Esben Bryndt Klinkby

Title: W mass measure-ment and simulation of the Transition Radiation Tracker at the ATLAS experiment

A thesis submitted for the degree of Doctor of Philosophy (PhD) on April 29, 2008

Experimental High Energy Physics Group, Niels Bohr Institute, University of Copenhagen

Supervisor:
Jørn Dines Hansen

Abstract

W mass measurement and simulation of the Transition Radiation Tracker at the ATLAS experiment 

At the time of writing, the final preparation toward LHC startup is ongoing. All the magnets of the machine have been installed and are currently being cooled. Most sub-detectors of the four experiments situated at the LHC ring, are installed in their final positions and are being integrated into their respective data aquisition systems.

This thesis concerns itself with the ATLAS experiment, focussing on a sub-detector named the Transition Radiation Tracker (TRT). Some attention is given to the hardware testing of the detector modules, but the main focus lies on the simulation of the detector and the comparison of the simulation with test-beam data, as well as with data collected during the commisioning phase using cosmic muons.

There is little doubt that LHC will bring insight with respect to the understanding of the universe on the fundamental level. In particular, it is anticipated that light will be shed on the origin of mass which according to our current understanding proceeds via the Higgs mechanism. Either the corresponding particle; the Higgs boson is discovered by the LHC experiments, or its existence will be strongly disfavoured. In either case, a key measurement to understand the origin of mass, is theW boson, since it is intimitely linked to the Higgs mechanism. By precise measuring the W boson mass, the allowed mass range for the Higgs boson can be constrained, both within the Standard Model and in its various extensions. Thus, regardless of the results of the Higgs search, a precise determination of the W mass is of paramount importance, and in this thesis methods are presented aiming at measurering the W
mass to the highest possible precision with the ATLAS experiment.

The thesis is structured as follows. In Part I the electroweak sector of the Standard Model is reviewed and various extensions are discussed. Emphasis is put on theW mass and its relations to other observables. After a short introduction to the ATLAS experiment, the prospects for a W mass measurement are reviewed in Part II. In Part III the simulation of the Transition Radiation Tracker is discussed and results are compared to test-beam measurements. Comparison of simulation to measurements proceeds in Part IV, in which a number of somewhat practical problems faced during the commissioning period are discussed.