Browsing by Author "Arce, Ayana T"

The J/ψ particle has been the source of much research since its

discovery in 1974. It provides an important probe of quantum

chromodynamics and has lead to many important insights into the

interactions of quarks and gluons in bound states. The rate of J/ψ

production was found to be much higher than expected at hadron

colliding experiments. Non-relativistic quantum chromodynamics was

developed in order to address these issues. This theory predicts a

strong spin-alignment not observed in data. All previous measurements

of J/ψ production have overlooked the hadronic environment the

J/ψ is produced in. This work is the first exploration of the

radiation surrounding J/ψ events measured at ATLAS at

√s=8 TeV. This is the first measurement of the

separation between the J/ψ and a matched jet and the second

measurement of the momentum fraction shared between the jet and the

J/ψ. These variables probe the radiation environment around the

J/ψ, and provide a new ways to understand quarkonia production.

A search for new heavy resonances decaying to boson pairs (WZ, WW or ZZ) using 20.3 inverse femtobarns of proton-proton collision data at a center of mass energy of 8 TeV is presented. The data were recorded by the ATLAS detector at the Large Hadron Collider (LHC) in 2012. The analysis combines several search channels with the leptonic, semi-leptonic and fully hadronic final states. The diboson invariant mass spectrum is studied for local excesses above the Standard Model background prediction, and no significant excess is observed for the combined analysis. 95$\%$ confidence limits are set on the cross section times branching ratios for three signal models: an extended gauge model with a heavy W boson, a bulk Randall-Sundrum model with a spin-2 graviton, and a simplified model with a heavy vector triplet. Among the individual search channels, the fully-hadronic channel is predominantly presented where boson tagging technique and jet substructure cuts are used. Local excesses are found in the dijet mass distribution around 2 TeV, leading to a global significance of 2.5 standard deviations. This deviation from the Standard Model prediction results in many theory explanations, and the possibilities could be further explored using the LHC Run 2 data.

A search for supersymmetry in pair-produced gluinos decaying via top squarks to the lightest neutralino is presented. Events with multiple hadronic jets, of which at least three must be identified as originating from b-quarks, and large amounts of missing transverse energy in the final state, are selected for study. The dataset utilized encompasses proton-proton collisions with a center-of-mass energy of sqrt(s) = 13 TeV and integrated luminosity of 79.9 fb-1 collected by the ATLAS experiment at the LHC from 2015 to 2017. The search employs a parameterized boosted decision tree (BDT) to separate supersymmetric signal events from standard model backgrounds. New methods for optimal BDT parameter point selection and signal region creation, as well as new soft kinematic variables, are exploited to increase the search's expected exclusion limit beyond prior analyses of the same dataset by 100-200 GeV in the gluino and neutralino mass plane. No excess is observed in data above the predicted background, extending the previous exclusion limit at the 95% confidence level by 250 GeV to approximately 1.4 TeV in neutralino mass. The analytical and machine learning techniques developed here will benefit future analysis of additional Run 2 data from 2018.

A search for single production of Vector-Like quarks decaying to the Higgs boson and a top quark, or a massive vector boson and a top quark is presented. Events with exactly one high momentum lepton, at least three hadronic jets, of which at least one originates from a b-quark, and large missing transverse energy are selected. Data was collected from proton-proton collisions at center of mass energy sqrt(s)=13 TeV and integrated luminosity of 139 fb -1 representing the full LHC Run 2 dataset, using the ATLAS detector. This search uses the event effective mass as a discriminating variable in a binned maximum likelihood fit. No significant excess of events is observed in the data above predicted background. Exclusion limits on the signal production cross section are set at 95% confidence level, extending the exclusion limits to masses of 1.9 TeV for singlet T models of Vector-Like quarks, with universal coupling strength κ > 0.6. These results also exclude single production of the exotically charged X^5/3 Vector-Like quark at a mass of 1.5 TeV for coupling κ = 0.5. We also present a study of jet charge in searches for high mass exotically charged particles. We expect little benefit from use of jet charge as studied here, but expect that future work can extract more value.