Muon identification in the ATLAS calorimeters

The idea of matter being composed of small elementary particle was already suggested by philosophers such as Leucippus, Democritus or Epicurus in ancient Greece. In the 19th century, John Dalton concluded that all kinds of matter were made up of a single kind of elementary particle. These particles came to be known as atoms. This name derived from the Greek word atomos meaning “indivisible”. However, before the turning of the century, scientists had already realised that atoms could be divided into smaller constituents. During the early years of the 20th century, nuclear physics experiments culminated in the discovery of nuclear fission and fusion, which made real (although not profitable) the old alchemistdream of transmuting lead into gold. During the years 1950s and 60s the world of physics was overwhelmed with the discovery of a large number of “elementary particles” which would be known as the particle zoo. In the mid 1970s, with the formulation of the Standard Model, the existence of all these particles could be explained in terms of few number of fundamental constituents. The Standard Model is a relativistic quantum field theory consistent with both quantum mechanics and special relativity. It describes the strong, weak, and electromagnetic fundamental forces which are carried by mediating gauge bosons. These interaction bosons are known as gluons for the strong nuclear force, as $W\pm$ and Z for the weak nuclear force, and as photons $(\gamma)$ for the electromagnetic force. In addition, the theory introduces 24 fundamental particles as the constituents of matter. The Standard Model also predicts the existence of another particle, known as the Higgs boson, which gives mass to the other particles. For a basic introduction see [1]. To date, almost all predictions of the Standard Model have been tested experimentally. As an example, the theory predicted the existence of the $W\pm$, Z bosons, the gluon and the top or charm quarks well before their existence was confirmed in high energy physics experiments. However, there are still a number of theoretical and experimental limitations to the theory. The most evident being that it only explains three of the four known fundamental forces in nature, leaving out the gravitational interaction. Also recent evidence of oscillations in the neutrino sector, predict that actually neutrinos are not massless, as is commonly assumed in the Standard Model. Other paradoxes like the hierarchy problem or the evidence of dark matter and dark energy are not addressed by the theory. Today the common belief is that the Standard Model is an effective theory valid only up to a certain energy scale, after which, a more general theory would take over. These extensions to the Standard Model are enclosed in what is commonly known as Beyond the Standard Model Physics. Such theories include Supersymmetry, Extra Dimensi ons or String Theory. The Higgs boson is the most important piece of the theory that has not been confirmed experimentally. It plays a very important role in the Standard Model since it provides a mechanism whereby particles acquire their mass. The Higgs mechanism can be pictured as a field covering the entire universe. This field would present “resistance” to the passage of particles, and it would be this “friction” what is perceived as the mass of the particle. The Standard Model, however, does not predict the mass of the Higgs boson, which remains a free parameter of the theory. The Higgs boson as predicted by the Standard Model is a highly unstable particle. Once enough energy is available to produce the Higgs particle, this will promptly decay through one of the possible decay channels. Some of these channels present special topologies that can be measured in high energy physics experiments. The first attempts to discover the Higgs boson were made at the LEP experiments at CERN, and although they were not successful, they provided many precision measurements that established limits to the Higgs mass. Namely, the LEP experiments excluded the existence of a Higgs mass below 114 GeV=c2 and put an indirect upper limit to 144 GeV=c2 both values obtained at a 95% confidence level. Currently, in the Tevatron accelerator at Fermilab, a proton anti-proton collider, the experiments CDF and DØ continue the pursuit of the dis covery of the Higgs. The next generation collider, the LHC, will provide proton-proton collisions with unprecedented luminosity and centre of mass energy. Its two general purpose experiments, CMS and ATLAS, will continue the search for the Higgs. In ATLAS, the channel where the Higgs boson decays into a pair of Z bosons, which further decay into either electrons or muons, presents the cleanest signature for a wide range of possible Higgs masses. The analysis of this channel relies on good lepton reconstruction and identification. This thesis presents an algorithm that attempts to identify muons in the ATLAS calorimeters using their distinctive energy deposition patterns. This algorithm, which is part of the ATLAS offline reconstruction chain, complements the standard muon reconstruction in the muon spectrometer, especially in the regions where the latter presents design limitations. This thesis is divided into six chapters. Chapter 1 introduces the Standard Model formalism and the concepts of the Higgs boson production mechanisms. Chapter 2 gives a general overview of the LHC particle accelerator and the different ATLAS subdetectors. Chapter 3 presents the concepts of track reconstruction in both the inner detector and the muon spectrometer. It summarises the performance of the different tracking systems and the muon reconstruction algorithms. Chapter 4 introduces a calorimeter muon tagger algorithm that identifies the muon amo ngst all the inner detector tracks using the energy deposition measurements in the calorimeters. This chapter also presents a comparison study with other lepton reconstruction and identification algorithms. Chapter 5 presents an analysis on the $H \to ZZ^(+) \to 4l$ decay channel where the improvements associated with the use the calorimeter muon tagger are evaluated. A discussion of the results obtained is given in Chapter 6.