Study of the heavy flavour fractions in z+jets events from $$p\bar{p}$$ collisions at energy √s = 1.96 TeV with the CDF II detector at the Tevatron collider

Thesis/Dissertation · Sun Jun 01 00:00:00 EDT 2008

DOI:https://doi.org/10.2172/945412· OSTI ID:945412

Mastrandrea, Paolo ^[1]

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1. Univ. of Siena (Italy)

The Standard Model of field and particles is the theory that provides the best description of the known phenomenology of the particle physics up to now. Data collected in the last years, mainly by the experiments at the big particle accelerators (SPS, LEP, TEVATRON, HERA, SLAC), allowed to test the agreement between measurements and theoretical calculations with a precision of 10^-3 / 10^-4. The Standard Model is a Quantum Field Theory based on the gauge symmetry group SU(3)_C x SU(2)_L x U(1)_Y , with spontaneous symmetry breaking. This gauge group includes the color symmetry group of the strong interaction, SU(3)_C, and the symmetry group of the electroweak interactions, SU(2)_L x U(1)_Y. The formulation of the Standard Model as a gauge theory guarantees its renormalizability, but forbids explicit mass terms for fermions and gauge bosons. The masses of the particles are generated in a gauge-invariant way by the Higgs Mechanism via a spontaneous breaking of the electroweak symmetry. This mechanism also implies the presence of a massive scalar particle in the mass spectrum of the theory, the Higgs boson. This particle is the only one, among the basic elements for the minimal formulation of the Standard Model, to have not been confirmed by the experiments yet. For this reason in the last years the scientific community has been focusing an increasing fraction of its efforts on the search of the Higgs boson. The mass of the Higgs boson is a free parameter of the Standard Model, but the unitarity of the theory requires values not higher than 1 TeV and the LEP experiments excluded values smaller than 115 GeV. To explore this range of masses is under construction at CERN the Large Hadron Collider (LHC), a proton-proton collider with a center of mass energy of 14 TeV and a 10³⁴ cm^-2 s^-1 peak luminosity. According to the present schedule, this machine will start to provide collisions for the experiments at the end of 2008. In the meanwhile the only running accelerator able to provide collisions suitable for the search of the Higgs boson is the Tevatron at Fermilab, a proton-antiproton collider with a center of mass energy of 1.96 TeV working at 3 • 10³²cm^-2s^-1 peak luminosity. These features make the Tevatron able for the direct search of the Higgs boson in the 115-200 GeV mass range. Since the coupling of the Higgs boson is proportional to the masses of the particles involved, the decay in b{bar b} has the largest branching ratio for Higgs mass < 135 GeV and thus the events Z/W + $$b\bar{b}$$ are the main background to the Higgs signal in the most range favored by Standard Model fits. In this thesis a new technique to identify Heavy Flavour quarks inside high - P_T jets is applied to events with a reconstructed Z boson to provide a measurement of the Z+b and Z+c inclusive cross sections. The study of these channels represent also a test of QCD in high transferred momentum regime, and can provide information on proton pdf. This new Heavy Flavour identication technique (tagger) provides an increased statistical separation between b, c and light flavours, using a new vertexing algorithm and a chain of artificial Neural Networks to exploit as much information as possible in each event. For this work I collaborated with the Universita di Roma 'La Sapienza' group working in the CDF II experiment at Tevatron, that has at first developed this tagger. After a brief theoretical introduction (chapter 1) and a description of the experimental apparatus (chapter 2), the tagger itself and its calibration procedure are described in chapter 3 and 4. The chapter 5 is dedicated to the event selection and the chapter 6 contains the results of the measurement and the study of the systematic errors.

• Research Organization: Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-07CH11359
• OSTI ID: 945412
• Report Number(s): FERMILAB-THESIS-2008-63; TRN: US0900828
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
A CENTERS
BRANCHING RATIO
CROSS SECTIONS
FERMILAB COLLIDER DETECTOR
FERMILAB TEVATRON
HIGGS BOSONS
NEURAL NETWORKS
PROTONS
QUANTUM CHROMODYNAMICS
QUANTUM FIELD THEORY
STANDARD MODEL
STANFORD LINEAR ACCELERATOR CENTER
STRONG INTERACTIONS
SYMMETRY
SYMMETRY BREAKING
SYMMETRY GROUPS
Experiment-HEP