Direct Search for Standard Model-Like Higgs Boson and Software Integration of Data Acquisition Cards

The Large Hadron Collider (LHC) accelerator at CERN near Geneva is designed to collide protons with a centre-of-mass energy up to 14 TeV. It was tested at lower energy in November 2009. The world record collisions of beams of 1180 MeV was achieved. The LHC has four interaction points for the four large experiments: ALICE, ATLAS, CMS, and LHCb. LHCb is the "LHC beauty" experiment, located at interaction point P8 (France). It is a single arm forward spectrometer dedicated to the b-hadron sector optimised to study CP-violating processes and rare decays. In particular the LHCb detector has the capability to measure decay vertices with a resolution of few tenths of microns. Two topics have been addressed in this thesis. In the first part we study the LHCb sensitivity to detect a Standard Model Higgs boson in the HW ± → bb̅ + l±  νl(—)  and HZ0 → bb̅ + l+l- channels (or events with similar topologies). Here we face the challenge to discriminate signal events from the huge background sources which have been identified to be tt̅, Z0W ±, Z0Z0, W± + 2b and γ*/ Z0 + 2b events. Several tools have been designed for this goal which requires the reconstruction of b-jets. In particular we have optimised a b-jet finder algorithm, and a Neural Network procedure for the b-jet energy measurement. We have achieved a dijet mass resolution (giving the Higgs mass) of 38%. The efficiency to identify b-jets is 70%, and we discard more than 70% c-jets (90 % for lighter quark-jets). The Higgs event selection uses another Neural Network based on the event topology. If we consider 5 years of data taking we obtain about 50 events and a statistical significance in the Higgs mass spectrum of about 1, assuming a SM Higgs with a mass of 120 GeV/c2. This statistics is in principle not enough to discover a SM Higgs. The only hope is that a new mechanism exists producing events with a similar topology. The second part of this thesis is a contribution to the integration of the Data Acquisition electronics boards built in Lausanne (the TELL1 boards) into the Experiment Control System (ECS). The LHCb detector consists of roughly one million detector channels which are readout by about 300 TELL1 boards. A complete framework has been developed allowing to configure, control, interact, "spy" and check the TELL1s from the ECS. This project is now used by all the different LHCb subdetectors using TELL1 boards.

Bay, Aurélio
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis4686-7

 Record created 2010-03-04, last modified 2018-01-28

Rate this document:

Rate this document:
(Not yet reviewed)