Theses and Dissertations - Department of Physics & Astronomy

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Browsing Theses and Dissertations - Department of Physics & Astronomy by Author "Agashe, Kaustubh"

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    125 gev higgs boson mass from 5d gauge-higgs unification
    (University of Alabama Libraries, 2015) Carson, Jason Carl; Okada, Nobuchika; University of Alabama Tuscaloosa
    In the context of a simple gauge-Higgs unification (GHU) scenario based on the gauge group SU(3)×U(1)′ in a 5-dimensional flat space-time, we investigate a possibility to reproduce the observed Higgs boson mass of around 125 GeV. We introduce bulk fermion multiplets with a bulk mass and a (half) periodic boundary condition. In our analysis, we adopt a low energy effective theoretical approach of the GHU scenario, where the running Higgs quartic coupling is required to vanish at the compactification scale. Under this "gauge-Higgs condition," we investigate the renormalization group evolution of the Higgs quartic coupling and find a relation between the bulk mass and the compactification scale so as to reproduce the 125 GeV Higgs boson mass. Through quantum corrections at the one-loop level, the bulk fermions contribute to the Higgs boson production and decay processes and deviate the Higgs boson signal strengths at the Large Hadron Collider (LHC) experiments from the Standard Model (SM) predictions. Employing the current experimental data which show the the Higgs boson signal strengths for a variety of Higgs decay modes are consistent with the SM predictions, we obtain lower mass bounds on the lightest mode of the bulk fermions.
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    Collider phenomenology of heavy neutrinos
    (University of Alabama Libraries, 2016) Das, Arindam; Okada, Nobuchika; University of Alabama Tuscaloosa
    The existence of the neutrino mass has been established by the neutrino oscillation experiments. The so-called seesaw extension of the Standard Model is probably the simplest idea to naturally explain the existence of tiny neutrino mass through the lepton number violating Majorana mass term. There is another alternative way, commonly known as the inverse seesaw mechanism, where the small neutrino mass is obtained by the tiny lepton number violating parameters. In this work we investigate the signatures of such heavy neutrinos, having mass in the Electroweak scale at the high energy colliders. Based on a simple realization of inverse seesaw model we fix the model parameters to reproduce the neutrino oscillation data and to satisfy the other experimental constraints. We assume two flavor structures of the model and the different types of hierarchical light neutrino mass spectra. For completeness we consider the general parameterization for the model parameters by introducing an arbitrary orthogonal matrix and the nonzero Dirac and Majorana phases. Due to the smallness of the lepton number violating parameter this model can manifest the trilepton plus missing energy at the Large Hadron Collider(LHC). Using the recent LHC results for anomalous production of the multilepton events at $8$ TeV with a luminosity of $19.5$ fb$^{-1}$, we derive the direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy neutrino mass. Using a variety of initial states such as quark-quark, quark-gluon and gluon-gluon as well as photon mediated processes for the Majorana heavy neutrinos we obtain direct upper bounds on the light-heavy neutrino mixing angles from the current LHC data at $8$ TeV. For the pseudo-Dirac heavy neutrinos produced from the various initial states using the recent anomalous multilepton search by the LHC at $8$ TeV with $19.5$ fb$^{-1}$ luminosity, we obtain upper bounds on the mixing angles.
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    A tale of two standard model extensions
    (University of Alabama Libraries, 2019) Villalba, Desmond; Okada, Nobuchika; University of Alabama Tuscaloosa
    The Standard Model (SM) has provided physicists with a nearly complete effective description of the fundamental building blocks for the Universe. While several questions regarding the makeup of our Universe have been resolved, many still remain. One such question deals with the vast energy difference between the electroweak scale and Planck scale (${\cal O}(10^{17}\, {\rm GeV})$). The origin of this large divide has physical implications affecting the running of the Higgs mass, as it receives quantum corrections that are quadratic. Affiliated with this large division of energy scales is an issue that came about upon detection of the Higgs boson at the Large Hadron Collider (LHC), which enabled us to infer the value of the Higgs self-coupling. As a result, the renormalization group equation for the Higgs self-coupling predicts a negative self-coupling at around energies of $10^9 - 10^{11} $ GeV. If true, this would indicate that our vacuum state is unstable. Taking our motivation from stringy effects by modifying the local kinetic term of an Abelian Higgs field by the Gaussian kinetic term, we show that the Higgs field does not possess any instability, and the beta-function of the self-interaction for the Higgs becomes exponentially suppressed at high energies, showing that such class of theory never suffers from a vacuum instability. Another interesting question to consider, is what might be the origin of the large mass difference between the fundamental fermions? As will be shown, the fermion mass hierarchy can be explained through the use of our formalism developed in our setup of the "Domain-Wall Standard Model in a non-compact 5-dimensional space-time", where all the SM fields are localized in certain domains of the 5th dimension. Reproducing the hierarchy is contingent upon the localization positions of the fermions along the extra flat dimension. As a result of these different localization points, the effective 4-dimensional Kaluza-Klein mode gauge couplings become non-universal. This allows for the possibility of interesting experimental considerations which will be discussed. Flavor Changing Neutral Current constraints provide stringent bounds on our model, through these constraints we can glean information about the extra dimension. We have found two possibilities that satisfy these constraints: (1) the KK mode of the SM gauge bosons are extremely heavy and unlikely to be produced at the LHC, however future FCNC measurements can reveal the existence of these heavy modes. (2) the width of the localized SM fermions is very narrow, meaning the 4D KK mode gauge couplings are almost universal. In this case the FCNC constraints can be easily avoided, even for a KK gauge boson mass of order TeV. Such a light KK gauge boson can be discovered at the LHC in the near future.