Development of a Sampling Calorimeter for the LHCb Upgrade

Since 2008 the Large Hadron Collider (LHC) at CERN, Geneva, has offered the ideal testing ground to probe the scientific knowledge of the fundamental interactions. There, the LHCb experiment performs precision measurements of CP violation and rare decays of B hadrons. LHCb will be upgraded to run at a luminosity of $1.5\times10^{34} \ \text{cm}^{−2} \ \text{s}^{−1}$ . This will require a substantial modification of its current electromagnetic calorimeter due to high radiation doses in the central region and increased particle densities. This PhD thesis presents an innovative technological solution based on spaghetti calorimeters (SPACAL). The candidate scintillators include both radiation-hard inorganic crystal garnets, for the region with the highest radiation dose, and organic dyes in polystyrene or polysiloxane hosts. The R&D on scintillating materials individuated garnets radiation-hard up to 1 MGy with timing capabilities close to LYSO:Ce. Their composition was then tuned to reduce their decay time at the level of plastic scintillators, keeping competitive timing. Samples of polysiloxane scintillators were tested with promising results. Prototypes of SPACAL with lead or tungsten absorbers were produced and tested at DESY and CERN. The energy resolutions showed sampling and constant contributions of ∼ 10%/ $\sqrt{E} \oplus$ 1% in line with the current LHCb modules. The spatial resolutions reached below 1 mm, and time resolutions at the level of 15 ps at high energies. Moreover, time resolution was studied with several PMTs coupled to the scintillators in direct contact or via light guides, with or without optical glues. A Monte Carlo simulation framework was developed, validated with testbeam results, and used to optimise the prototypes. It relies on a hybrid approach to describe the transport of the optical photons, retaining the precision of ray tracing but reducing by orders of magnitude the computation time. The framework was employed to study the deterioration caused by the LHCb background to the time resolution of a SPACAL with tungsten absorber and inorganic crystals, giving further input to the scintillators R&D.