Search for Heavy Resonances Decaying into Two Higgs Bosons or into a Higgs Boson and a W or Z Boson in the $q\bar{q}$ ($b\bar{b}$) $\tau^{+}\tau^{-}$ Final State with the CMS Detector

This thesis presents a search for potential signals of new massive particles decayingto pairs of W, Z, and Higgs bosons that are predicted by beyond the standard modeltheories. The data analyzed have been collected with the CMS detector at the Large???Hadron Collider (LHC) during pp collisions at center of mass energies of s = 8 TeV???in 2012 (Run 1) and s = 13 TeV in 2016 (Run 2), corresponding to an integratedluminosity of 19.7 fb???1 and 35.9 fb???1 , respectively. Such new particles are the prominent feature of many theoretical models that aim to clarify some of the questionsunanswered by the standard model, such as the apparently large difference betweenthe electroweak and the gravitational scales. The final states analyzed in this workare compatible with the presence of a new massive resonance that decays to a Higgsboson and a W, Z, or Higgs boson, with one of the Higgs bosons further decaying to ??leptons. Since ?? leptons are unstable, they can decay further into either lighter leptons(`), electrons and muons, and neutrinos or into neutral and charged hadrons (??h ) andneutrinos. Therefore, they can generate a plethora of final states. The other boson canbe a W, Z, or H boson and is required to decay hadronically to a pair of quarks.The first study is focused on the search for a resonance decaying to HH and subsequently to ?? + ?? ??? bb in the final state where one of the tau leptons decays to hadronsand a neutrino; and the other ?? to a lighter lepton, either an electron or a muon, andtwo neutrinos. This analysis is based on data recorded in Run 1 (2012) of the LHC.The second study searched for WH, ZH, or HH resonances decaying to quarks and ??leptons in data recorded during Run 2 (2016) of the LHC. In the Run 2 analysis, inorder to extend the sensitivity, additional final states are considered in which both ??leptons from the H boson decay hadronically. In addition, the search is extended toconsider hadronic decays of the W and Z bosons to qq.These final states are particularly challenging because, for large resonance masses, thebosons are highly energetic and the final products from their decay are separated by asmall angle in space. This collimation implies that the quarks from the hadronicallyboson decay are reconstructed in one large-cone jet. Novel jet-substructure techniquesand dedicated algorithms for the mass reconstruction and flavor identification of thejets are applied to distinguish W, Z, and H bosons. The ?? pair produced from the Hboson decay has a high Lorentz boost and the final decay products are also collimated.Special techniques were developed as part of this doctoral work to correctly reconstructand identify the ?? lepton pairs in this particularly boosted topology.The search is performed by scanning the distribution of the reconstructed mass of theiii