Performance of the ATLAS forward calorimeter and search for the invisible Higgs via vector boson fusion at ATLAS

The ATLAS detector will examine proton-proton collisions at 14 TeV provided by CERN's Large Hadron Collider (LHC). ATLAS is a general purpose detector with tracking, calorime- try and a large muon system. The calorimeter system provides hermetic coverage of a large fraction of the solid angle of the detector. In the region close to the beam line, the calorimeter components are the FCal detectors which provide additional rj coverage im- proving the jet tagging efficiency and the missing energy resolution. The performance of the FCal calorimeter for both electrons and hadrons is one of the major topics of this thesis. The measured electromagnetic response for the FCal 1 module was 12.14±0.06 ADC/GeV which is in good agreement with the predicted value of 12 ADC/GeV from IE the simulation which will be used to provide the initial electromagnetic response for the FCal modules during the early stages of ATLAS data taking. The hadronic per- formance was investigated using two calibration schemes: flat weights and transverse weights. The energy resolution for hadrons using transverse weights was found to be aE/E = (72.7 ± 1.5)%/-/E&© (2.85 ± 0.28)%, here Eb is in GeV, exceeding design require- ments. In addition to good resolution, the transverse weights provided a uniform energy response, within ~ 3%, for energies greater than 40 GeV. In some extensions of the Standard Model the Higgs may predominately decay to par- ticles not visible in the detector. An upper limit on the sensitivity to observe an invisible Higgs with the ATLAS detector via the vector boson fusion production channel was inves- tigated. Two complementary studies were undertaken: a signal-to-background analysis and a shape analysis. The results from the signal-to-background analysis using 30 fb _1 of data for a 130 GeV mass Higgs show that it is possible to detect an invisible Higgs if the beyond the Standard Model cross-section is greater than ~ 72% of the Standard Model cross-section at 99% confidence level assuming a 100% Higgs branching ratio to invisi- ble particles. For the shape analysis the sensitivity is ~ 104%. The uncertainty in the jet energy scale and resolution are the dominant systematic errors in these studies.