First Measurement Of Top Quark Pair Production Cross-Section

ABSTRACT

This dissertation presents the first measurement of top quark pair production cross-section in events containing a muon and a tau lepton. The measurement was done with 1 fb?1 of data collected during April 2002 through February 2006 using the DØ detector at the Tevatron protonantiproton collider, located at Fermi National Accelerator Laboratory (Fermilab), Batavia, Illinois.

Events containing one isolated muon, one tau which decays hadronically, missing transverse energy, and two or more jets (at least one of which must be tagged as a heavy flavor jet) were selected. Twenty-nine candidate events were observed with an expected background of 9.16 events. The top quark pair production cross-section is measured to be _(t¯t) = 8.0+2.8 ?2.4 (stat)+1.8 ?1.7 (syst) ± 0.5 (lumi) pb. Assuming a top quark pair production cross-section of 6.77 pb for Monte Carlo signal top events without a real tau, the measured _ × BR is _(t¯t) × BR(t¯t ! ? + _ + 2_ + 2b) = 0.18+0.13 ?0.11 (stat)+0.09 ?0.09 (syst) ± 0.01 (lumi) pb.

CHAPTER 1
FOUNDATIONS

1.1 The Standard Model of Particle Physics

This dissertation presents an investigation in the field of elementary particle physics, whose goal is to understand the fundamental constituents of matters and the laws governing them. The standard model (SM) is the currently accepted theory which describes a vast range of phenomena in elementary particle physics, including the strong, weak, and electromagnetic interactions. It is consistent, renormalizable, and has been tested in precision to a high degree of accuracy.

There are two key features of the standard model. The first one is gauge invariance, which means the physics described by the standard model should not change under phase transformations which are functions of space-time coordinates. The second is the Higgs mechanism, the formalism which endows the particles of the standard model with mass.

The gauge invariance principle constrains the particle contents and the interactions between them in a unique way. Given a set of particles, the symmetry group of transformation, and the gauge bosons, the principle defines the form of interactions between the particles mediated by the bosons. The standard model is a theory based on the SU(3)C ×SU(2)L×U(1)Y symmetry groups. The subscript C, L, and Y refers to the color group, left-handed, weak isospin group, and weak hypercharge group, respectively.

A theory of fundamental particle interactions built from the gauge invariance principle alone doesn’t allow the existence of massive gauge bosons. In the standard model, the masses of the gauge bosons of weak interactions are generated by the Higgs mechanism. It transforms degrees of freedom in the Higgs field(s), which are scalar field(s), into masses of the weak gauge bosons. In elementary particle theory, the Higgs mechanism that generates these masses is also known as electroweak symmetry breaking. Within the electroweak theory, the Higgs mechanism is also resposible for generating the masses of the fermion contents of the standard model.

Figure 1.1: Plots of electroweak constraints of the mass of the standar model Higgs particle from other standard model measurement. Top left: constraints from the mass of W bosons, mW; top right: constraints from the mass of the top quark, mt; bottom left: constraints from both mass of the top quark and the W bosons, bottom right: constraints from the global electroweak fit. Figures are taken from the report of LEP Electroweak Working Group [1].

There is no unique formulation of the Higgs mechanism in the standard model. It is therefore more custom to use the term Higgs sector when discussing the more general aspects of electroweak symmetry breaking and the origin of mass of standard model fermion. In the standard model, the minimal implementation of the Higgs mechanism is an SU(2) doublet which corresponds to the existence of an electrically neutral, scalar particle.

A tremendous amount of effort has been put to explore and understand the Higgs sector of the standard model. As of the year 2007, there is no direct experimental confirmation of the physics of the Higgs sector, be it the minimal one or any of the extended ones. The available knowledge about the Higgs sector is obtained by constraints from other parts of the standard model. Figure 1.1 shows four graphs that show the constraints on the minimal standard model Higgs from other well-measured parameters of the standard model.

An important property of the Higgs particles1 is that the couplings between Higgs particles and other particles are proportional to the other particles’ masses. The top quark is currently the heaviest known particle, and is expected to play important role in the exploration of electroweak symmetry breaking mechanism. The next section discusses top quark production and decays at the Tevatron.