Evolution of the CMS ECAL Performance and R&D Studies for Calorimetry Options at High Luminosity LHC

During the past years the Large Hadron Collider (LHC) at CERN operated with a maximum center-of-mass energy of $\sqrt{s} = 8$~TeV, a peak luminosity of around $7\times 10^{33}$~cm$^{-2}$s$^{-1}$ and collected about $23$~fb$^{-1}$ of data which lead to the discovery of a Higgs Boson in July 2012. To further constrain the properties of the newly discovered Higgs boson, the decision to extend the LHC program has recently been made. In this framework, a major upgrade of the beam optics in the interaction region will take place around 2022 to achieve a leveled peak luminosity of $\mathcal{L} = 5\times10^{34}$~cm$^{-2}$s$^{-1}$. These will be the operating conditions during the High Luminosity LHC (HL-LHC) which is expected to deliver an integrated luminosity of 3000~fb$^{-1}$ by 2035. During HL-LHC phase the radiation levels will become much higher with respect to the nominal values for which the CMS detector was designed. Therefore it is of crucial importance to identify and quantify the effects ofradiation damage on the current detector in order to prepare an adequate upgrade of its components and maintain a good performance throughout its whole operation. In this thesis, a detailed study of the radiation damage effects on the performance of the CMS Electromagnetic Calorimeter (ECAL) is presented. A dedicated campaign of irradiation with 24 GeV protons has been performed at CERN Proton Synchrotron during 2011 and 2012. Lead tungstate (PbWO$_4$) crystals have been irradiated to different proton fluences up to $14 \times 10^{13}$~cm$^{-2}$ which is close to the integrated fluences expected at the end of HL-LHC in the ECAL endcaps (EE). Degradation of crystal properties such as light output, energy linearity and energy resolution has been studied combining laboratory measurements and test beam results, obtained with 10-150 GeV electrons provided by the CERN H4 facility. Results have been compared with a simulation model which properly reproduce the experimental data and allows to extrapolate the performance of the CMS ECAL throughout the whole HL-LHC phase. Especially in the forward region of the detector, where radiation levels are higher, the constant term of the energy resolution will increase up to $\sim 10 \%$ (at $|\eta| = 2.2$) after 3000~fb$^{-1}$ of data, foreseen for the end of HL-LHC. A similar degradation of the calorimeter resolution will not allow a proper operation. %(limiting the trigger capabilities and jet identification) Since also the Hadronic Calorimeter endcap (HE) will be strongly damaged by radiation, a general upgrade of the whole endcap calorimeter (EE+HE) will be required during the long shutdown foreseen around 2022. In this framework, a set of more radiation hard calorimeter designs and technologies which are able to operate under HL-LHC conditions are currently being investigated for the upgrade of the calorimetric system. Among the different options, several of them require the developments of radiation hard scintillators and wavelength shifters. In this work, R\&D studies on several scintillators candidates for calorimetry applications are presented. Attention is focused on inorganic crystals (LuAG, YAG, LSO, YSO) doped with rare earth ions (Ce$^{3+}$, Pr$^{3+}$) and cerium doped quartz (SiO$_2$:Ce) and barium disilicate (DSB:Ce). In addition to a full characterization of optical and scintillation properties, a dedicated study of radiation hardness was perfomed on a set of samples. Irradiation with $\gamma$ and proton sources were perfomed in different facilities leading to the identification of the most radiation tolerant samples and the best production parameters. Particular efforts were made on the study of LuAG crystals directly grown in a fiber shape by micro-pulling-down technique. The possibility to use heavy crystals in fiber geometry represents a flexible and powerful tool for many calorimeter options. Such fibers are good candidates for the replacement of the current HE plastic scintillator. In addition, doped crystal fibers can provide a radiation hard wavelength shifter to read out the signal from Shashlik-based detectors or can be used as scintillator in a SpaCal calorimeter design. Results obtained from several test beams performed at CERN and FNAL facilites are presented. Different configuration of sampling calorimeter modules made of brass and LuAG fibers have been tested with electrons and pions beams. The potential of crystal fibers for calorimetry applications has been demonstrated and further tests on larger calorimeter prototypes are foreseen for the early future.