Probing the Heavy Flavor Content of Top quark events produced in proton-proton collisions at the Large Hadron Collider with the CMS detector

The Large Hadron Collider ( LHC ) installed at CERN will accelerate and collide bunches of protons at unprecedent energies and luminosities. Collisions with a center of mass energy of 14 TeV and luminosity of the order of $10^{34}cm^{-2}s^{-1}$ are expected to occur with a 25 ns periodicity once full operation mode is attained. The LHC is expected to clarify open issues in the current understanding of the fundamental interactions, namely the mechanism that breaks the Electroweak symmetry. The Compact Muon Solenoid experiment ( CMS ) is one of the general purpose experiments installed at the LHC . Among its sub-detectors, the Electromagnetic Calorimeter ( ECAL ) is expected to have an excellent mass resolution obtained from its fine granularity composed of 76000 high purity PbWO4 crystals. In the ECAL the crystal scintillation light is converted into electrical signals, digitized and transferred from the detector to the readout system through 3500 high speed optical links (800 MBit/s). This thesis describes two personal contributions from the author in the context of preparing the data taking startup with the CMS experiment at the LHC . A dedicated test system, used to validate the “Off-detector electronics” in charge of reading out the trigger and event data produced by the ECAL, is presented and its performance is evaluated. The architecture of the corresponding monitoring system is also described and a special emphasis is put in synchronization of the calorimeter trigger data in parallel with its transmission to the upper level of decision. The possibility of measuring R = B(t -> Wb)/B(t ->Wq) and the efficiency to tag b quark jets (b-tagging) in the top dilepton channel with $e\mu$ final states is discussed. The heavy flavor content of the $t\bar{t}$ events is studied and a strategy to understand the background contributions to this measurement from data is outlined. The uncertainties on the measurement are estimated for different b-tagging working points and for an integrated luminosity of 250 $pb^{-1}$ in proton-proton collisions at $\sqrt{s}$=10 TeV.