Master thesis defense by Peter Winkel Rasmussen – University of Copenhagen

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Master thesis defense by Peter Winkel Rasmussen

Title:  Elastic pp Scattering at ALFA - with √s = 13 TeV and ß*=2.5 km

In this thesis elastically scattered protons measured with the ALFA detector, a subdetector of ATLAS, is analyzed. The measured protons were collided at √s = 13 TeV at the LHC during a special run with ß*=2.5 km. A total integrated luminosity of 50.81 1/µb is used in the analysis. Improvements in the collimator settings and a very small beam emittance allowed ALFA to obtain an almost background free sample. The special optics and improvements in the data taking should allow ALFA to measure total proton cross section, σ_tot, using the optical theorem and the ρ parameter, which is the ratio between the real and imaginary scattering amplitude. ALFA might even be sensitive to the coulomb region which allows for a precise measurement of the integrated luminosity.
Elastic candidates were selected, background was estimated, experimental effects and detector efficiencies were quantified in the analysis. All of these components are needed to determine σ_tot and ρ.
However, unexpected inefficiencies in the detectors complicated the determination of detector efficiencies. Solutions to these problems were suggested but they are not yet implemented in the analysis.
The inefficiencies along with preliminary values for the integrated luminosity and positioning of the ALFA detector means that the physics result is incomplete at this point. The final results are:
σ_tot = 104.64 +/- 1.77 mb, ρ = 0.036 +/- 0.013