| The Higgs Boson, as the final piece of the Standard Model, was observed by the ATLAS and CMS collaborations at LHC in 2012. Its mass was measured to be around 125 GeV. The measurements of its properties show the consistence with the Standard Model predictions. Some extensions of the Standard Model predict the existence of additional heavy Higgs partners aiming for solving the known problems, that can not be properly explained by the Standard Model. Searches for those particles are crucial for testing the new physics beyond the Standard Model.The ATLAS detector is a general purpose detector at LHC. It is designed for the mea-surements of the Standard Model particles properties, and more ambitious and interesting mission, investigation of new physics. The ATLAS detector had great successful runs before end of 2012 and collected 4.7 fb-1 data at (?)= 7 TeV and 20.3 fb-1 data at (?)= 8 TeV. After two years maintenance and upgrade, ATLAS starts the new runs with higher energy at (?)= 13 TeV and collects 3.2 fb-1 data in 2015 and up to 35 fb-1 data in 2016. The higher energy open a new era for the new physics searches, especially for heavy particles.This thesis presents a search for heavy Higgs boson in H ? ZZ ? llvv final state with ATLAS detector at (?)= 13 TeV. The data luminosity used for this search corresponds to 13.3 fb-1. A neutrino escapes the detector without interaction. Its energy is not directly measured, and is the main contribution to the missing transverse momentum (ETmiss), making this quantity crucial for this search. In addition, new physics beyond the Standard Model can produce non-interacting particles, such as neutralinos, which contribute even more to the ETmiss.This thesis also presents the methodology of the ETmiss reconstruction based on the ATLAS inner detector measured tracks and other reconstructed objects. Using tracks in ETmiss measurements help to identify whether the interaction originates from hard scattering vertex or pileup vertices. Two ETmiss reconstruction algorithms, explicitly using tracks, were developed for Run-II high luminosity:the track-based ETmiss (pTmiss) and the track-based soft term (TST) ETmiss. The performance of ETmiss using data collected with the ATLAS detector at (?)= 13 TeV is presented in this thesis along with its systematic uncertainty. The TST ETmiss has better performance on the resolution with respect to pileup and similar performance on the scale measurement comparing to the cluster-based soft term ETmiss. The improvement is crucial in the high pileup environment. The TST ETmiss is recommended and widely used in ATLAS physics analyses using 13 TeV data. The pTmiss performs the best pileup resistance, while the scale measurement is biased by the missing neutral particles contribution.Choosing llvv final state as a search channel is due to its clearer event topology than the llqq and vvqq final states, and larger production cross section than the 4l final state. The main backgrounds in this search are dibosons.W+jets, Z+jets, tt and single top quark productions. A mix of data-driven and Monte Carlo methods were used to reduce and estimate the contributions from all background processes in the signal search region. The Higgs boson transverse mass (mT), calculated from ETmiss and the two final state leptons, is used as discriminating variable to separate the signal from backgrounds. A maximum binned likelihood fit is performed on mT distribution to extract the observed number of signal events from data. The search is performed in a Higgs boson mass range 300-1000 GeV. No significant excess is observed and the data agree with the backgrounds from the Stand Model predictions. A model-independent 95% confidence level upper limits on the cross section of pp?H?ZZ process are derived and presented as a function of the Higgs boson mass. For mH= 300 GeV, the observed cross section limit is 1343 fb, and is 49 fb for mH= 1000 GeV. Moreover, two beyond the Standard Model benchmark models are tested with current cross section limits. |
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