Search for direct production of top quark pairs at sqrt(s) = 8 Tev using topological variables

Abstract

When though the Standard Model (SM) has had a great success in the physical description of particles and its interactions, given the fact that all experimental measurements agree with its predictions, there are many well-founded reasons to believe that it is not a complete theory. Among these are the hierarchy problem as well as gravitation and dark matter which are not explained by the SM. Supersymmetry (SUSY) is an extension of the SM that could provide natural solution to the hierarchy problem: the cancellation of the quadratic divergences on the Higgs bosom mass (coming from the top quark loops) is achieved through the contribution of new loops from the supersymmetric particles. Furthermore, another strength of SUSY is that, if R-parity is satisfied in nature, the LSP (lightest SUSY particle) could be a good candidate from dark matter. The search for top quarks (stops) with masses below 1 TeV is motivated by many Super-symmetric models that provide a natural solution to the hierarchy problem of the Standard Model. Searches for direct production of pairs of stops at sqrt(s) = 8 TeV have been already performed by ATLAS and CMS experiments using cut & count and multivariate analysis techniques, based on kinematic variables to maximize the signal of background ratio. We report here the results of a search for direct production of stop pairs with the subsequent decay of each stop to a top quark and a neutralino, assuming a branching ratio of 100%, based on topological variables not used in previous analysis. We focus our search on the semileptonic channel of the top quark pairs produced, having as final state one single isolated lepton, more than three jets (at least one tagged as b-jet) and missing transverse energy

The data analyzed correspond to an integrated luminosity of 19.5 fb-1 of proton-proton collisions at sqrt(s) = 8 TeV, collected by the CMS experiment. The topology of the event is defined as the most likely permutation of the objects in the final state corresponding of the Feynman diagram studied. This is accomplished by maximizing likelihood function. An additional discriminant is obtained by finding the matrix elements weight of the most probable permutation by using MADWEIGHT. We define event selection criteria based on correlations of topological and kinematic variables. We show that this technique, based on the topology of the event, competes with the exclusion limits already obtained by previous analyses and has the potential to become a powerful tool for future searches. The document is organized in the following way: First, a brief introduction to the LHC and some of its operation details is given, this is followed by a description of the CMS detector and its sub-detectors. Then the Standard Model is reviewed as well as the reasons why new physics is expected. After this, a brief introduction to SUSY is given, showing the main implications that it has, and some of the solutions given by this theory to the SM limitations. Second, the last, the current status of some SUSY searches is described, their actual limits are shown, and certain strategies used by the CMS experiment to search to SUSY are described. Finally, the analysis performed by us, the results, the conclusions and the future developments are presented