Probing new physics at the LHC: searches for heavy top-like quarks with the ATLAS experiment

Is our Standard Model (SM) of the fundamental particle interactions complete? Apparently, the answer is “no”. Many theories have been proposed to explain what is currently not understood, like the nature of Dark Matter, or the reason why the Higgs boson is so light. Now that the Large Hadron Collider (LHC) at CERN is fully operational, it is possible for experiments like ATLAS to explore very high-energy regimes where new physics can be probed. The work presented in this dissertation consists of two analyses aimed at the discovery (or exclusion) of a signal from a new particle: a quark similar to the top quark (the heaviest particle of the Standard Model) but with a larger mass. This new “top-like” quark could be a simple replica of the SM top quark, just with higher mass, i.e. a chiral fourth-generation up-type quark, or it could have exotic features. The latter hypothesis is particularly interesting as many “beyond-Standard Model” theories predict new heavy so-called vector-like quarks. Both searches rely on a partial dataset of proton-proton collisions collected by the ATLAS experiment during the year 2012. These results are documented in two ATLAS public notes (ATLAS-CONF-2013-018 and ATLAS-CONF-2013-060) and are currently being updated with the full statistics. They are also part of a combined effort of different research teams within the ATLAS Exotics working group to explore additional signatures, including those from heavy vector-like bottom partners. The first analysis described focuses on pair production of heavy top-like quarks decaying into a W boson and a bottom quark, exactly like the top quark. In this case kinematic features of the decay process, namely the fact that the W boson from the heavy top will receive more boost than the W boson from the top, are exploited to reconstruct the mass of the particle and discriminate between a signal from new physics and background from Standard Model processes. The second analysis instead studies pair production of heavy top-like quarks, with at least one of them decaying into a Standard Model Higgs boson and a top quark. Here the complete reconstruction of the mass of the particle is difficult, since many jets are present in the final state, resulting in a high combinatorial background. This apparent disadvantage is instead used to build a discriminating variable sensitive to multiple decay modes of the heavy top-like pair, and exploiting the fact that few Standard Model processes can produce such high jet multiplicity, many of which originate from bottom quarks. Both searches are and kept independent from theoretical assumptions and orthogonal to each other so that it is possible to combine them and perform a quasi-model independent analysis, resulting in the most stringent limits on the mass of the heavy vector-like top partner. Indeed, this particle is excluded at 95% CL, independently on the model, for masses from 350 GeV and up to 550 GeV (almost up to 600 GeV). If instead an assumption on the model is made and vector-like tops are considered to be singlets of isospin, then we are able to exclude them at 95% CL for masses up to 670 GeV. A previous analysis that was performed in 2011 on data from proton-proton collisions at a center-of-mass energy of 7 TeV and that set the strategy for vector-like quark searches is not detailed in this dissertation but is documented in a paper published on Phys. Lett. B 718 (2013) 1284.