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Search for Susy through Vector Boson Fusion Processes in CMS detector at LHC
The Standard Model (SM) of particle physics is the most successful theory to date as it holds every single quantum observation in a single framework. Despite being successful, it fails to explain many mysteries like the unification of forces, absence of dark matter and dark energy and hierarchy prob...
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Lenguaje: | eng |
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2021
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Acceso en línea: | http://cds.cern.ch/record/2753842 |
Sumario: | The Standard Model (SM) of particle physics is the most successful theory to date as it holds every single quantum observation in a single framework. Despite being successful, it fails to explain many mysteries like the unification of forces, absence of dark matter and dark energy and hierarchy problem, etc. There are many theories beyond the Standard Model (BSM) that explain very well about these fundamental problems. Supersymmetry (SUSY) is one such theory that answers these questions. However, for all of its attractive features, there is as yet no experimental evidence. So to verify experimentally, and to explore New Physics, the world's most powerful accelerator Large Hadron Collider (LHC) is built at CERN. Experimental observation of Higgs boson has been given by CERN two general-purpose experiments namely Compact Muon Solenoid (CMS) and A Toroidal LHC ApparatuS (ATLAS) at Large Hadron Collider (LHC) in July 2012.\\ Minimal Supersymmetric Standard Model (MSSM) conserving R-parity leads to the outcome that Lightest Supersymmetric Particle (LSP) is the dark matter candidate and is stable. In this Thesis, the search for SUSY particles (chargino and neutralino) is being presented through the Vector Boson Fusion (VBF) processes in the single hadronic tau ($\tau_{h}$) final state along with two forward jets and missing energy due to LSP and neutrinos. The proton-proton collision dataset used for the present analysis corresponds to the integrated luminosity of 35.87 $fb^{-1}$ at a center-of-mass energy of 13 TeV collected by the CMS detector at the LHC. The signal process involves the pair production of charginos ($\tilde\chi_{1}^{\pm}$) and neutralinos ($\tilde\chi_{2}^{0}$) by the fusion of vector bosons radiated by two incoming partons. Then chargino and neutralino decay to staus ($\tilde{\tau}$) that further decays to LSP ($\tilde\chi_{1}^{0}$) and tau lepton which will finally decay to hadronic tau ($\tau_{h}$). The LSP is {\it bino} like and chargino/neutralino are {\it wino} like SUSY particles. Many SM background contains hadronic taus, jets and $\met$ which can easily fake our signal. Some of the major backgrounds like QCD multijet, W+jets and $t\bar{t}$ are estimated using data-driven and semi-data driven techniques respectively. The $\mt$ ($m_{jj}$) variable is used to distinguish the signal from the background in 1-lepton+jj (0-lepton+jj) channel. Unfortunately, we have not observed excess of data over SM background predictions in this analysis. The results obtained from single hadronic tau final state is combined with the results from the other three channels: single muon, single electron, and 0-lepton channels that are analyzed by other group members. Hence, by using a combined likelihood technique in the bins of $m_{T}$ ($m_{jj}$) in 1-lepton+jj (0-lepton+jj), an upper limit at 95\% confidence level (CL) is set on the production cross section of charginos and neutralinos. Charginos and neutralinos are considered as mass degenerate as they belong to the same Gauge group. Two scenarios are considered: 1) ``slepton'' model i.e. decay of $\tilde\chi_{1}^{\pm}$/$\tilde\chi_{2}^{0}$ through slepton 2) ``WZ'' model i.e. decay through W$^*$ and Z$^*$. The difference in both the models is because of the branching ratio of $\tilde\chi_{1}^{\pm}$/$\tilde\chi_{2}^{0}$ into leptonic final states. In the slepton model, for compressed mass scenarios where the mass difference between chargino/neutralino and LSP is less i.e. $\Delta m $ =1 (30) GeV, exclusion limits of 112 (215) GeV are set on the masses of charginos and neutralinos, whereas in WZ model, for same mass gap i.e. 1 (30) GeV, an upper limit of 112 (175) is obtained. The present analysis obtains the most stringent limit to date on the production of charginos and neutralinos decaying to leptons in compressed mass spectrum scenarios defined by the mass separation $1 < \Delta m < 5 $GeV and $25 < \Delta m < 50$ GeV. On the hardware part, the aging study of resistive plate chamber (RPC) detectors have been performed which are already installed at the CMS muon system. During the high Luminosity (HL-LHC) phase, the luminosity will increase, so to study present RPC detectors' survival, the chambers are placed under continuous radiation in Gamma Irradiation Facility (GIF++) at CERN in same background condition as would be available in HL-LHC. After monitoring the detector parameters and detector performance study, we have not observed any evidence of aging. |
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