Theses and Dissertations from UMD

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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

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Now showing 1 - 5 of 5
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    DETERMINING THE NEUTRINO LIFETIME FROM COSMOLOGY
    (2021) Dev, Abhish; Chacko, Zackaria ZC; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Neutrinos are the most mysterious particles in the standard model. Many of theirfundamental properties such as their masses, lifetimes, and nature (Dirac or Majorana) are yet to be pinned down by experiments. Currently, the strongest bound on neutrino masses comes from cosmology. This bound is obtained by scrutinizing the gravitational effect of the cosmic neutrinos on the evolution of structure in our universe. However, this bound assumes that the neutrinos from the Big Bang have survived until the present day. In this dissertation, the unstable neutrino scenario is studied in light of current and near-future cosmological experiments. We show that the current cosmological bound on the neutrino masses can be relaxed significantly in an unstable neutrino scenario. We further show that near-future experiments offer the possibility of independently measuring both the masses of the neutrinos and their lifetimes. We consider an elusive scenario in which the cosmic neutrinos decay into invisible radiation after becoming non-relativistic. The Boltzmann equations that govern the cosmological evolution of density perturbations in the case of unstable neutrinos are derived and solved numerically to determine the effects on the matter power spectrum and lensing of the cosmic microwave background (CMB). A Markov-Chain Monte-Carlo (MCMC) analysis is done on the current cosmological data and mock future data to obtain its sensitivity to the neutrino masses and lifetimes. We show that the effect of the neutrino masses on large scale structures is dampened by the decay of neutrinos, which leads to a parameter degeneracy between the neutrino masses and lifetimes inferred from the cosmological data. This degeneracy allows for a significant relaxation of the current cosmological upper bound on the sum of neutrino masses from about 0.2 eV in the stable neutrino case to 0.9 eV in the unstable neutrino scenario. This window is important for terrestrial experiments such as KATRIN which are seeking to independently measure the neutrino masses in the laboratory. We further show that near-future large scale structure measurements from the Euclid satellite, when combined with cosmic microwave background data from Planck, may allow an independent determination of both the lifetimes of the neutrinos and the sum of their masses. These parameters can be independently determined because the Euclid data will cover a range of redshifts, allowing the growth of structure over time to be tracked. If neutrinos are stable on the timescale of the age of the universe, we show that these observations can improve the lower limit on the lifetimes of the neutrinos by seven orders of magnitude, from O(10) years to 2 × 108 years(95%C.L.), without significantly affecting the measurement of the neutrino masses. On the other hand, if neutrinos decay after becoming non-relativistic but on timescales less than O(100) million years, these observations may allow for, not just the first measurement of the sum of neutrino masses, but also the determination of the neutrino lifetime from cosmology.
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    Left-right symmetric model and its TeV-scale phenomenology
    (2017) Lee, Chang Hun; Mohapatra, Rabindra Nath; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Standard Model of particle physics is a chiral theory with a broken parity symmetry, and the left-right symmetric model is an extension of the SM with the parity symmetry restored at high energies. Its extended particle content allows us not only to find the solution to the parity problem of the SM but also to solve the problem of understanding the neutrino masses via the seesaw mechanism. If the scale of parity restoration is in the few TeV range, we can expect new physics signals that are not present in the Standard Model in planned future experiments. We investigate the TeV-scale phenomenology of the various classes of left-right symmetric models, focusing on the charged lepton flavour violation, neutrinoless double beta decay, electric dipole moments of charged leptons, and leptogenesis.
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    Limits on Neutrino Emission from Gamma-Ray Bursts with the 40 String IceCube Detector
    (2012) Meagher, Kevin James; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Cosmic rays have been observed on Earth with energies in excess of 1020 eV. Because cosmic rays are charged particles and are bent by galactic magnetic fields, the origin of these particles has remained a mystery. Gamma-ray bursts are one of a few astronomical sources containing an environment capable of accelerating charged particles to the energies observed. In addition, gamma-ray bursts are the leading candidate due to the fact that the total aggregate power observed in gamma-ray bursts and ultra high energy cosmic rays are the same order of magnitude. Neutrinos can only be created by hadronic interactions, so an observation of neutrinos in coincidence with a gamma-ray burst would provide compelling evidence that hadrons are accelerated in gamma-ray burst fireballs and hence the origin of cosmic rays. Using the IceCube Neutrino Observatory in its 40 string configuration, a stacked search was performed to look for the simultaneous occurrence of muon neutrinos with 117 gamma-ray bursts. This analysis is optimized on the assumption that order TeV neutrinos are produced in pγ interactions during the prompt phase of the GRB, when gamma-rays coexist with protons that are assumed to be the source of the observed extragalactic cosmic ray flux. With half the detector complete, this is the first analysis sensitive to the flux predicted by fireball phenomenology and the assumption that GRBs are the sources of the highest energy cosmic rays. No evidence for neutrino emission was found, placing a 90% CL upper fluence of 1.1 × 10-3 erg cm-2 in the energy range of 37 TeV - 2.4 PeV or 82% of the predicted fluence.
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    The Search for Neutralino Dark Matter with the AMANDA Neutrino Telescope
    (2008) Ehrlich, Ralf; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole. Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and  electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton. AMANDA - the Antarctic Muon and Neutrino Detector Array - makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected. If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation.
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    A SUSY SO(10) GUT Model with Lopsided Structure
    (2007-02-27) Li, Yingchuan; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The standard model (SM) of elementary particles has been established for more than 30 years and tested by a large number of experiments. However, because of the naturalness problem of the electroweak symmetry breaking scale and a large number of unexplained parameters in SM, physicists have been looking for a more fundamental theory. Supersymmetry (SUSY) and grand unification are two appealing concepts that have been mostly implemented to build candidates for beyond SM theories. SUSY helps to stabilize the scale of electroweak symmetry breaking, and grand unification embeds the SM gauge groups into larger and more fundamental gauge groups. Neutrino oscillations, signaling massive neutrinos, are the first direct evidence of beyond SM physics. A tiny neutrino mass can be elegantly explained by the seesaw mechanism. The neutrino masses from this mechanism are of Majorana type and therefore break the B-L (baryon number minus lepton number) symmetry. A favorable framework of studying neutrino masses and oscillations is the SO(10) grand unification theory (GUT) which naturally accommodates a B-L breaking. The same B-L breaking can also facilitate baryogenesis via a leptogenesis scenario. This provides an interesting correlation between these two pieces of phenomenology. This thesis presents a realistic SUSY SO(10) GUT model with lopsided structure, which generates the correct masses and mixing of neutrinos and produces the right amount of baryon asymmetry. One of the most characteristic features of this model is the lopsided mass matrices structure. We examine observables in B decays that are sensitive to this structure, and find a specific pattern of predictions that can be used to test this type of models.