Identification of bottom-quarks in searches for new heavy resonances decaying into boosted top-quarks with the ATLAS detector and a development of an improved $b$-tagging algorithm

From all the so far discovered elementary particles, the top-quark is the heaviest. Its large mass of $173.34\GeV$ is unexplained and suggests that the top-quark may play a special role in nature, as it occurs in many beyond the Standard Model predictions. Several of these theories anticipate, the existence of heavy particles that decay predominantly into top-quark pairs. Searches for such new particles have been already performed by the experiments of the TEVATRON collider located at the Fermi National Accelerator Laboratory as well as by the two largest LHC experiments, ATLAS and CMS. Meanwhile, the exclusion limits on some of these models extend already up to $\TeV$ mass scales. Thus the ongoing searches for new heavy particles decaying into top-quark pairs focus more strongly on events that contain high-$\pt$ top-quarks. The decay products of a boosted top-quark (or any other highly boosted particle) can be strongly collimated and their signatures in the detector system might even have a significant overlap. In such a case the standard reconstruction techniques might fail to resolve the decay products of the top-quarks individually and dedicated algorithms are needed in order to recover the loss of sensitivity. \\ The identification of bottom-quark decays, called $b$-tagging, is an important tool for the selection and reconstruction of top-quarks in order to suppress most of the relevant background processes. Boosted particle decays are a particularly challenging environment, as the performance of the currently used $b$-tagging algorithms are strongly reduced under these conditions. Thus dedicated $b$-tagging algorithms are required that are more stable inside dense jet environments in order to recover for the effects that are connected to the performance degradation. \\ This thesis describes the development, calibration and application of a new $b$-tagging algorithm, that takes the conditions of dense jet environments better into account than other algorithms. In addition, it is shown how a connection between the topology of the reconstructed $t\bar{t}$ candidate decays and the $b$-tagged jets contained in the studied events can be exploited to improve the sensitivity to search for new heavy particles decaying into top-quark pairs.