SUSY Search In Final States With Tau Leptons With The ATLAS Detector

The Standard Model(SM)was built up since the later half of the 20th century.It describes the mass and interactions among fundamental particles.Experimentally,ATLAS and CMS announced that they found a new particle with mass around 125 GeV.The particle has been proved to be the standard model Higgs boson.The next year,Noble price for Physics was awarded jointly to Francios Englert and Peter Higgs for theoretical prediction of Higgs.Although the Standard Model has been proved to be very successful in both experimental and theoretical sides,there are lots of problems that can not be solved by SM,such as the gauge mass hierarch problem and the grand unification issue.To solve these problems,a new theory-supersymmetry has been introduced.Supersymmetry introduces a symmetry between fermionic and bosonic particles.The only difference between them is spin.Supersymmetry also provides a suitable dark matter candidate-the lightest supersymmetric particle(LSP).Ditau analysis is based on the phenomenological Minimal Supersymmetric Stan-dard Model.It focus on light stau models,the charginos and neutralinos are assumed to decay with 100%BR to final states containing tau leptons.The first scenario is the direct production of chargino pairs decaying into the lightest neutralino via an interme-diate on-shell τ slepton,referred as ClC1.The second scenario is the pair production of degenerate charginos and next-to-lightest neutralinos,referred as C1N2.The main signatures of the studied final states are:low jet activity,large ETmiss and two or more taus.During the period of master,I mainly working on ditau analysis.The combination of 2015 and 2016 data are used in this analysis,with the total integrated luminosity around 36.1 fb-1.The triggers used are ditau asymmetry trigger and ditau plus ETmiss trigger.The main contribution from my side is the estimation of SM background except for multi-jet contribution.For W + jets,.we design a specialized control region and validation region to correct the contribution in signal region and validate it in validation region.For the other processes,including Z + jets,Top,and diboson WW and ZZ process,we design a validation region for each of them.I am also involved in the sample production and validation.Another study is τ decay mode classification efficiency in Z→ττsignal region.In this study,I use Track Multiplicity Fit Method and BDT method to select variables with high separation power in five decay mdoes.The selected variables will be later used in BDT for further efficiency enhancement.