Observation of top quarks and first measurement of the t anti t production cross section at a centre-of-mass energy of 7 TeV with the CMS experiment at the LHC

Like the fictional Doctor Faustus, particle physicists devote themselves to the understanding of the elementary building blocks of matter and the nature of the forces acting between them. Over the last six decades, great success has been made on this field as a result of a fruitful interplay between improved experimental techniques and ingenious theoretical insights which enabled a comprehensive description of the observed phenomena. In the early seventies, the Standard Model (SM) of elementary particle physics was derived, summing up our current understanding of the particles composing all matter and their interactions. According to this theory, the fundamental fermions, namely six flavours of quarks and leptons, respectively, interact via the exchange of force-mediating bosons which couple to the charges of the fermions. Since its formulation, the Standard Model has been subject to a large number of experimental tests and highest-precision data which collectively confirmed its predictions. One major success for the predictive power of the Standard Model was the experimental observation of the top quark in 1995. With an exceptionally large mass of m_t=(173.3±1.1) GeV/c², the top quark is nearly as massive as a gold nucleus and outweighs the next-heaviest fermion, the b quark, by about a factor of 40. This characteristic property implies that top quarks on average decay before top-flavoured hadrons can be formed, offering a unique possibility to study quasi-free quarks. The Standard Model predicts two mechanisms for the production of top quarks, either in pairs of top and antitop quarks via the strong interaction or singly in charged-current weak interactions. While the pairwise production led to its discovery in 1995, the second production mechanism was experimentally observed only about two years ago. Once produced, top quarks almost exclusively decay via the weak interaction into a b quark and a W boson. While all ordinary matter is composed of the two lightest quarks, the up and down quarks, the four heavier quark flavours only appear in collisions of high-energy particles. Therefore, their production under laboratory conditions requires modern particleaccelerator and collider facilities. Until last year, only the Fermilab Tevatron collider near Chicago, USA, provided sufficient centre-of-mass energy for the production of top quarks and thus paved the way for its discovery about sixteen years ago. Since the Tevatron was built, substantial technological progress in the intervening years has enabled the design and construction of the Large Hadron Collider (LHC) of the European Organisation for Nuclear Research (CERN) near Geneva, Switzerland. Extensively using superconducting magnets, the LHC provides hitherto unachievable centre-of-mass energies for the production of heavy particles in proton-proton collisions. After long design and construction phases, the first pp collisions at a centre-of-mass energy of @√(s)=7 TeV were achieved on March 30th 2010, turning the LHC to the most powerful collider ever operated. Given this huge centre-of-mass energy and instantaneous luminosity, the LHC can be considered a top quark factory and will thus facilitate scrutiny of top quark properties and interactions. The two general-purpose detectors ATLAS and CMS have been built to detect and analyse pp collisions provided by the Large Hadron Collider. Driven by the intention to exploit the full discovery potential of the LHC, the designs of ATLAS and CMS concentrated on the discovery of the as yet unobserved Higgs boson, which constitutes the last particle predicted by the Standard Model, and on the search for experimental evidences of phenomena predicted by extensions of the Standard Model. The CMS apparatus is located near Cessy, France, in an underground cavern about 100 m below surface. Since the first pp collision event recorded by the CMS apparatus, a data set corresponding to an integrated luminosity of L=(36.1@±4.0) pb^-1 was acquired with a fully-operational detector until November 2010. Due to the extremely short lifetime of top quarks, only its decay products can be experimentally observed. Therefore, electronic signals recorded by the CMS apparatus are subjected to reconstruction algorithms which aim for the detailed extraction of information available on the interactions occurred in pp collision events. Simulations further serve for the comparison of these reconstructed events with expectations based on theoretical and phenomenological models. Given these simulated samples of signal and background events, criteria for the selection of candidate events can be chosen, driven by the requirement for a good signal-to-background ratio while maintaining high signal selection efficiency. Moreover, analysis strategies can be developed and their expected performance tested, prior to their application to observed pp collision data. Based on the early data set provided by the CMS collaboration, and employing all the techniques mentioned above, the analysis presented in this thesis is dedicated to the search for top quarks at the LHC and to perform a first measurement of the top quark pair production cross section at a centre-of-mass energy of 7 TeV.