Master thesis defense by Jack Christopher Hutchinson Rolph – University of Copenhagen

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Master thesis defense by Jack Christopher Hutchinson Rolph

Title: Measurement of the total p-p cross section using the ALFA detector at √s = 13 TeV


The total proton-proton cross-section was measured using data from the ALFA (Absolute Luminosity for ATLAS) detector using special high β∗ optics and the optical theorem. This was achieved by measuring the differential elastic cross-section as a function of the momentum- transfer Mandelstam variable t and fitting to a Monte Carlo simulation including experimental effects by minimization of the χ2 goodness-of-fit value with respect to parameters of interest. The preliminary total cross-section was measured to be:

σT = 105.716 ± 0.129(stat.) ± 1.295(syst.) mb 

The overall fit to data was poor. The measured cross-section was found to vary significantly depending on the time period chosen from data. The primary contribution to the poor χ2 goodness-of-fit test was found to be in Arm 2 for the period 8 < l < 107 over the entire fitting range. Furthermore, the background contribution exhibited anomalous peaks at approximately ±5 mm in LHC x co-ordinates. This effect was also observed in unpaired bunches. No clear candidates for the causes of these effects were found.