ATLAS Electromagnetic Calorimeter performance measurements, search for the Higgs boson in the $H \to Z\gamma$ channel and detector development for position reconstruction of noble liquid scintillation

In this thesis, a method for the recovery of QED Final State Radiation photons emitted from muons at small (collinear, ∆R < 0.15) angles is extended to include photons emitted at larger angles (∆R ≥ 0.15) from both electrons and muons. The method is used in the search for Higgs boson decays to 4-leptons, H → ZZ∗ → 4ℓ, in ATLAS, correcting 3 out of 60 candidate events. It is also applied in the search for Higgs decays to a Z boson and a photon, H → Zγ, introducing a 2% improvement in the upper limit set by the analysis, yielding 11 × S M at mH = 125.5 GeV (95% CL). The method is also used in the measurement of the photon electromagnetic scale to provide a precision better than 0.5%, reducing the measured Higgs mass systematic uncertainty obtained from the H → γγ analysis. Data-Monte Carlo comparisons are performed to ascertain the validity of the procedure before its application to the different measurements. The collinear photon selection has an efficiency of 70% and a purity of 85%, and a collinear photon is found in 4% of Z → µ+µ−events. The noncollinear selection has an efficiency of 60% and a purity > 95%, and a photon is found in ∼ 1% of events. The second part of the thesis presents new results from a developed prototype Gaseous Photomultiplier detector based on a cascade of Thick GEM structures intended for gamma-ray position reconstruction in liquid argon. The detector has a MgF2 window, transparent to VUV light, and a CsI photocathode deposited on the first THGEM. A 10 cm2 area is instrumented with four readout channels. A gain of 8 · 105 per photoelectron and ∼100% photoelectron collection efficiency are measured at stable operation settings. A ∼100 µm position resolution at 100 kHz readout rate is demonstrated at room temperature. Structural integrity tests of the detector and seals are successfully performed at cryogenic temperatures by immersing the detector in liquid Nitrogen, laying a good foundation for future operation tests in noble liquids. This new type of device provides a low cost solution for large-area real-time gamma-ray imaging.