UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Cosmological Phase Transition of Composite Higgs Confinement
    (2021) Ekhterachian, Majid; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We study the cosmological confinement-deconfinement phase transition (PT) of nearly conformal, strongly coupled large N field theories, applicable to composite Higgs models. We find that despite strong coupling, aspects of the PT can be analyzed when the confinement is predominantly spontaneous. In this scenario, the leading contribution to the transition rate can be computed within effective field theory of dilaton-- the pseudo Nambu-Goldstone boson associated with the spontaneous breaking of conformal symmetry. We then show how the holographic dual formulation in terms of 5D warped compactifications allows for qualitative understanding of the missing pieces of the earlier described 4D picture and a quantitative improvement of the calculations. In this description the PT is from a high-temperature black-brane phase to the low-temperature Randall-Sundrum I phase, and the transition proceeds by percolation of bubbles of IR-brane nucleating from the black-brane horizon. We show that the bubble configuration interpolating between the two phases can be smooth enough to be described within 5D effective field theory. We find that cosmological PT in the minimal models can complete only after a large period of supercooling, potentially resulting in excessive dilution of primordial matter abundances. We then show how generic modifications of the minimal models can result in a much faster completion of the PT. We also study the stochastic gravitational wave background produced by the violent bubble dynamics and discuss the implications of the PT for baryogenesis.
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    H (sub) alpha & Neutral Density Scaling in the Maryland Centrifugal eXperiment
    (2009) Clary, Ryan; Ellis, Richard; Hassam, Adil; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Maryland Centrifugal eXperiment (MCX) is a hydrogen plasma confinement experiment with a rotating mirror magnetic configuration. This experiment was designed to test the concepts of centrifugal confinement and velocity shear stabilization which may allow scaleability to a fusion reactor. These two concepts, however, rely on supersonic plasma fluid velocities, which, apart from possible plasma instabilities, could be greatly reduced by fluid drag with neutral hydrogen, leading to decreased confinement. Resonant charge exchange between a hydrogen ion and a hydrogen atom is believed to be the dominant drag mechanism on the rotating plasma. Neutral hydrogen emission lines (particularly the Balmer-alpha line, H (sub) alpha) are therefore of primary interest in diagnosing how neutral hydrogen affects plasma confinement. For this purpose, a multi-chord H (sub) alpha emission detector (multi-chord HED) was designed and constructed by the author in order to measure emissivity profiles. These profiles, together with an atomic collisional-radiative model, provide estimates of neutral hydrogen density and local charge-exchange times. Varied experimental parameters were applied to MCX discharges and the resulting variations in neutral density are compared to theoretical scaling laws. The charge-exchange times are compared to the measured momentum confinement time. We find that the inner and outer-most flux surfaces are not distinctly identified by the emissivity profile and the emissivity is dominant at the vacuum chamber wall. We also find that, while the overall emissivity profile does not match theoretical prediction, neutral density scaling is approximately described by the models. In addition, charge-exchange times are found to be much smaller than the momentum confinement time as well as to scale differently than the momentum confinement time. This dissertation includes a detailed description of the multi-chord HED system and its calibration, both spectrally and absolutely. We also present models based on neutral and plasma interaction which provide the scaling laws used to compare to experimental results.