Central Exclusive Production with Dimuon Final States at LHCb

The Standard Model of particle physics combines the theories of the electroweak and strong forces to describe the interactions of the elementary building blocks of matter, quarks and leptons. Along with the massless mediator of the electromagnetic force, the photon, the theory successfully predicts the existence of three massive bosons, W$^\pm$ and Z, which mediate the weak interaction and eight massless electrically neutral bosons, the gluons, which mediate the strong interaction. However the Higgs Mechanism of the Standard Model, which is required to give mass to the elecroweak bosons and the fermions, predicts the existence of a Higgs Boson which has yet to be observed experimentally. Also the fourth fundamental force of nature, gravitation, is not accounted for within the Standard Model. There is a violation of CP symmetry in the weak interaction of the Standard Model which reflects a fundamental asymmetry between matter and anti-matter in nature. However the predicted level of CP violation is insufficient to fully account for the experimentally observed asymmetry in nature. The LHCb experiment is dedicated to the study of CP violation in the b-physics sector and precision tests of the Standard Model. It is located at CERN’s Large Hadron Collider (LHC) which is a 27 km ring located 100 m below the French Swiss border near Geneva, Switzerland. Two proton beams are accelerated around the ring in opposite directions and are collided at interaction points where the detector experiments are located. On the 30th March 2010 proton collisions at an energy of 3.5 TeV per beam began at the LHC. The work contained in this thesis is based on data collected throughout the running period of 2010. It is concerned with the identification and measurement of processes resulting in the exclusive production of dimuons. These exclusive processes involve colourless exchange between the protons resulting in regions devoid of particles called rapidity gaps. Chapter 2 gives an overview of the Standard Model of Particle Physics. Brief descriptions of the structure of the field theories and the Higgs mechanism are given. The theory describing exclusive dimuon production is discussed and the motivations for measuring these processes at LHCb are given. In Chapter 3 an overview of the structures of the LHCb detector and the LHC accelerator is given. The particle tracking and identification detectors are discussed in detail. Chapter 4 presents the results of measurements of the charged particle detection efficiency of the Vertex Locator (VELO). The chapter begins with an overview of the theory of charged particle detection using silicon detectors. The silicon sensors and the overall VELO structure are then described in detail. Finally the performance of the clustering and the charge collection of the VELO is discussed. As the VELO is used to identify exclusive events through the use of VELO defined rapidity gaps it is important that it operates with a high charge collection and clustering efficiency. In Chapter 5 preliminary results for the measurement of exclusive dimuon production are presented. Cross-section measurements for exclusive $J/\psi, \psi(2S)$ and non-resonant production $p(\gamma\gamma+\mu^{+}\mu^{-}+p$ are presented and comparisons made with theory. The additional requirement of a single photon allows the cross-section for exclusive $\chi_c$ production to be measured and also the relative contribution of $\chi^0_c, \chi^1_c, \chi^2_c$ to be determined. Finally Chapter 6 presents a summary of the results of the exclusive dimuon production cross-section measurements.