The Endcap Timing Layer of the CMS experiment: detector development and impact on physics analyses

The CMS experiment will be upgraded with a MIP Timing Detector (MTD) for the high-luminosity phase of the Large Hadron Collider (HL-LHC). The MTD will allow measuring the time of passage of charged particles with a resolution of 30-40~ps, disentangling in this way particles coming from interesting events from those originated in uncorrelated, but overlapping, scattering processes. Thanks to the MTD, the CMS detector will maintain its excellent performance while operating in an environment with an integrated luminosity a factor 6-7 higher than the present one. This work focuses in particular on the sub-detector that will instrument the endcap region of MTD, the Endcap Timing Layer (ETL). ETL will be equipped with thin planar silicon sensors based on the Low-Gain Avalanche Diode (LGAD) technology, the Ultra-Fast Silicon Detectors (UFSDs), which combine a moderate internal gain (10-30) with an active thickness of only 50~\mum~to provide excellent timing performance. The first chapter introduces the HL-LHC and the physics motivation for it, along with an overview of the MTD. The second chapter describes in detail the principles of operation of the UFSDs, while a summary of the productions can be found in chapter~\ref{ch:productions}. The experimental techniques used to characterize the sensors are reported in chapter~\ref{ch:experimental_techniques}. Chapters~\ref{ch:interpad} and \ref{ch:beta} present the results on two measurement campaigns that led to the definition of an almost final design for the UFSDs to be installed at the ETL. The first campaign focused on the development of the inter-pad region of multi-pads UFSDs: it demonstrated the importance of having low-doped and small-area $p$-stops to avoid the premature breakdown of the arrays and the micro-discharge effect. The grid guard-ring design of the $p$-stops proved to be the most reliable and the least sensitive to floating pads. The second measurement campaign, instead, proved that the UFSD gain layer design most suited for the ETL needs is a deep carbonated boron implant, with both the boron and the carbon diffused at low thermal load: such design is able to achieve the target time resolution up to a radiation fluence of~\fluence{2.5}{15}, delivering a charge $\geq$~5~fC, with low sensitivity to both non-uniform biasing conditions and non-uniform irradiation. Finally, the last chapter presents the analysis, using simulated data, of a Higgs bosons pair decaying in two $\mathrm{b}\overline{\mathrm{b}}$ pairs, in the vector-boson-fusion (VBF) production mode. This is one of the most interesting physics channels for CMS at the HL-LHC, as it allows measuring the Higgs self-coupling. The analysis focuses, in particular, on the impact of the MTD timing, which improves the primary vertex tagging by \simile~2\%, and the jet reconstruction efficiency and purity by 0.5 and 6~\%, respectively.