Calibration of the ATLAS central hadronic Tile Calorimeter in LHC Run-2 and beyond

The Tile Calorimeter is a sampling hadronic calorimeter covering the central region of the ATLAS experiment, with steel as the absorber and plastic scintillators as an active medium. The scintillators are read out by wavelength shifting fibres to 9852 photomultiplier tubes. The analogue signals from the photomultiplier tubes are amplified, shaped, and digitised on the detector every 25 ns. The data are then read out to the off-detector systems for further processing. The Tile Calorimeter employs several calibration systems that, together with the collected collision data, provide the basis for response equalisation and monitoring at each stage of the readout path, from scintillation light production to energy and time reconstruction. A Laser system is used to monitor the stability and to perform a calibration of the Tile Calorimeter, from the photomultipliers to the read-out electronics. This dissertation provides a detailed description of the calculation of the energy calibration factors from Laser data. These calibration factors, produced weekly, have been applied to calorimeter channels in order to reduce the channel-to-channel spread and to recover the global calorimeter response disturbed by the photomultiplier tube drifts during the LHC Run-2. The observed PMT response evolution as a function of time over the entire Run-2 period is also shown. The response of the PMTs depends on their past operation, the amount of light collected, and the current delivered in hours, days, and weeks prior to its current use. The TileCal PMTs were already operating for about 14 years and are expected to function for about that long more. This dissertation discusses quantitative models to quantify the evolution of the PMT response as a function of luminosity during LHC Run-2 and to forecast how this response will change over time during Run-3 and Run-4. For the Long Shutdown III, it has been decided to replace about 1000 PMTs with new ones with better properties. In this thesis, a brief overview of the test bench built at CERN is given. Using this test bench, the responses of a set of PMTs will be studied in controlled conditions.