Track reconstruction in dense environments and the search for new physics in the fully hadronic diboson channels with the ATLAS detector

With the increase in center-of-mass energy of the LHC to $\sqrt{s}=13$ TeV for Run 2, events with dense environments are produced much more abundantly. In the core of highly energetic hadronic jets, the average separation of charged particles is comparable to the size of individual ATLAS inner detector elements. These dense environments may be produced by new physics processes or objects, including massive particles that decay to highly boosted bosons. However, this density can create confusion within the algorithms reconstructing charged particle trajectories (tracks), so careful optimization must be carried out to ensure that the track reconstruction performance in dense environments is not adversely affected. Such optimization will increase the possibility of discovery of new phenomena and allow higher precision measurements of the newly opened kinematic regime. This work describes a series of improvements to the ATLAS offline track reconstruction to enhance its performance in dense environments. The effects of these improvements are demonstrated using both Monte Carlo simulation and data. Using data alone, residual inefficiencies of the track reconstruction in the core of jets are quantified as a function of the transverse momentum of the jet. The fraction of lost tracks is presented using the energy loss in silicon. It varies from $0.061 \pm 0.006\textrm{(stat.)} \pm 0.014\textrm{(syst.)}$ to $0.093 \pm 0.017\textrm{(stat.)}\pm 0.021\textrm{(syst.)}$ between a transverse jet momentum of 200 to 400 GeV and 1400 to 1600 GeV, respectively. With this improved track reconstruction performance, and vastly smaller uncertainties through the data-driven measurement of the track reconstruction inefficiency, ways to reconstruct the masses of jets with higher precision become possible. It is demonstrated that combining the strengths of both the calorimeter and the tracker into a combined jet mass provides the most performant method currently available in ATLAS, reducing both the resolution of the reconstructed mass of the jet and its uncertainty. Employing these novel reconstruction methods, a search for resonances with masses in the range $1.2 < m < 3.5$ TeV in the hadronically decaying $WZ$, $WW$, or $ZZ$ final state, is performed in \lumi\ of $\sqrt{s}=13$ TeV proton-proton collision data. No significant deviations from the Standard Model background expectations are observed. An additional charged or neutral heavy vector boson, as predicted by the Heavy Vector Triplet phenomenological Lagrangian (assuming $g_V = 1$), decaying through $W'\rightarrow WZ$ (or $Z'\rightarrow WW$), is excluded in the mass range 1.2-2.0 (1.3-1.7) TeV at the $95\%$ confidence level.