Measurement of the Inclusive pp->Z/gamma*->e+e- Cross Section at sqrt(s) = 7 TeV with the ATLAS Experiment and Design Studies for a First Level Track Trigger for the ATLAS Trigger Upgrade at the future High Luminosity LHC

Abstract

This dissertation presents in the first part a measurement of the inclusive pp->Z/gamma*->e+e- production cross section with the ATLAS experiment at the Large Hadron Collider (LHC). For this, proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV collected in 2011 corresponding to an integrated luminosity of 4.6 fb^-1 are analyzed. The cross section is determined in three regions of Z/gamma* mass, 46-66 GeV, 66-116 GeV and 116-150 GeV; and in addition differentially in Z/gamma* rapidity. The mass determines the scale of the interaction, whereas the rapidity gives information about the momentum fractions of the initial protons the interacting partons carry. This makes the measurement valuable for the study of the proton structure, i.e. as input to fits of parton density functions (PDF). Backgrounds are taken from simulation with the exception of the multi-jet background which is estimated using a data-driven technique. The resulting cross sections are compared to predictions of next-to next-to-leading order QCD calculations using different PDFs. Differences are observed and only some PDFs show good agreement with the data. The presented measurement can thus be used in future PDF fits to better constrain the quark and gluon densities in the proton. The second part studies a possible design for adding track information to the first level trigger of the ATLAS detector in the scope of the proposed upgrade of the LHC, the High Luminosity LHC (HL-LHC). The planned increase in luminosity by a factor 5-10 w.r.t. the nominal LHC conditions puts strong demands on the rejection capability of the trigger. Using track information in conjunction with information from the calorimeter and muon system helps to maintain pT thresholds at the electroweak scale. A fast decision within the trigger latency can be achieved exploiting hardware based pattern matching using Content-Addressable-Memories. The number of necessary patterns and the expected number of fake tracks per event are studied with dedicated simulations. To reduce the data volume that needs to be transferred to the foreseen track finding units, techniques are developed to reject hits from low transverse momentum particles. The studies performed within this thesis indicate that a track trigger finding all tracks with pT > 10 GeV can be built. If realized it will help ATLAS to continue a versatile physics program also in the HL-LHC era.