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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Item A Search for Muon Neutrinos in Coincidence with Gamma-Ray Bursts in the Southern Hemisphere Sky Using the IceCube Neutrino Observatory(2016) Maunu, Ryan Edward; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The origin of observed ultra-high energy cosmic rays (UHECRs, energies in excess of $10^{18.5}$ eV) remains unknown, as extragalactic magnetic fields deflect these charged particles from their true origin. Interactions of these UHECRs at their source would invariably produce high energy neutrinos. As these neutrinos are chargeless and nearly massless, their propagation through the universe is unimpeded and their detection can be correlated with the origin of UHECRs. Gamma-ray bursts (GRBs) are one of the few possible origins for UHECRs, observed as short, immensely bright outbursts of gamma-rays at cosmological distances. The energy density of GRBs in the universe is capable of explaining the measured UHECR flux, making them promising UHECR sources. Interactions between UHECRs and the prompt gamma-ray emission of a GRB would produce neutrinos that would be detected in coincidence with the GRB’s gamma-ray emission. The IceCube Neutrino Observatory can be used to search for these neutrinos in coincidence with GRBs, detecting neutrinos through the Cherenkov radiation emitted by secondary charged particles produced in neutrino interactions in the South Pole glacial ice. Restricting these searches to be in coincidence with GRB gamma-ray emis- sion, analyses can be performed with very little atmospheric background. Previous searches have focused on detecting muon tracks from muon neutrino interactions fromthe Northern Hemisphere, where the Earth shields IceCube’s primary background of atmospheric muons, or spherical cascade events from neutrinos of all flavors from the entire sky, with no compelling neutrino signal found. Neutrino searches from GRBs with IceCube have been extended to a search for muon tracks in the Southern Hemisphere in coincidence with 664 GRBs over five years of IceCube data in this dissertation. Though this region of the sky contains IceCube’s primary background of atmospheric muons, it is also where IceCube is most sensitive to neutrinos at the very highest energies as Earth absorption in the Northern Hemisphere becomes relevant. As previous neutrino searches have strongly constrained neutrino production in GRBs, a new per-GRB analysis is introduced for the first time to discover neutrinos in coincidence with possibly rare neutrino-bright GRBs. A stacked analysis is also performed to discover a weak neutrino signal distributed over many GRBs. Results of this search are found to be consistent with atmospheric muon backgrounds. Combining this result with previously published searches for muon neutrino tracks in the Northern Hemisphere, cascade event searches over the entire sky, and an extension of the Northern Hemisphere track search in three additional years of IceCube data that is consistent with atmospheric backgrounds, the most stringent limits yet can be placed on prompt neutrino production in GRBs, which increasingly disfavor GRBs as primary sources of UHECRs in current GRB models.Item A Search For Muon Neutrinos Coincident With Gamma-Ray Bursts Using IceCube(2015) Richman, Michael David; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present constraints derived from a search of four years of IceCube data for a prompt neutrino flux from gamma-ray bursts (GRBs). A single low-significance neutrino was found in coincidence with one of the 506 observed bursts, consistent with the expectation from atmospheric backgrounds. Although GRBs have been proposed as candidate sources for ultra-high energy cosmic rays, our limits on the neutrino flux disfavor much of the parameter space for the latest models. We also find that no more than ~1% of the recently observed astrophysical neutrino flux consists of prompt emission from GRBs that are potentially observable by existing satellites. These results currently represent world-leading constraints on a prompt neutrino flux from GRBs. In this thesis, we also introduce an original machine learning software package called pybdt. This implementation is now the de facto standard tool for machine-learning-based classification in IceCube analyses. Finally, we describe an extension of the unbinned likelihood method used in past searches to allow for the combination of data from different detector configurations with different background characteristics in the calculation of model constraints.Item 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.Item A Search for Relativistic Magnetic Monopoles with the IceCube 22-String Detector(2011) Christy, Brian John; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic monopoles are particles which act as a source for divergent magnetic fields, equivalent to a proton's electric field. Beyond simply adding the final symmetry to Maxwell's equations, their existence would solve numerous outstanding problems in the particle physics community. However, no conclusive evidence for their existence has been found. Magnetic monopoles possess many unique characteristics that allow for detection from a variety of experimental methods. One property is the large scaling of the Cherenkov radiation (∼ 8300) compared to electrically charged particles. Magnetic monopoles are postulated to be extremely heavy (∼ 104−1017 GeV). However, they would be topologically stable and accelerated via magnetic field lines throughout the universe, potentially reaching energies ∼ 1015 GeV. Therefore, searches for relativistic magnetic monopoles incident on Earth are an important piece to the overall experimental search. The IceCube neutrino observatory, located at the South Pole, offers a novel environment to search for these particles. IceCube is a km37 GeV, E & 1011 GeV) can travel completely through the Earth while remaining relativistic. This dissertation details the first search performed for these relativistic magnetic monopoles with IceCube data. The data is from 2007, when IceCube operated as a partially completed detector with an instrumented volume of ∼0.2 km3. It considers monopoles at four discrete speeds: β = 0.76, 0.8, 0.9, 0.995, ranging from just above the Cherenkov threshold in ice to a boost factor of 10. Discrimination between a potential magnetic monopole signal and background is achieved by considering the brightness and direction of the event. After an initial search revealed deficiencies in the simulated background model, a more conservative analysis produces limits that are ∼ 10 x better than previous searches. The final limits are then transformed to be a limit on an isotropic flux at the Earth's surface, due to the dependence on direction to the overall sensitivity of the analysis.Item A search for muon neutrinos coincident with Gamma-ray Bursts with the IceCube 59-String detector(2011) Redl, Peter Christian; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gamma-Ray Bursts (GRBs) are believed to be prime candidates to produce the cosmic ray flux above 10^18 eV. Cosmic rays are deflected by galactic and inter-galactic magnetic fields and do not point back to their source, therefore cosmic ray observations cannot confirm or rule out GRBs as a source. Leading theories predict that if GRBs are indeed responsible for the highest energy cosmic rays, then they would produce a detectable TeV-scale neutrino flux in a km^3 sized neutrino detector. Neutrinos are not deflected by magnetic fields and point back to their source, making it possible to correlate a neutrino flux with its source. The detection of a neutrino flux from GRBs would be strong evidence that GRBs are a source of the highest energy cosmic rays. IceCube is the first km^3 sized neutrino detector in the world and is therefore sensitive to the predicted TeV neutrino flux from GRBs. The finished detector consists of 5160 light-sensitive Digital Optical Modules (DOM) arranged on 86 Strings. There are 60 DOMs on a single string deployed at depths between 1450 and 2450 meters below the surface. The first IceCube String was deployed during the South Pole summer of 2004-2005 with construction of the IceCube detector finishing during the austral summer of 2011. The results presented here are from the 59-string detector, which operated from May 2009 to May 2010. IceCube is able to detect charged particles moving through its instrumented volume near the speed of light by detecting the Cherenkov light given off by those charged particles. Muon and anti-muon neutrinos produce secondary muons if they interact with a nucleon. If this interaction happens in or near the instrumented volume IceCube can detect those secondary muons. By searching for a neutrino signal coincident in time and space with satellite detected gamma rays from GRBs, the analysis presented here pushes the sensitivity for neutrinos from GRBs to 0.46 times the theoretically predicted neutrino flux. The result is combined with the previous search and a combined 90% upper limit of 0.22 times the theoretical predicted flux is set. The implication of this stringent limit on the model is discussed and future IceCube sensitivities are outlined. IceCube is the largest neutrino detector in the world and with this result has entered the era of neutrino astrophysics by constraining long standing astrophysical neutrino production models.Item A Search for Muon Neutrinos from Gamma-Ray Bursts wih the IceCube 22-String Detector(2009) Roth, A Philip; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two searches are conducted for muon neutrinos from Gamma-Ray Bursts (GRBs) using the IceCube detector. Gamma-Ray Bursts are brief and transient emissions of keV/MeV radiation occuring with a rate of a few per day uniformly in the sky. Swift and other satellites of the Third Interplanetary Network (IPN3) detect these GRBs and send notices out via the GRB Coordinate Network (GCN). The fireball model describing the physics of GRBs predicts the emission of muon neutrinos from these bursts. IceCube is a cubic kilometer neutrino detector buried in the deep antarctic ice at the South Pole that can be used to find these prediceted but still unobserved neutrinos. It is sensitive to them by detecting Cherenkov light from secondary muons produced when the neutrinos interact in or near the instrumented volume. The construction of IceCube has been underway since the austral summer of 2004-2005 and will continue until 2011. The growing IceCube detector will soon be sensitivite to the high energy neutrino emission from GRBs that is predicted by the fireball model. A blind and triggered search of the 22-string IceCube data for this neutrino emission was conducted. The principal background to the observation of neutrinos in IceCube is muons generated in cosmic-ray air-showers in the atmosphere above the detector. Atmospheric neutrinos make up a separate irreducible background to the detection of extraterrestrial neutrinos. A binned stacked search of 41 bursts occuring in the northern hemisphere greatly reduces the muon background by looking for tracks moving up through the detector. The atmospheric neutrino background is greatly reduced by the temporal constraints of the search, making it effectively background free. 40 individual unbinned searches of bursts occuring in the southern hemisphere extend IceCube's sensitivity to the higher background regions above the horizon. No significant excesses over background expectations are found in either search. A 90% confidence upper limit on the neutrino fluence from northern hemisphere bursts is set at 6.52 x 10-3 erg cm-2 with 90% of the expected signal between 87.9 TeV and 10.4 PeV.