Reconstruction and applications of QED final state radiation photons in $Z \to \mu\mu$ decays produced in 7 TeV proton-proton collisions at the Large Hadron Collider

This thesis presents a method for including the final state radiation (FSR) photon in the reconstruction of Z bosons decaying into pairs of muons (Z → µµγ ). An algorithm for the reconstruction of FSR photons has been developed. The FSR photon candidates with ET,γ >1.3 GeV are reconstructed within a ∆Rγ,µ < 0.15 cone about the axis defined by the muon momentum direction at the interaction point, using the liquid argon calorimeter of ATLAS and a special clustering efficient at low energies. Based on an integrated luminosity of 164 pb−1, the crosssection of the Z → µµγ events with at least one FSR photon within fiducial region (ET,γ >1.3 GeV and ∆Rγ,µ <0.15) is measured: σ f id Z→µµγ= 0.073±0.001(stat)±0.006(sys)±0.002(lum) nb. This is consistent with the expectation from a NNLO QCD calculation including QED FSR. Photons within the fiducial region are reconstructed with a 70% average effi- ciency which increases to 85% for ET > 4 GeV. Inclusion of these photons in the dimuon invariant mass calculation was shown to lead to improved Z-boson invariant mass resolution and scale: a Gaussian fit shows that the peak of the distribution moves from 89 GeV to 91.31±0.06 GeV with a standard deviation of 2.5±0.07 GeV. A '20% of all inclusive Z → µµ events are corrected in the tail of the invariant mass (M<80 GeV). Moreover, based on a test with simulated Higgs → ZZ(∗) → 4µ samples, more than 20% of the events are expected to find a reconstructed FSR photon around. The improvement of the Higgs invariant mass resolution and scale by adding the reconstructed FSR photons is presented.