Search for the ttHH(bbbb) nonresonant production in the leptonic final states using machine learning techniques at the CMS Experiment

Date

2024-1

Author

Sökmen Şahin, Gamze

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This thesis presents a search for the production of a top quark-antiquark pair asso- ciated with a pair of Higgs bosons in both semileptonic and dileptonic final states using machine learning techniques. The candidate t ̄tHH events are selected with cri- teria both targeting a lepton and jets decay channels and two leptons and jets decay channels of the tt system and the decay of the double Higgs bosons into two bot- tom quark-antiquark pairs. The dilepton (DL) study is performed for the first time by using the proton-proton collision data collected between the years 2016 and 2018 at the CERN Large Hadron Collider (LHC) in the Compact Muon Solenoid (CMS) experiment at a center-of-mass energy of √13 TeV. The semileptonic (SL) study is also performed for the first time with the upgraded CMS detector at the CERN High- Luminosity(HL)-LHC using proton-proton collisions at a center-of-mass energy of √14 TeV by using simulated samples. In order to increase the sensitivity of both searches, selected events are fed into a multi-classifier deep neural network. For the SL channel, the discriminant outputs of the DNN are split into several b jet multiplic- ity categories with different expected signal and background rates. A simultaneous maximum likelihood fit is performed to evaluate the expected sensi- tivity reach for each channel. For the Run 2 study in the DL channel, no deviation from the background-only hypothesis is observed. A 95% confidence level upper limit on the t ̄tHH production cross section is observed at 94.23 times the standard model (SM) prediction for an expected value of 69.25 for the collision data collected at an integrated luminosity of 41.5 fb−1. The HL-LHC study in the SL channel is expected to exclude t ̄tHH production down to 3.14 times the SM cross section with 3000 fb−1 of data. The sensitivity for Minimal Composite Higgs Model scenarios is also presented.

Subject Keywords

LHC, CMS, Higgs boson, Top kuark, Leptonic

URI

https://hdl.handle.net/11511/108713

Collections

Graduate School of Natural and Applied Sciences, Thesis