OU - Dissertations

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Open Access
The Initial-Final Mass Relation Revisited: A Monte Carlo Approach with the Addition of the M67 White Dwarf Population
(2018-12) Canton, Paul; Kilic, Mukremin; Williams, Kurtis; Baron, Edward; Wisniewski, John; Abbott, Braden; Miller, David
The initial-final mass relation (IFMR) describes how the initial mass of a stellar object maps to its mass as a remnant. The focus of this work is to re-examine this relation for low to intermediate mass stars. Studies of the IFMR have been used in derivations of the age of the galactic components, to measure the total integrated mass loss from stars, to bound the initial mass which will form a white dwarf as opposed to a core collapse supernova, and to study the effects of post-main sequence evolution on exoplanet systems. Given stellar lifetimes far exceed what we can observe in real time it is impossible to measure both a star's mass as it once was when it came to be as a star and its mass as a stellar remnant. For this reason, we look to objects which are white dwarfs (WDs) to get the latter piece of information and use the fact that they are in clusters of known age to get the prior piece of information. With both the initial and final mass of the object in hand it is then possible to fit a function to the data to obtain the IFMR. This dissertation presents the first spectroscopic analysis of WDs in the aged solar metallicity open cluster M67, a reanalysis of the M34, M35, NGC6633, and NGC7063 WD samples, and an IFMR from the application of the spectroscopic technique for deriving WD masses with a Monte Carlo approach to explore the uncertainties in the initial and final masses in an improved way.
Open Access
J/psi meson production in association with a W boson: Cross section ratio measurement with the ATLAS detector using 8 TeV pp data from the Large Hadron Collider at CERN
(2016-05-13) Bertsche, David; Abbott, Braden; Conlon, Paula; Strauss, Michael; Baer, Howard; Santos, Michael
The production mechanism of charmonium particles in hadronic collisions is not fully understood and is an active area of research. These mechanisms can be studied by examining the associated production of charmonium particles in association with vector bosons particles. This thesis examines the associated production of J/psi mesons (the charmonium particle) along with W boson particles using 20.3 fb^-1 of data collected using the ATLAS detector at CERN’s Large Hadron Collider (LHC) accelerator from proton proton collisions at center of mass energy 8 TeV. A measurement of the prompt J/psi + W production rate as a ratio to the inclusive W production rate is made and the results used to provide some insights into the double parton scattering processes.
Open Access
Measurement of the top quark pair production cross section with a tau lepton in the final state at the center of mass energy of 8 TeV
(2016) Hasib, Ahmed; Gutierrez, Phillip; Kantowski, Ronald; Hawthorne, James; Abbott, Braden; Kao, Chung
This dissertation presents the measurement of the inclusive cross section of the top quark pair production in the final state of a tau lepton with associated jets. The dataset used in this measurement is collected by the ATLAS detector from proton-proton collisions during the 2012 operation of the Large Hadron Collider at the center-of-mass energy, $\sqrt{s} =$ 8~\tev. This dataset corresponds to an integrated luminosity of 20.2~\ifb. The inclusive \ttbar production cross section, $\sigma_{\ttbar}$ is found to be $\sigma_{\ttbar} = 231 \pm 3 {\rm (stat.)}^{+25}_{-25} {\rm (syst.)}\pm 3{\rm (lumi.)}~{\rm pb}$ for a top quark mass of 172.5~\gev. This result is consistent with the theoretical prediction $\sigma^{\mathrm{SM}}_{\ttbar} = 253^{+13}_{-15}~{\rm pb}$. Statistical analysis is performed to set a model independent upper limit on the visible cross section of any non Standard Model processes following frequentist probability. An upper limit on the branching ratio of the flavor changing neutral Higgs decay of the top quark to a charm quark is also calculated. The observed (expected) limit on the branching ratio at 95\% CL is $\text{BR}(t \rightarrow ch^0) <$ 10\% (15\%).
Open Access
Measurement of the Top Quark Pole Mass and QCD Radiation in Top Quark Pair Production using Electron-Muon Events with b-Tagged Jets in Proton-Proton Collisions at 7-8 TeV with the ATLAS Detector at the LHC
(2015) Bertsche, Carolyn; Strauss, Michael; Abbott, Braden; Kao, Chung; Murphy, Sheena; Lobban, Lance
As heavy as an atom of gold but much smaller than a proton, the top quark is the most massive fundamental particle known in the universe and an integral part of current particle physics research. The characteristics of the top quark, such as its high mass—close to the scale of electroweak symmetry breaking—and its dominance in the running of the recently discovered Higgs boson’s self-coupling, make the top quark a cornerstone of current research. Studies of the top quark are probes of new physics, essential tools to understand the Higgs boson, and valuable tests of the Standard Model. This thesis presents two measurements of top quark pair production events collected with the ATLAS Detector at the CERN Large Hadron Collider (LHC) in proton-proton collisions at a center-of-mass energy 7-8 TeV. Both measurements select top-antitop quark events with an opposite-sign electron-muon pair and one or two b-tagged jets in the final state. A study is first presented of the jet activity arising from quark and gluon radiation produced in association with top quark pairs, in events with an opposite-sign electron-muon pair as well as at least two b-tagged jets in the final state. It includes the complete 2012 ATLAS data sample of 20.3 fb-1 integrated luminosity of proton-proton collision data at 8 TeV. The fraction of events that does not contain additional jet activity in a central rapidity region is measured as a function of a) the minimum transverse momentum of any additional jet in the event, and b) the minimum scalar transverse momentum sum of all additional jets in the event, and the results given for four central rapidity regions and four regions of the invariant mass of the electron, muon and two b-jet system. Compensation for detector effects is applied to the experimental measurement and the results compared at the particle level to simulations by several next-to-leading order (NLO) and leading order (LO) Monte Carlo generators. The resulting gap fraction measurements, in comparison with simulation, can be used to tune Monte Carlo modeling of quantum chromodynamic (QCD) radiation and reduce associated modeling uncertainties in ATLAS physics measurements involving top quark pair production, such as top quark measurements and Higgs boson measurements with top quark backgrounds. Secondly, the top quark pole mass (mpole) was extracted from top quark pair top events having an opposite-sign electron-muon pair in the final state and one or two b-tagged jets. This measurement includes the complete 2012 ATLAS data sample, as well as the complete 2011 ATLAS data sample of 4.6 fb-1 integrated luminosity of proton-proton collision data at 7 TeV. This is combined with the theoretical cross section prediction including QCD corrections at next-to-next-to-leading order (NNLO+NNLL), with three different sets of parton distribution functions. The results are: top quark pole mass (7 TeV) = 171.40+2.58-2.56 GeV top quark pole mass (8 TeV) = 174.10 ± 2.64 GeV top quark pole mass (7-8 TeV) = 172.87+2.51-2.63 GeV in which the uncertainties include data statistics, experimental systematic effects, the knowledge of integrated luminosity and LHC beam energy, as well as PDF, QCD scale, and strong coupling constant uncertainties. An examination of consistency between the 7 and 8 TeV results found agreement within approximately 1.6 sigma.
Open Access
Naturalness and Novel Statistical Methods on the String Landscape
(2024-08) Martinez, Dakotah; Baer, Howard; Sinha, Kuver; Blume, Doerte; Abbott, Braden; Petrov, Nikola
In this work, we formalize the concept of naturalness in supersymmetric effective field theories, as well as introduce novel methods for performing statistical analyses in the string landscape. We revisit the various measures of practical naturalness for models of weak-scale supersymmetry (SUSY) including: 1. electroweak (EW) naturalness; 2. naturalness via sensitivity to high-scale (HS) parameters [Ellis-Enquist-Nanopoulos-Zwirner/Barbieri-Giudice (EENZ/BG)]; 3. sensitivity of Higgs soft terms due to high-scale radiative corrections; and 4. stringy naturalness (SN) from the landscape. We debut a new numerical routine for calculating these measures from any SUSY Les Houches Accord file. A vast array of (metastable) vacuum solutions arise from string compactifications, each leading to different 4-d laws of physics. The space of these solutions, known as the string landscape, allows for an environmental solution to the cosmological constant problem. We argue that the landscape favors natural softly broken supersymmetric (SSB) models over particle physics models containing quadratic divergences, such as the Standard Model or unnatural SSB models by presenting a computable measure. An anthropic selection of the weak scale to within a factor of a few of our measured value — in order to produce complex nuclei as we know them (atomic principle) — provides statistical predictions for Higgs and sparticle masses in accord with LHC measurements. The predicted Higgs and superparticle spectra might be testable at HL-LHC or ILC via higgsino pair production but is certainly testable at higher energy hadron colliders with $\sqrt{s}\sim$ 30–100 TeV.
Open Access
Observation of opposite sign WW with an associated photon production at a center of mass energy of 13 TeV with the ATLAS detector at the LHC
(2022-12) Lambert, Joseph; Abbott, Braden; Baer, Howard; Magruder, Kerry; Mason, Bruce; Stupak, John
The analysis presented in this thesis expects to provide the first observation of the opposite sign WW with an associated photon process and to measure its fiducial cross section. The analysis uses proton-proton collision data collected between 2015 and 2018 using the ATLAS detector at a center of mass energy of 13 TeV corresponding to an integrated luminosity of 139 inverse fb. Only events where one W boson decays to an electron and the other W boson decays to a muon are considered. Significant deviations from the measured opposite sign WW with an associated photon production cross section and the Standard Model (SM) prediction can provide evidence for beyond the Standard Model (BSM) physics. In addition, any deviation from the SM prediction can be parameterized in the framework of an effective field theory (EFT) and limits can be placed on these EFT parameters. The results of the EFT measurement are beyond the scope of this thesis and will be presented in another thesis. The majority of the background events arise from processes with prompt photons including tt with an associated photon and Z boson production with an associated photon. The analysis also considers backgrounds from non-prompt photons from electrons or hadrons. To improve the measurement sensitivity, a BDT (boosted decision tree) is trained using several well modeled input variables. The BDT combines the information from the input variables into a single variable: the BDT score. The opposite sign WW with an associated photon fiducial cross section and statistical significance are calculated by performing a binned maximum likelihood fit using the BDT output distribution. The expected statistical significance of the measurement is 7.2 σ and the expected fiducial cross section is 10.5 ± 14% (cross section) ± 15% fb (measurement). Where the cross section uncertainty is associated with the determination of the fiducial volume and the measurement uncertainty is associated with the determination of the signal strength using the binned maximum likelihood fit.
Open Access
Observation of the triboson process pp → W±W∓γ and limits on anomalous quartic gauge couplings with the ATLAS detector
(2023-05-12) Wilbern, Daniel; Abbott, Braden; Stupak, John; Kao, Chung; Bumm, Lloyd; Fagg, Andrew
This thesis presents a search for evidence of $W^\pm W^\mp \gamma$ production from $p-p$ scattering with $\sqrt{s}$ = 13 TeV at the Large Hadron Collider using 140 fb$^-1$ of integrated luminosity. The case where the $W$ bosons decay to opposite-flavor light leptons is considered, as other decay channels are dominated by backgrounds. Monte Carlo simulations are used to estimate the contributions of the $W^\pm W^\mp \gamma$ process as well as various background processes to the $e^\pm \mu^\mp \gamma$ channel. The contribution to the $e^\pm \mu^\mp \gamma$ channel of processes with a misidentified or non-prompt photon in the final state are estimated with data-driven methods. A machine learning algorithm trained on Monte Carlo simulations is used to further increase the purity of \wwy events in the dataset. A maximum likelihood fit to the binned distribution of the machine learning discriminant is performed to determine the best-fit value of the \wwy contribution to the \emuy channel. From this, the expected \wwy production cross section in a fiducial region is determined to be $10.5^{+17\%}_{-11\%}\ (\mathrm{theory}) \pm 15\%\ (\mathrm{experiment})$ fb. The observed cross section in the fiducial region will be measured after the \emuy dataset is unblinded. Additionally, to study potential deviations from the Standard Model prediction in the \emuy channel, the ATLAS run-2 dataset is used to set upper and lower limits at the 95\% confidence level on 13 Wilson coefficients of an effective field theory extending the Standard Model with dimension-8 operators. Two methods to restore unitarity to the effective field theory are investigated: the clipping method as well as a dipole form factor model. The dependence of the Wilson coefficients' expected upper and lower limits on these two methods' parameters are presented.
Open Access
Phenomenology of the String Theory Landscape
(2022) Salam, Mohammed Shadman; Baer, Howard; Barboza, Bruno; Abbott, Braden; Baron, Eddie; Jablonski, Michael
In this dissertation, we perform a thorough phenomenological study of the string theory landscape. To this end, we compare and contrast the data collected from particle accelerators and detectors against various models of observable particle phenomena. One stark indirect evidence of underlying string theory is the existence of supersymmetric (SUSY) particles, a variety of new particles resulting from a symmetry between the bosons and fermions observed in nature: i.e. every boson should be paired with a fermionic partner and vice versa. The discovery of the Higgs boson at the LHC in 2012, the particle responsible for giving mass to matter particles (e.g. electrons) and the massive gauge bosons, has provided us with strong bounds on the masses of these yet unobserved superpartner particles, which when combined with string theory landscape arguments, can yield strong statistical predictions for observing SUSY in future upgrades to particle accelerators. Various SUSY models are explored in the context of string landscape statistics by which we can rule some models out as realistic extensions to the Standard Model (SM). We also argue how realistic SUSY models requires the Higgs boson mass to be around 125 GeV with superpartners beyond current energy limits of the LHC - just what is observed experimentally. Additionally, we also analyze the prospect of detecting dark matter particles which only gravitate and exhibit at best only weak interactions. The emergence of SUSY also equips us with such Weakly Interacting Massive Particles (WIMPs), whose mass range can then be statistically predicted using string landscape arguments.
Open Access
Probing Flavor Changing Higgs Interactions at LHC and Future Hadron Colliders
(2020-08) Jain, Rishabh; Kao, Chung; Petrov, Nikola; Abbott, Braden; Baer, Howard; Mullen, Kieran
In this dissertation, we discuss the prospect of discovering flavor changing neutral Higgs (FCNH) interactions with quarks and leptons at current and future hadron colliders. Particularly, we have looked for $t \to c h^0$ and $\phi^0 \to \tau^{\pm} \mu^{\mp}$, where $\phi^0$ could be a CP-even scalar \bigg[$h^0$ (lighter), $H^0$ (heavier) \bigg ] or a CP-odd pseudo-scalar ($A^0$). A general two Higgs doublet model (gTHDM) is used to simulate $t \to c h^0$ and $\phi^0\to \tau^{\pm} \mu^{\mp} $ decays. The LHC measurements of the light Higgs boson ($h^0$) favor the decoupling limit of gTHDM, in which the couplings of $h^0$ approach Standard Model values. In this limit, FCNH couplings of the light Higgs boson $h^0$ are naturally suppressed by a small mixing parameter $\cos(\beta-\alpha)$, while the FCNH couplings of heavier neutral Higgs bosons $H^0, \mathrm{and} A^0$ are sustained by $\sin(\beta-\alpha) \sim 1$. Promising results are found for the LHC collision energies $\sqrt{s} = 13$ TeV and 14 TeV. In addition, we study the discovery potential of future pp colliders, with $\sqrt{s} = $ 27 TeV and 100 TeV. For $\phi^0 \to \tau^{\pm}\mu^{\mp}$, we evaluate the production rate of physics background from dominant processes ($\tau^+\tau^-, WW, ZZ, Wq, Wg, t\bar{t}$) with realistic acceptance cuts and tagging efficiencies. For $t \to c h^0$, where top is coming from top pair production, we consider $h^0 \to WW^* \to \ell^+ \ell^- + \slashed{E}_T$ and $h^0 \to \tau^+ \tau^- \to \ell^+ \ell^- + \slashed{E}_T$ and another top decaying hadronically to a b quark and two light jets. For this report we have studied $h^0 \to WW^*$ and $h^0 \to \tau^+ \tau^-$ separately. Our analysis suggests a reach of 5$\sigma$ or better, with integrated luminosity $\mathcal{L}$ = 3 $ab^{-1}$ and $\sqrt{s}$ = 13, 14 and 27 TeV for $\lambda_{tc}\leq 0.064$, under the current ATLAS limits for both light Higgs decay mode separately. For $h^0 \to WW^*$ we have also presented the discovery potential at 100 TeV.
Open Access
Radiatively-driven Natural Supersymmetry
(2014-08-15) Mickelson, Daniel; Baer, Howard; Milton, Kimball; Abbott, Braden; Mason, Bruce; Petrov, Nikola
Within the framework of supersymmetric theories, a question arises: how can the W, Z, and h masses be so low (∼ 100 GeV) when the superpartner masses are so high (mSUSY > 1−2 TeV)? This is the little hierarchy problem, and can be quantified by studying the fine-tuning of a particular model. Quantifying a model in such a way provides a unique opportunity to give upper bounds on the supersymmetric particle masses. Introduced in this dissertation is the model called Radiatively-driven Natural Supersymmetry, wherein low fine-tuning is achieved while maintaining a light Higgs scalar ≃ 125 GeV. In addition, RNS offers a particle spectrum that evades searches at all current collider experiments, and satisfies cosmological constraints. It is shown that RNS could be discovered with high luminosity at LHC14 in multiple channels, having a soft trilepton + MET signature, a unique same-sign diboson signature accompanied by jets, and gluino cascade decays in the trilepton+jets channel. An International Linear Collider operating at √s = 600 GeV would either discover RNS or rule it out as a feasible model. Dark matter direct and indirect detection experiments also offer a means of discovery, with a 1-ton noble gas detector effectively probing the entirety of RNS parameter space.
Open Access
Search for third generation vector-like leptons with the ATLAS detector
(2022-03-09) Muse, Joseph; Gutierrez, Phillip; Abbott, Braden; Sinha, Kuver; Leighly, Karen; Kornelson, Keri
The Standard Model of particle physics provides a concise description of the building blocks of our universe in terms of fundamental particles and their interactions. It is an extremely successful theory, providing a plethora of predictions that precisely match experimental observation. In 2012, the Higgs boson was observed at CERN and was the last particle predicted by the Standard Model that had yet-to-be discovered. While this added further credibility to the theory, the Standard Model appears incomplete. Notably, it only accounts for 5% of the energy density of the universe (the rest being ``dark matter'' and ``dark energy''), it cannot resolve the gravitational force with quantum theory, it does not explain the origin of neutrino masses and cannot account for matter/anti-matter asymmetry. The most plausible explanation is that the theory is an approximation and new physics remains. Vector-like leptons are well-motivated by a number of theories that seek to provide closure on the Standard Model. They are a simple addition to the Standard Model and can help to resolve a number of discrepancies without disturbing precisely measured observables. This thesis presents a search for vector-like leptons that preferentially couple to tau leptons. The search was performed using proton-proton collision data from the Large Hadron Collider collected by the ATLAS experiment from 2015 to 2018 at center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 139 inverse femtobarns. Final states of various lepton multiplicities were considered to isolate the vector-like lepton signal against Standard Model and instrumental background. The major backgrounds mimicking the signal are from WZ, ZZ, tt+Z production and from mis-identified leptons. A number of boosted decision trees were used to improve rejection power against background where the signal was measured using a binned-likelihood estimator. No excess relative to the Standard Model was observed. Exclusion limits were placed on vector-like leptons in the mass range of 130 to 898 GeV.
Open Access
Searches for a high-mass Higgs-like diboson resonance in the H-->WW-->lvqq decay channel using pp collisions at both sqrt(s) = 8 and 13 TeV with the ATLAS detector at the LHC
(2016-08-16) Pearson, Benjamin; Strauss, Michael; Abbott, Braden; Kao, Chung; Watson, Deborah; Harwell, Jeffrey
This work presents two searches for a high-mass Higgs boson in the H-->WW-->lvqq decay channel using the ATLAS detector to analyze the high-energy proton-proton collisions provided by the Large Hadron Collider at two different center-of-mass energies, sqrt(s) = 8 TeV in 2012 and sqrt(s) = 13 TeV in 2015, corresponding to two independent datasets with sizes given by their integrated luminosities of 20.3 fb-1 and 3.2 fb-1, respectively. No significant excess of data above the expected background is observed in either analysis, so upper limits are set on the production cross-section times branching ratio, as a function of the hypothesized boson mass, for the various signal models tested. The derived limits substantially improve upon previous results.