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 decaying to pairs of W, Z, and Higgs bosons that are predicted by beyond the standard model theories. 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 $\sqrt{s}= 8$ TeV in 2012 (Run~1) and $\sqrt{s}= 13$ TeV in 2016 (Run~2), corresponding to an integrated luminosity 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 questions unanswered by the standard model, such as the apparently large difference between the electroweak and the gravitational scales. The final states analyzed in this work are compatible with the presence of a new massive resonance that decays to a Higgs boson and a W, Z, or Higgs boson, with one of the Higgs bosons further decaying to $\tau$ leptons. Since $\tau$ leptons are unstable, they can decay further into either lighter leptons ($\ell$), electrons and muons, and neutrinos or into neutral and charged hadrons ($\tau_{h}$) and neutrinos. Therefore, they can generate a plethora of final states. The other boson can be 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 $\tau^{+}\tau^{-}b\bar{b}$ in the final state where one of the tau leptons decays to hadrons and a neutrino; and the other $\tau$ to a lighter lepton, either an electron or a muon, and two 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 $\tau$ leptons in data recorded during Run 2 (2016) of the LHC. In the Run 2 analysis, in order to extend the sensitivity, additional final states are considered in which both $\tau$ leptons from the H boson decay hadronically. In addition, the search is extended to consider hadronic decays of the W and Z bosons to $q\bar{q}$. These final states are particularly challenging because, for large resonance masses, the bosons are highly energetic and the final products from their decay are separated by a small angle in space. This collimation implies that the quarks from the hadronically boson decay are reconstructed in one large-cone jet. Novel jet-substructure techniques and dedicated algorithms for the mass reconstruction and flavor identification of the jets are applied to distinguish W, Z, and H bosons. The $\tau$ pair produced from the H boson 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 reconstruct and identify the $\tau$ lepton pairs in this particularly boosted topology. The search is performed by scanning the distribution of the reconstructed mass of the resonance, looking for a local excess in data in comparison with the background prediction. Theoretical scenarios of new particles with a spin of 0, 1, and 2 are investigated, and upper limits are set on their cross-section as a function of mass. These are the first searches for heavy resonances decaying to bosons pair with $\tau$ leptons in the final state of Run 2 of the LHC.