One of the important early searches at the Large Hadron Collider at CERN is for high mass Z-like objects. Such objects are predicted in a number of extensions to the Standard Model and the analyses may become sensitive to their decays at fairly modest integrated luminosities. This thesis decribes a search for a neutral, heavy gague boson decaying into boosted top quark pairs using the first 5.0 fb-1 of sqrt(s) = 7 TeV data from the 2011 physics run of the LHC. By exploiting the semi-muonic decays of the top pairs and using boosted decision trees to perform the final event selection we set upper limits on the cross section of a heavy, Z-like gague boson of 5 pb at the lowest masses (500 GeV) to less than 1 pb for masses above 1 TeV.

Within the standard model top quarks are predicted to be most often produced in pairs

via the strong interaction. However they can also be produced singly through the weak

interaction. This is a rarer process with many experimental challenges. It is interesting

because it provides a new window to search for evidence of physics beyond the standard

model picture, such as a fourth generation of quarks or to search for insight into the Higgs

Mechanism. Single top production also provides a direct way to calculate the CKM matrix

element Vtb.

This thesis presents new measurements for single top quark production in the s+t, s and

t channels using 5.4 fb-1 of data collected at the DØ detector at Fermilab in Batavia, IL. The analysis was performed using Boosted decision trees to separate signal from background and Bayesian statistcs to calculate all the cross sections. The results obtained are:

s channel : σ (ppbar → tb + X) = 0.68 +0.41 - 0.39 pb,

t channel : σ (ppbar → tqb + X) = 3.03 +0.78 - 0.66 pb,

Combined s+t channels : σ(ppbar → tb + X, tqb + X) = 3.01 +0.80 - 0.75 pb.

The s channel has a Gaussian significance of 1.6σ, the t channel 5.5σ, and the s+t 4.3σ. The results are consistent with the standard model predictions within one standard deviation. By combining these results with the results for two other analyses (using different MVA techniques) improved results of:

Combined s+t channels : σ(ppbar → tb + X, tqb + X) = 3.43 +0.73 - 0.74 pb,

t channel : σ(ppbar → tqb + X) = 2.90 +0.59 - 0.59 pb.

were obtained. These give a 5.6σ significance for the combined s + t channel and 5.5σ for the t channel.

The search for the production of four top quarks decaying in the dileptonic channel in proton-proton collisions at the LHC is presented. The analysis utilises the data recorded by the CMS experiment at sqrt{s} = 13 TeV in 2015, which corresponds to an integrated luminosity of 2.6 inverse femtobarns. A boosted decision tree algorithm is used to select signal and suppress background events. Upper limits on dileptonic four top quark production of 14.9 times the predicted standard model cross section observed and 22.3 (+16.2) (-8.4) times the predicted standard model cross section expected are calculated at the 95% confidence level. A combination is then performed with a parallel analysis of the single lepton channel to extend the reach of the search.

A search for a rare Standard Model Process, four top quark production in the di-lepton plusjets final state (e e, μ μ, and e μ), is presented. The analysis utilizes data equivalent to 101.5 fb−1 of luminosity at √s = 13 TeV recorded by the CMS experiment during 2017 and 2018. Following a baseline selection to choose events with a pair of opposite sign charged leptons, four or more hadronic jets, of which at least 2 jets are b-tagged, the data are classified in bins of jet multiplicity (4, 5, 6, 7, and 8 or more jets), and b-tag multiplicity (2, 3, and 4 or more b-tagged jets). Events with at least 7 jets and 3 b-tagged jets are the most sensitive to the tttt signal. The HT distributions for the 15 categories per year and decay channel are fitted to simulation using Maximum Likelihood Estimation to obtain an upper limit on the cross section σtt tt . An a-priori expected limit of 3.3+4.6−1.7 × σSMtt tt (39+56−20fb) is obtained at the 95% CL, with an a-priori expected signal significance of 0.65σ. The a-posteriori asymptotic limits are 3.0+4.4−1.6 × σSMtt tt (36+53−19fb). The asymptotic observed limits are 6.0 × σSMtt tt (72fb) with a significance of 1.6σ.

In this analysis we present the first results of the measurement of the shape parameter rb in the Lund-Bowler fragmentation function for the b quark in a ttbar environment. The analysis uses charmed mesons produced in the leptonic decays of ttbar at sqrt(s)=13 TeV in the CMS detector using the full 2016 dataset with an integrated luminosity of 35.9 fb^-1. The charged particle decays of J/Psi -> mu^+ mu^- and D^0 -> K pi are used as proxies for the parent b quark via the ratio xB (the pT of the charmed meson divided by the sum of the transverse momentum of all charged particles in the jet containing the meson) to measure the shape parameter rb. The shape parameter rb is measured to be rb = 0.841 +/- 0.014 (stat) +/- 0.031 (syst).

This dissertation presents a search for the Standard Model production of four top quarks (pp$\rightarrow$\tttt) using events with opposite-sign dilepton ($\mu^+\mu^-$, $\mu^\pm e^\mp$, or $e^+e^-$)+jets signatures. The data used in this analysis are from proton-proton collisions at a center-of-mass energy $\sqrt{s}=13$ TeV recorded by the CMS detector at LHC during year 2016. The total integrated luminosity of the data corresponds to 35.8 $\mathrm{fb}^{-1}$. A multivariate analysis method is used to discriminate \tttt signal events from background events based on global event and jet properties. No significant deviation is observed in data from the prediction of the background. An upper limit is set on the cross section of Standard Model \tttt production to be 64 fb at 95\% confidence level. A combination of the measurements with results from single-lepton ($\mu$, $e$)+jets final states and a previous measurement from the CMS experiment is performed and the combined cross section measurement is $13^{+11}_{-9}$ fb, with an observed signal significance of 1.4 standard deviations.