$b$-tagging in dense environments

This note note describes the performance of the common ATLAS $b$-tagging algorithms in the context of dense environments as they can be found in the decay of boosted top quarks into jets and addresses the problems related to such conditions. A new multivariate analysis based $b$-tagging algorithm called MVb is presented additionally. The training of this new $b$-tagger is modified with respect to that of the current tools to take the conditions of boosted topologies better into account.