• b-jet
• b-tagging
• CMS upgrades
• Higgs boson
• Phase 1 upgrade
• pixel sensor
• silicon tracker

The aim of this Thesis is to study the Higgs boson coupling to bottom quarks in the next Runs of the Large Hadron Collider (LHC). In particular the performance of a simplified analysis in the context of the CMS experiment is studied in different scenarios, starting from the upcoming LHC Run 2 and extending to long term runs, after upgrades of the LHC machine and of the CMS detector.
This Thesis work also includes contribution to the qualification of silicon pixel modules for the new CMS pixel tracker that will replace the current one at the end of 2016.
For a Higgs boson of 125 GeV, the decay to a bottom quark pair (bb) is the dominant mode, with a branching ratio of approximately 58%. However, the prevailing QCD multijet background events prevent an inclusive search of the Higgs boson in the bb decay channel.
Therefore searches of the H(bb) decay at the LHC consider particular production modes of the Higgs boson: the vector boson fusion and the associated production with a vector boson or with a top quark pair.
In this Thesis, the associated production of the Higgs with a Z boson has been studied, when the latter decays into a pair of opposite sign muons or electrons (referred to as leptons hereafter).
In such final state the signature of two isolated opposite sign leptons from the Z boson decay helps for triggering and rejecting background events. Due to the nearly hermetic coverage of the CMS detector and thanks to its silicon tracker, its high granularity electromagnetic calorimeter and its excellent muon system, leptons originating from Z bosons can be efficiently identified and precisely reconstructed.
The reconstruction of the Higgs boson in the bb final state relies on the identification of b-jets, originating from the hadronization of b-quarks into B hadrons.
Therefore, dedicated b-tagging algorithms have been developed to identify b-jets, being based on distinguishing features of the B hadrons.
The latter are characterized by relatively long lifetimes, such that they decay inside the tracker volume and tracks of daughter charged particles identify a secondary displaced vertex within the jet. Moreover, such tracks also tend to have larger impact parameters.
A good ability in b-jet identification relies on efficient and high quality tracking and vertexing, that can only be achieved with an appropriate performance of a pixel tracking detector.
Therefore, the second main topic of this Thesis deals with activities that contribute to upgrade the CMS silicon pixel tracker to ensure and improve its performance.
The current CMS silicon pixel tracker was designed to sustain a maximum fluence of 1.6*1015 neq/cm2 and to operate at a maximum instantaneous luminosity of 1034 cm-2s-1. As the latter limits will be exceeded at the end of the LHC Run 2 in 2016, the pixel tracker must be replaced with a new upgraded one by then. The so called Phase 1 upgrade pixel tracker is currently being built and is expected to be installed at the end of 2016. It will have one additional silicon layer with respect to the current one and the innermost layer closer to the interaction point, without increasing the amount of material.
Such improvements will result in a higher tracking efficiency, which is important for tracking performance and hence for b-jet identification.
Additional extended upgrades of the CMS tracker, usually referred to as Phase 2 upgrades, are foreseen around 2023 before the beginning of High Luminosity Phase of the LHC, needed to retain the same excellent detector performances in an even more challenging environment.
This Thesis is organized in seven Chapters.
Chapter 1 introduces the Higgs-Brout-Englert mechanism of spontaneous symmetry breaking in the Standard Model of Particle Physics and summarizes the phenomenology of Higgs boson production and decays at hadron colliders. Latest experimental searches for the Higgs boson at the LHC leading to its discovery in 2012 are also cited.
In Chapter 2 the layout and operation of the LHC machine are briefly summarized. Then the subsystems of the CMS detector are described, including the main upgrades planned for the coming years, highlighting the improvements in the inner and outer silicon tracking detector.
Chapter 3 focuses on the building and commissioning of the Phase 1 upgrade pixel tracker, that shall be installed at the end of 2016. After illustrating the components of pixel barrel modules, activities currently going on in the Pisa production center are described.
In Chapter 4 as an introduction to the analysis work the main strategies and algorithms for particle identification and reconstruction adopted by the CMS collaboration are presented.
The first Section of Chapter 5 summarizes the strategy and results for the search of the Higgs boson produced in association with a vector boson and decaying to a bb pair, performed on data collected with the CMS detector until the end of 2012. The second Section describes the preparation of the analysis for the LHC run at a center of mass energy of 13 TeV. First, some basic kinematics studies comparing Monte Carlo samples at 8 and 13 TeV are performed. Then, the performance of a simple cut-based invariant mass analysis is evaluated and the choice of different b-tagging discriminators and working points is evaluated.
In Chapter 6 the impact of the b-tagging performance on the Z(ll)H(bb) sensitivity is estimated in future detector upgrade scenarios. Due to the impossibility of producing large full simulation samples for the most important background processes in such future scenarios, the studies are based on a two-dimensional parametrization of the b-tagging efficiency, derived from fully simulated ttbar samples, available for all the considered scenarios.
Such parametrization is then used to estimate the sensitivity of the Z(ll)H(bb) analysis in those future upgrade scenarios, highlighting the improvements that are expected to be achieved thanks to better tracking capabilities.
In the last Chapter 7 the most relevant results of this Thesis are summarized.