Introduction

Introduction

The Standard Model represents the most complete model of physical phenomena at the sub-nuclear scale. The model describes the electroweak interaction, based on the gauge symmetry group SU(2)_L× U(1). The electroweak model postulates the existence of left handed fermion doublets

n_{i iL}    and  u_i d_iL

for leptons and quarks respectively, which transform under SU(2)_L [1]. In the model, the gauge bosons for the electroweak interactions W_µⁱ, B_µ are massless, and such are the leptons and the quarks. However, in the real world we observe massive leptons and quarks and linear combinations of the electroweak gauge bosons:

Aµ	=	BµcosqW+Wµ3sinqW
Wµ±	=	(Wµ1 iWµ2)/2
Zµ0	=	-BµsinqW+Wµ3cosqW

where q_W is the electroweak mixing angle. Since the W^±,Z⁰ bosons are massive while the photon (A_µ) is not, the electroweak symmetry is broken. The Standard Model postulates the existence of a scalar field, the Higgs field which couples to the electroweak bosons, thus breaking the symmetry. In the Higgs mechanism, the masses of bosons and fermions emerge as the coupling strengths of the interaction with the Higgs field.

The parameters of the electroweak model have been measured with great precision in past experiments; the parameter with the worst relative precision is the mass of the top quark (see Table 0.1). A very precise measurement of the top quark is required to reduce the theoretical uncertainties in the electroweak sector; in particular, since the top quark is the heaviest of all fundamental fermions and therefore couples strongly to the Higgs field, it plays a major role in the understanding of the Higgs mechanism.

The top quark can be produced at hadron colliders either by QCD processes --- in tt pairs --- or electroweak processes --- where a single top emerges. The Large Hadron Collider (LHC) currently being built at CERN will produce, due to its high design luminosity of 10³⁴ cm²s^-1, up to 80 million tt pairs as well as 30 million single top quarks per year, allowing to reduce the statistical error on the top mass measurement.

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Observable	Measurement	Precision
MW (GeV)	80.425±0.034	0.04 %
sin2qW	0.23150±0.00016	0.07 %
GZ (GeV)	2.4952±0.0023	0.09 %
MZ (GeV)	91.1875±0.0021	0.002 %
mt (GeV)	178.0±4.3	2.4 %

Table 0.1: Precision measurements of electroweak observables. Table taken from [6].

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The aim of the study presented in this thesis is to simulate the single top production process and subsequent top decay in the ATLAS detector, which is one of the two multipurpose experiments at the LHC. The single top channel is statistically less significant, compared to tt production; however, its less complex signature allows for a better understanding of the systematic effects, resulting in a precision of the mass measurement that is competitive with that in the pair production channel.

Chapter 1 of this thesis outlines the current knowledge on top quark physics and briefly describes the recipe for a detailed analysis; Chapter 2 illustrates the structure of the ATLAS detector and discusses the effect of detector performance on the analysis. Chapter 3 is devoted to the description of the trigger and data acquisition of the detector, focusing on the trigger efficiencies for top physics channels. Chapter 4 discusses in detail the readout electronics and data acquisition chain for the Monitored Drift Tube (MDT) chambers. NIKHEF is involved in the development and production of MDT chambers and the associated read-out electronics. Chapter 5 describes the tools used to generate Montecarlo data for top physics processes, the software simulation of the ATLAS detector and the algorithms used to reconstruct physical objects from raw simulated data. Finally, Chapter 6 discusses the analysis on simulated data and gives an outlook towards real data taking in 2007.

--Barison 20:06, 18 Jul 2005 (MET DST)