An investigation of the sensitivity of LHC dark matter searches at $\sqrt{s}$ = 13 TeV to dark Higgs models

The ‘Higgs Portal Dark Matter’ model is introduced in this thesis. A dark Higgs boson is responsible for generating masses for dark sector particles. Since this thesis focuses on dark Higgs decaying to a pair of b-tagged jets, the experimental signature is one boosted double b-tagged jet with large missing energy. Two different and complementary search methods have been investigated in this thesis: the largeradius mono-jet search and the more “generic” missing energy + jets search using the CMS αT analysis. The former is carried out by the ATLAS experiment, searching for a boosted double b-tagged large-radius jet with large missing energy. The latter is based on a CMS search, searching for resolved b-jets with specific kinematic variables and large missing energy. In the recast of the large-radius search, all signal events are generated by MadGraph5 aMC@NLO v2.3. 3 in the two coupling scenarios presented in Section 3.2. Pythia 8 and Rivet v2.5. 2 are used for showering and event selection. Finally, HiggsCombineTool from CMSSW 8 1 0 is applied to calculate the expected exclusion upper limits at 95% confidence level (CL) with an integrated luminosity of 35.9 fb−1. The recast of the large-radius search shows that signals with couplings according to the LHC recommendation can obtain significant exclusion limits. When the mass of the dark Higgs (ms) is 50, 70 and 90 GeV, the mass of the Z ′ (mZ′ ) up to 1800, 2000 and 1800 GeV are respectively excluded. The recast of the largeradius search shows that signals with couplings set to fulfil the DM relic abundance constraint can obtain significant exclusion limits. For ms = 50, 70 and 90 GeV, mZ′ up to 3300, 3450 and 3500 GeV are respectively excluded. In the reinterpretation of the CMS αT search, all signal events are generated by MadGraph5 aMC@NLO v2.3. 3 using the same coupling scenarios as the largeradius search recast. To estimate backgrounds in this reinterpretation, we use the data recorded by the CMS experiment in 2016 together with the estimated background numbers provided in Ref. [12]. Pythia 8 and Rivet v2.5. 2 are used for showering and event selection. We use the same event selection as in Ref. [12]. As the dataset in Ref. [12] is binned in 224 bins, we select the most sensitive ones for our analysis using the Asimov Test. Finally, HiggsCombineTool from CMSSW 8 1 0 is again applied here to calculate the expected and observed sensitivities at 95% CL with an integrated luminosity of 35.9 fb−1 based on the selected most sensitive bins. The reinterpretation of the CMS αT search shows that signals with couplings according to the LHC recommendation cannot obtain significant exclusion limits. However, the reinterpretation of the CMS αT search shows that signals with couplings set to fulfil the DM relic abundance constraint can obtain significant exclusion limits. For ms = 50, 70 and 90 GeV, mZ′ up to 3600, 3700 and 3700 GeV are respectively excluded. Both the recast of the large-radius search and the reinterpretation of the CMS αT search give new experimental sensitivities to dark matter searches through the ‘Higgs Portal Dark Matter’ model.