Theses and Dissertations from UMD
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Item New Messengers & New Physics: A Survey of the High-energy Universe(2023) Crnogorcevic, Milena; Ricotti, Massimo; Caputo, Regina; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying the origins of the high-energy emission in the Universe can profoundly affect our fundamental understanding of the cosmic origin and its evolution at the most extreme scales. In this dissertation, I explore the high-energy observations of different astrophysical systems to inform our understanding of the theoretical frameworks used to describe them. I harness the current multimessenger infrastructure to investigate questions ranging from new physics and transient astronomy to compact objects and extended emission in the gamma-ray, gravitational-wave, and neutrino skies. The focus in the first part of this dissertation is on utilizing the Fermi Large Area Telescope (LAT) low-energy (LLE) technique to search for the light axion-like-particle (ALP) within the MeV gamma-ray emission of long gamma-ray bursts (GRBs). We perform a data-driven sensitivity analysis to determine distances for which detection of an ALP signal is possible with the LLE technique, which, in contrast to the standard LAT analysis, allows for a larger effective area for energies down to 30 MeV. Assuming an ALP mass $m_a \lesssim 10^{-10}$~eV and ALP-photon coupling $g_{a\gamma} = 5.3\times 10^{-12}$ GeV$^{-1}$, we find that the distance limit ranges from $\sim\!0.5$ to $\sim\!10$~Mpc. We demonstrate that the sensitivity of the LLE technique to detecting light ALPs is comparable to the standard LAT analysis, making it an excellent complementary---yet independent---way to search for ALPs with \textit{Fermi}. Next, we select a candidate sample of twenty-four GRBs and conduct a model comparison analysis in which we consider different GRB spectral models with and without an ALP signal component. We find that including an ALP contribution does not result in any statistically significant improvement of the fits to the data. Motivated by the delay between the ALP emission time and the time of the jet break-out associated with its ordinary long-GRB emission, we conduct a novel search for ALPs within time windows that precede the main-episode gamma-ray emission of a long GRB, focusing on the sample of sources with known precursor emission detected with LAT and LLE. We report no statistically significant detection of ALPs within the GRB precursor emission and discuss the parts of the ALP parameter space probed with this method. Multimessenger astronomy is at the heart of the remainder of this dissertation. First, I present a follow-up search for excess emission of X-rays with the Swift Burst Alert Telescope (Swift-BAT) and that of gamma rays with the Fermi Gamma-ray Burst Monitor (Fermi-GBM), in spatial and temporal correspondence to gravitational-wave events reported by the LIGO/Virgo/Kagra (LVK) Collaboration. In collaboration with the Fermi-GBM Team, we combine the observations from these two instruments to determine whether there is any statistically significant excess emission around the given gravitational-wave trigger. We report no new joint detections but present the joint flux upper limits. Finally, I present the results of the cross-correlation studies between the unresolved Fermi-LAT gamma-ray and the IceCube neutrino skies. We report no positive cross-correlation in the real-data sky maps. We then combine simulation and observation techniques to place upper limits on the fraction of neutrinos produced in proton-proton or proton-gamma interactions that occur in blazars. Assuming all gamma rays from unresolved blazars are produced from neutral pions via proton-proton interactions, we find that---for energies above 10~GeV---up to 60 % of the unresolved blazar population may contribute to the diffuse neutrino background (the fraction is 30 % for proton-gamma interactions). We also include predictions for the improved sensitivity considering 20 years of IceCube data.Item SEARCH FOR GAMMA-RAY COUNTERPARTS OF GRAVITATIONAL WAVE EVENTS AND OTHER TRANSIENT SIGNALS WITH HAWC(2019) Martinez Castellanos, Israel; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source. The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales. We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger. We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.Item Neutrino Mass and Proton Lifetime in a Realistic Supersymmetric SO(10) Model(2015) Severson, Matthew Michael; Mohapatra, Rabindra N; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents a complete analysis of fermion fitting and proton decay in a supersymmetric $SO(10)$ model previously suggested by Dutta, Mimura, and Mohapatra. A key question in any grand unified theory is whether it satisfies the stringent experimental lower limits on the partial lifetimes of the proton. In more generic models, substantial fine-tuning is required among GUT-scale parameters to satisfy the limits. In the proposed model, the {\bf 10}, $\overline{\bf{126}}$, and {\bf 120} Yukawa couplings contributing to fermion masses have restricted textures intended to give favorable results for proton lifetime, while still giving rise to a realistic fermion sector, without the need for fine-tuning, even for large $\tan\beta$, and for either type-I or type-II dominance in the neutrino mass matrix. In this thesis, I investigate the above hypothesis at a strict numerical level of scrutiny; I obtain a valid fit for the entire fermion sector for both types of seesaw dominance, including $\theta_{13}$ in good agreement with the most recent data. For the case with type-II seesaw, I find that, using the Yukawa couplings fixed by the successful fermion sector fit, proton partial lifetime limits are readily satisfied for all but one of the pertinent decay modes for nearly arbitrary values of the triplet-Higgs mixing parameters, with the $K^+ \bar\nu$ mode requiring a minor ${\cal O}(10^{-1})$ cancellation in order to satisfy its limit. I also find a maximum partial lifetime for that mode of $\tau(K^+ \bar\nu) \sim 10^{36}$\,years. For the type-I seesaw case, I find that $K^+ \bar\nu$ decay mode is satisfied for any values of the triplet mixing parameters giving no major enhancement, and all other modes are easily satisfied for arbitrary mixing values; I also find a maximum partial lifetime for $K^+ \bar\nu$ of nearly $10^{38}$\,years, which is largely sub-dominant to gauge boson decay channels.Item Search for Quantum Gravity with IceCube and High Energy Atmospheric Neutrinos(2010) Huelsnitz, Warren; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)IceCube is a cubic-kilometer neutrino telescope nearing completion in the South Pole Ice. Designed to detect astrophysical neutrinos from 100 GeV to about an EeV, it will contribute to the fields of high energy astrophysics, particle physics, and neutrino physics. This analysis looks at the flux of atmospheric neutrinos detected by IceCube while it operated in a partially-completed, 40-string configuration, from April 2008 to May 2009. From this data set, a sample of about 20,000 up-going atmospheric muon neutrino events with negligible background was extracted using Boosted Decision Trees. A discrete Fourier transform method was used to constrain a directional asymmetry in right ascension. Constraints on certain interaction coefficients from the Standard Model Extension were improved by three orders of magnitude, relative to prior experiments. The event sample was also used to unfold the atmospheric neutrino spectrum at its point of origin, and seasonal and systematic variations in the atmospheric muon neutrino flux were studied. A likelihood method was developed to constrain perturbations to the energy and zenith angle dependence of the atmospheric muon neutrino flux that could be due to Lorentz-violating oscillations or decoherence of neutrino flavor. Such deviations could be a signature of quantum gravity in the neutrino sector. The impact of systematic uncertainties in the neutrino flux and in the detector response on such a likelihood analysis were examined. Systematic uncertainties that need to be reduced in order to use a two-dimensional likelihood analysis to constrain phenomenological models for Lorentz or CPT violating neutrino oscillations were identified.Item The upside of minimal left-right supersymmetric seesaw in deflected anomaly mediation(2008-05-30) Spinner, Sogee; Mohapatra, Rabindra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The state of the standard model of particle physics is reviewed focusing on two of it's major issues: the hierarchy problem and its inconsistency with observed neutrino masses. Supersymmetry, an elegant solution to the former, and the seesaw mechanism in left-right models, a natural solution to the latter, are then introduced. The work then focuses on a specific supersymmetric left-right models, which has an additional discrete symmetry allowing a prediction of the seesaw scale at around 1011 GeV--consistent with neutrino oscillation data. It also solves the μ problem and guarantees automatic R-parity conservation and a pair of light doubly-charged Higgses which can be searched for at the LHC. This model has interesting properties in the context of anomaly mediated supersymmetry breaking (AMSB). After a brief introduction to this topic, it is shown that this model is an instance of the Pomarol Rattazzi model of deflected AMSB. The tachyonic slepton problem of AMSB is solved in a combination of two ways: the right-handed sleptons are saved by their couplings to the low energy doubly-charged fields while the left-handed sleptons receive positive contributions from the partially decoupled D-terms. The resulting phenomenology is similar to that of mimimal AMSB due to the gaugino spectrum; however, same generation mass differences in the sfermion sector are much larger than that of mAMSB and the right-handed selectron can be as massive as the squarks. Finally, this model also contains a mechanism for solving the EWSB problem of AMSB and a dark matter candidate.