Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
8 results
Search Results
Item Search for emerging-jet signatures in pp collisions at 13 TeV with CMS using a fully data-based method for background extraction(2023) Chen, Yi-Mu; Belloni, Alberto; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For this thesis, I present a search for emerging jets produced in proton-proton collisions at a center-of-mass energy of 13 TeV. This search examines a hypothetical dark QCD sector that couples to the standard model through a scalar mediator. The scalar mediator decays into a standard model quark and a dark sector quark. As the dark sector quark showers and hadronizes, it produces long-lived dark mesons that subsequently decay into SM particles, resulting in a jet with multiple displaced vertices, known as an emerging jet. This search extends the existing efforts of CMS by including the possibility of a flavored coupling between the standard model sector and dark sector, which results in emerging jets containing both long-lived and prompt decays. This search looks for pair production of the scalar mediators at the LHC, yielding events with two SM jets and two emerging jets. As the detector signature left by such dark sector showers significantly deviates from the assumptions made when designing the detector, a fully data-based method is used for evaluating the number of standard model events that can be mistaken as signal events. The search is carried out on data collected by the CMS experiment corresponding to an integrated luminosity of 138 /fb , and we observe no significant excess. The results are interpreted using two dark sector models and exclude mediator masses up to 1750 GeV for an unflavored dark QCD model and up to 2000 GeV for a flavored-aligned dark QCD model.Item DIRECTED FLOW MEASUREMENTS USING THE SPECTATOR PLANE WITH THE CMS DETECTOR IN THE 2018 PB-PB RUN AT √SNN = 5.02 TEV(2022) Adams, Eric Brian; Mignerey, Alice C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Compact Muon Solenoid (CMS) is used along with the Spectator Reaction PlaneDetectors (SRPD) to measure the directed flow signal, using spectator neutrons, in the 2018 √sNN = 5.02 TeV Pb-Pb run with the Large Hadron Collider (LHC) at CERN. The Hadron Forward detector (HF), along with the tracker, is also used to measure the directed flow signal but instead uses participant nucleons. The results from the CMS 2018 Pb-Pb spectator and participant measurements of the directed flow signal are compared. These are the first spectator plane directed flow measurements from the CMS detector. The results are compared to the Pb-Pb collisions as reported by the A Large Ion Collider Experiment (ALICE) collaboration in 2013 [1].Item Measurement of Wγγ Cross Section and Limits on Anomalous Quartic Gauge Couplings in Proton-Proton Collisions at 8 TeV with the CMS detector(2016) Anelli, Christopher Ryan; Belloni, Alberto; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Present the measurement of a rare Standard Model processes, pp →W±γγ for the leptonic decays of the W±. The measurement is made with 19.4 fb−1 of 8 TeV data collected in 2012 by the CMS experiment. The measured cross section is consistent with the Standard Model prediction and has a significance of 2.9σ. Limits are placed on dimension-8 Effective Field Theories of anomalous Quartic Gauge Couplings. The analysis has particularly sensitivity to the fT,0 coupling and a 95% confidence limit is placed at −35.9 < fT,0/Λ4< 36.7 TeV−4. Studies of the pp →Zγγ process are also presented. The Zγγ signal is in strict agreement with the Standard Model and has a significance of 5.9σ.Item Search for Pair Production of Top Squarks in Proton-Proton Collisions at $\sqrt{s} = 8$ TeV(2015) Calvert, Brian Michael; Hadley, Nicholas J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Supersymmetric extensions to the standard model can solve a number of current, unresolved issues in particle physics. In most of these models, the top-squark, the supersymmetric partner to the top quark, plays an integral role in fixing some of these issues. Although the existence of many supersymmetric particles have been strongly constrained by experiments, currently the existence of the top-squark remains largely unconstrained. This dissertation presents several searches for top-squark pair-production in $R$-parity conserving supersymmetry where the lightest neutralino is assumed to be stable. The data utilized in this search corresponds to 19.66 fb$^{-1}$ of proton-proton collision data collected by the CMS experiment at $\sqrt{s} = 8$ TeV during the 2012 LHC run. The main focus of the dissertation is a search in the dileptonic final state, where the experimental final state is two leptons, two bottom quarks, and missing transverse momentum. Using a cut-based approach, no excess of events above the nominal background expectations is observed. This result is combined with a top-squark search in the semi-leptonic final state to exclude top-squark pair-production at the 95\% confidence level for top-squark masses up to 700 GeV. when the lightest neutralino's mass is below 260 GeV. This dissertation also presents a powerful new approach to the dileptonic top-squark search. Shape-based comparisons, using three complementary discriminating variables, between the observed data and the nominal background expectations achieve much better statistical sensitivity to top-squark pair-production in comparison with the cut-based search. Notably, the shape analysis excludes the existence of top-squarks that are nearly mass-degenerate with the top quark. Currently, no other direct top-squark search can achieve this exclusion. As well, there are a number of observed excesses in the shape analysis. The statistical significances of these excesses are tested against top-squark pair-production models. The subset of models where top-squark decays to a top quark and the lightest neutralino and the mass-splitting between the top-squark and the lightest neutralino is $(150\pm12.5)$ GeV are found to fit with a statistical significance of $\sim 3.5$--$4\sigma$. The global significance of these excesses is quantified by correcting for the look-elsewhere effect; the highest post-correction significances are found to be $\sim 2.5$--$3\sigma$.Item Directed Flow In Heavy Ion Collisions at sqrt{s_NN}=2.76 TeV(2015) Gomez, Jaime Arturo; Mignerey, Alice L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV relative to the collision Event Plane, defined by the participant plane using various subdetectors. The rapidity-even directed flow component is measured for the first time using an Event Plane weighted by the transverse momentum of the emitted charged hadrons and found to be largely independent of pseudorapidity with a sign change at transverse momenta $p_{T}$ between 1.2 and $1.7$ GeV/c. These results are compared to measurements made by two other experiments at the LHC. Combined with the observation from ALICE of a vanishing rapidity-even $p_{T}$ shift along the spectator deflection plane is strong evidence for dipole like initial density fluctuations in the overlap zone of the nuclei. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.Item Search for Pair Production of Third-Generation Scalar Leptoquarks and R-Parity Violating Top Squarks in Proton-Proton Collisions at sqrt(s) = 8 TeV(2014) Pedro, Kevin; Eno, Sarah; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a search for pair production of third-generation scalar leptoquarks or top squarks in R-parity violating supersymmetry decaying through the coupling λ′333, with the new scalar particle decaying to a tau lepton and a bottom quark. The data used in this search were collected by the Compact Muon Solenoid experiment and comprise 19.7 fb-1 of proton-proton collision data from the Large Hadron Collider at a center-of-mass energy of √(s) = 8 TeV. No excess of events above the standard model background prediction is observed. The existence of third-generation scalar leptoquarks or top squarks with masses below 740 GeV is excluded at the 95% confidence level, assuming the branching fraction for the decay to a tau lepton and a bottom quark is 100%. This is currently the strongest limit on this kind of leptoquark or top squark. The search is extended to address a chargino-mediated decay of the top squark involving the R-parity violating coupling λ′3jk, producing a tau lepton, a bottom quark, and two light quarks. The existence of top squarks in this model with masses below 580 GeV is excluded at the 95% confidence level. This is the first search for the pair production of top squarks with the decay involving the R-parity violating coupling λ′3jk.Item Search for Pair Production of First-Generation Scalar Leptoquarks in Proton-Proton Collisions at √s=7 TeV(2011) Ferencek, Dinko; Eno, Sarah C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A search for pair production of first-generation scalar leptoquarks in a final state containing an electron, a neutrino, and at least two jets is performed using proton-proton collision data at √s=7 TeV. The data corresponding to an integrated luminosity of 36 pb-1 were collected by the CMS detector at the CERN LHC. The number of observed events is in good agreement with the predicted yield for Standard Model processes. Prior results from CMS in the dielectron channel are combined with this electron+neutrino search. A combined 95%-confidence-level lower limit of 339 GeV is set on the mass of a first-generation scalar leptoquark for β of 0.5, where β is the branching ratio for the leptoquark to decay to an electron and a quark. These results represent the most stringent direct limits to date for values of β greater than 0.35.Item SEARCH FOR FIRST GENERATION SCALAR LEPTOQUARKS WITH THE CMS DETECTOR AT THE LARGE HADRON COLLIDER(2011) Twedt, Elizabeth Walsh; Baden, Andrew; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Leptoquarks are particles predicated to exist by many models beyond the standard model. A search is presented for the production of first generation scalar leptoquarks with a branching ratio into charged leptons of β = 1 with the Compact Muon Solenoid detector at the Large Hadron Collider at CERN, at a center of mass energy of 7 TeV.