses that seek to constrain both Effective Field Theory (EFT) models and simplified models of DM. The ATLAS mono-Z(ll) analysis uses 20.3 fb−1 of data produced at 8 TeV and selects events with a leptonically-decaying Z boson produced back-to-back with a large amount of MET. A cut-and-count method finds that no excess above the SM prediction is observed, and so constraints are calculated for the suppression scale Λ of the EFTs, and for the quark-DM-mediator coupling of a simplified model with a scalar mediator exchanged in the t-channel. The ATLAS mono-W/Z(jj) analysis uses the first 3.2 fb−1 of data produced at 13 TeV, and selects events with a single large-radius jet produced in association with MET. A profile likelihood fit of the SM background estimation and data is used to extract a limit on the signal strength for a vector mediator s-channel simplified model, and converted to a limit on the suppression scale Λ for a ZZχχ contact operator. A reinterpretation of Run I results from ATLAS for three common simplified models is also presented, including a comparison of the results from the mono-jet, mono-Z(l+l−) and mono-W/Z(jj) channels. Limits on the model coupling strengths are discussed. The strongest constraints are obtained with the mono-jet channel, however the leptonic mono-Z channel is able to remove the large multi-jet background to attain limits that are weaker by only a factor of a few. It is essential that the reconstruction of objects within the ATLAS detector, along with their energy measurement and calibration, is well understood and that the performance is optimised. Along with a general discussion of the relevant objects in the detector (leptons, jets and MET), the in situ measurement of corrections to the energy scale of hadronically-decaying tau leptons is described