Measurement of the charged particle multiplicities in central diffraction with forward proton measured in ALFA detector in the ATLAS experiment

This dissertation presents a study of inclusive charged particle production in proton proton central diffraction using data collected during the 2015 run of the ATLAS experiment at the Large Hadron Collider, with proton tagging provided by the ALFA detector. The analysis focuses on the characterisation of the signal and on an attempt to estimate the background contributions affecting the measurement.Monte Carlo simulations were performed using the Pythia 8.3 generator at the particle level, employing a fast modelling of the Inner Detector and calorimeter response together with a simplified treatment of detector resolution.The background studies are based on the correlation between the proton fractional momentum loss determined from proton kinematics and its calorimetric estimate, evaluated as the difference between the two quantities. An important element of the analysis is the use of the Minimum Bias Trigger Scintillators, which are employed at the trigger level to detect inelastic proton proton interactions. The MBTS veto is defined as the absence of activity in both forward inner MBTS rings, which together cover the pseudorapidity range from 2.76 to 3.86 in absolute value. The application of this veto enhances the selection of events with large rapidity gaps, characteristic of diffractive processes and, in particular, of central diffraction at relatively low diffractive masses.Several data samples corresponding to different trigger configurations and signal selections were analysed. The ALFA sample is based solely on the requirement of a proton reconstructed on both sides of the ATLAS detector. The Central Exclusive Production sample includes an additional MBTS veto, providing a dataset enriched in low diffractive masses. The inclusive Central Diffraction ALFA TE3 sample is selected using a calorimetric trigger with a total transverse energy threshold of 3 GeV in the central region, resulting in the selection of higher diffractive masses. The ALFA and ALFA TE3 samples are further subdivided according to the application of the MBTS veto.Multiple background scenarios were investigated, including single and double accidental proton coincidences, as well as combinations of non diffractive and diffractive processes. Signal to background fits were performed, allowing for the extraction of signal fractions and the study of the proton fractional momentum loss, the product of its calorimetric estimates from both sides, and charged particle multiplicity distributions. The signal was found to populate predominantly the region where the difference between the fractional momentum loss calculated from proton kinematics and that inferred from calorimetric information is greater than minus 0.02, with an enhanced concentration observed when the MBTS veto is applied. While a promising description was obtained for the ALFA and Central Exclusive Production samples, no satisfactory fit was achieved for the ALFA TE3 sample. The remaining discrepancies highlight limitations of the background modelling and motivate further studies based on data driven techniques.