The new laser calibration system of the ATLAS hadron calorimeter for High Luminosity LHC

The research project was developed within the framework of the upgrade of TileCal, the ATLAS central hadron calorimeter, for the HL-LHC (High Luminosity Large Hadron Collider) phase. ATLAS is a general-purpose detector at LHC, CERN, which focuses on studying the Standard Model of particle physics and searching for physics beyond the Standard Model. TileCal is a sampling calorimeter composed of alternating steel layers as absorber material and plastic scintillator tiles as active material. The light produced by the incoming particles interacting with the scintillating tiles is guided through wavelength-shifting fibers to photomultiplier tubes (PMTs). This thesis work is focused on the laser calibration system, which ensure TileCal performance stability for calibrating the response of PMTs. In particularly, it is focused on the preparation of a new optical line, whose main change with respect to the current laser system is the introduction of an adjustable continuous light component superimposed to the pulsed laser light and mixed by an integrating sphere, to better reproduce the PMT operating conditions during pp collisions and to allow for calibrating the PMT response to laser pulses at different level of average anode current. The original contribution of this thesis to the project consisted in defining a geometry for the new optical line. A sequence of measurements to assess light transmission stability and efficiency was performed by using monitors located along the optical line in two setups, in Pisa and at CERN. The results described in this thesis work were discussed with an official ATLAS review committee, which approved the implementation of the project as part of the TileCal upgrade.