Search for Heavy Resonances in Four-Top-Quark Final States in $pp$ Collisions at $\sqrt{s}$ = 13 TeV With the ATLAS Detector

A search for heavy resonances with a mass of 1 TeV or more in four-top-quark events is presented. This search uses proton-proton-collision data collected with the ATLAS detector at the Large Hadron Collider (LHC) during Run 2 in the years 2015-2018 at a center-of-mass energy of $\sqrt{s}$ = 13 TeV. The rare four-top-quark final state can be investigated for the first time with this data set corresponding to an integrated luminosity of 139 fb$^{-1}$. It offers a unique way to probe top-philic resonances $Z^{\prime}$ which would predominantly be produced in association with top quarks $t\bar{t} Z^{\prime} \rightarrow t\bar{t} t\bar{t}$ and are inaccessible by conventional searches. Events with exactly one reconstructed lepton are selected in this search. The particular configuration where the resonance decays fully hadronically is investigated by explicitly reconstructing the resonance. The mass spectrum $m_{\mathrm{JJ}}$ is obtained from two reconstructed jets collecting the decay products of the two top quarks from the resonance and is investigated in the range between 1 TeV and 3.2 TeV. In a model-independent interpretation, the data is compared to the expected $m_{\mathrm{JJ}}$ distribution. No excess is observed. In a model-dependent interpretation, upper limits on the cross section are computed for six resonance masses between 1 TeV and 3 TeV assuming a simplified model with fixed coupling of the top quark to the resonance. The observed (expected) limits range between $59~(52)$ fb for a resonance mass of 1 TeV and $11~(10)$ fb for a resonance mass of 3 TeV. Furthermore, upper limits are presented for different values of the coupling between top quarks and the resonance. The reach of the LHC is extended to open up more possibilities for discoveries by increasing the precision of measurements and investigating even rarer processes. The LHC and its detectors are scheduled for an extensive upgrade, enabling them to collect a total integrated luminosity of 3 ab$^{-1}$. A major upgrade of ATLAS is the replacement of its inner detector. The new Inner Tracker (ITk) consists of a pixel silicon detector at small radius and a silicon strip detector at larger radius. The service trays are components of the mechanical structure of the ITk strip detector endcaps in the forward regions. They hold the cooling services and guide the electrical services to the detector components. The development of the final design of the service trays is presented. The quality assurance tests of the service trays are discussed and confirm that the produced service trays meet the required specifications. All service trays for the endcaps are built and ready for assembly with the other structural components.