Search for doubly charged scalar bosons decaying to $W^{\pm}W^{\pm}$ at LHC

The existence of massive neutrinos is strong evidence for physics beyond the Standard Model. The Standard Model (SM) Higgs sector, extended by one weak gauge triplet of scalar fields with a very small vacuum expectation value, is a quite promising setting to account for neutrino masses through the so­called type ­II seesaw mechanism. Such a model is called Doublet­Triplet­Higgs­Model including singly­charged and doubly­charged bosons, which can be directly accessible in TeV­scale experiments, leading to a distinctive phenomenology at the Large Hadron Collider. This thesis presents a search for charged Higgs bosons decaying into $W^{\pm}W^{\pm}$ or $W^{\pm}Z$ bosons, involving experimental signatures with two leptons of the same charge, or three or four leptons with various electric charge combinations, missing transverse momentum, and jets. A data sample corresponding to 139 $fb^{-1}$ integrated luminosity collected by the ATLAS detector at the LHC at a center­of­mass energy of 13 TeV from 2015 to 2018 is used. Two scenarios are explored, corresponding to the pair production of doubly charged $H^{\pm\pm}$ bosons or the associated production of a doubly charged $H^{\pm\pm}$ boson and a singly charged $H^{\pm}$ boson. Three channels are defined as same­charge­two­lepton ($2\ell^{sc}$) channel, three­lepton ($3\ell$) channel, and four­lepton ($4\ell$) channel depending on the lepton multiplicity of final states. In $2\ell^{sc}$ and $3\ell$ channels, the most dominant irreducible background, $W^{\pm}Z$, is constrained by data in a dedicated control region in order to suppress the uncertainty of the normalization. Important reducible backgrounds such as non­prompt leptons and charged flip electrons are estimated by data­driven methods. Only a few SM processes can produce 4ℓ final states; hence the non­prompt leptons in the $4\ell$ channel are estimated by a semi­data­driven method due to the statistics limit. Other backgrounds from the SM are estimated by the Monte­Carlo simulation. The signal regions of three channels are separately designed with rectangular cuts generated by Simulated Annealing (SA) algorithm with several discriminating variables. No significant deviations from the Standard Model predictions are observed. The CLs method is used to derive the upper limits on the signal production cross­section. $H^{\pm\pm}$ bosons are excluded at 95% CL up to 350 GeV and 230 GeV for the pair and associated production modes.