Z $\rightarrow$ $\tau\tau$ Cross Section Measurement and Liquid-Argon Calorimeter Performance at High Rates at the ATLAS Experiment

In this study, a measurement of the production cross section of Standard Model $Z$ bosons in proton-proton collisions in the decay channel $Z\rightarrow\tau\tau$ is performed with data of 1.34$\,\mathrm{fb^{-1}}$ - 1.55$\,\mathrm{fb^{-1}}$ recorded by the ATLAS experiment at the LHC at a center-of-mass energy of $\sqrt{s}=7\,\mathrm{TeV}$. An event selection of the data is applied in order to obtain a sample enriched with $Z\rightarrow\tau\tau$ events. After background estimations using data and Monte Carlo (MC) simulations, the fiducial cross sections in the sub-channels $Z\rightarrow\tau\tau\rightarrow e\tau_h + 3\nu$ and $Z\rightarrow\tau\tau\rightarrow \mu\tau_h + 3\nu$ are measured. Together with the geometrical and kinematical acceptance, $A_Z$, and the well known tau lepton branching fractions, these results are combined to a total inclusive $Z\rightarrow\tau\tau$ cross section. $A_Z$ is obtained from MC studies only, and the combination of the channels is done including statistical and systematical uncertainties using the BLUE method. The result is a measured total inclusive cross section of $\sigma^{inc}_{tot}(Z\rightarrow\tau\tau) = { 914.4 \pm 14.6\textrm{(stat)} \pm 95.1\textrm{(syst)} \pm 33.8\textrm{(lumi)} \textrm{ pb}}$. This is in agreement with theoretical predictions from NNLO calculations of $\sigma^{theory}(Z\rightarrow\tau\tau)=964\pm48\,\mathrm{pb}$ and also with measurements previously performed by the ATLAS and CMS experiments. With the increased amount of data, the statistical uncertainty could be reduced significantly compared to previous measurements. Furthermore, a testbeam analysis is performed to study the operation of the electromagnetic and hadronic endcap calorimeters, EMEC and HEC, and of the forward calorimeter, FCal, in the high particle fluxes expected for the upgraded LHC. The high voltage return currents of the EMEC module are analysed in dependence of the beam intensity. The results are compared to model predictions and simulations to extract the point of critical operation. Overall, the results for the critical beam intensities and the critical high voltage currents are in agreement with the predictions, but the assigned uncertainties are rather large. The general behaviour of the high voltage current in dependence of the beam intensity above the critical intensity could be confirmed very well. The testbeam data show that the EMEC can be operated up to highest LHC luminosities, and that ATLAS conserves its excellent calorimeter performance in this detector area.