Search For Heavy Majorana Or Dirac Neutrinos And Right-handed W Gauge Bosons In Final States With Two Charged Leptons And Two Jets With The ATLAS Detector

The Standard Model(SM)of particle physics is the theory describing all the fundamental particles and the interactions between each other.Although it has performed extremely well in either predicting properties of particles or explaining experimental results in the past decades,there are still open questions that it leaves unanswered.One of these questions is that neutrino masses have been known to be nonzero from neutrino oscillation experiments but they are predicted to be zero in the Standard Model.The Left-Right Symmetry Models(LRSMs)extend the Standard Model by introducing a right-handed counterpart to the left-handed weak interaction,and predict the heavy right-handed W boson(WR)and right-handed Majorana or Dirac neutrinos(NR).Such that the broken parity symmetry of the Standard Model is restored and the tiny masses of the known neutrinos could be explained through seesaw mechanisms.A search for the WR bosons and NRl(l＝e,μ)neutrions based on the LRSMs,with a final state containing two opposite-sign same flavor leptons(e,μ)and two jets,is presented in this thesis.The search was conducted using the proton-proton collison data with an integrated luminosity of 36.1 fb-1.The data was collected during 2015 and 2016 by the ATLAS detector at the Large Hadron Collider(LHC)with a center-of-mass energy of(?)TeV.The major background events arise from the Z+jet(s)and top quark processes.The contribution of the two dominant backgrounds were estimated with data-driven method,while other less important backgrounds were directly evaluated from Monte Carlo simulation.No significant deviation from the Standard Model expectation is observed in any channel.95%confidence level exclusion limits are set in the plane of WR mass and NR mass for both Majorana and Dirac NR hypotheses,in electron and muon channels.95%upper limits are also derived for the Keung-Senjanovic process(pp→WR→lNR→llWR→lljj)on the cross section.A statisical combination was performed using the results from this opposite-sign analysis and a separate search targeting the same-sign lepton pair final state.The improved exclusion limits reach to about 4.7 TeV for the WR mass and to about 1-3 TeV for the NR mass.