Search for astrophysical neutrino sources using IceCube low-energy neutrinos

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Chen, Chujie
Taboada, Ignacio
Otte, A. Nepomuk
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Astrophysical neutrinos can escape from radiation-opaque regions and act as messengers, allowing us to explore some of the most extreme environments in the universe. The IceCube Neutrino Observatory is designed to observe the universe via neutrinos from deep within the Antarctic ice. More than 300 people from 56 institutions in 14 countries have formed the IceCube collaboration to work on different aspects of research using the detector. Much of the work in this thesis can be attributed to collaborators in IceCube. My original work focuses on data analysis after event selection. DeepCore, a densely instrumented sub-detector of IceCube, extends IceCube’s energy reach down to about 10 GeV, enabling the search for astrophysical transient neutrino sources. I aim to utilize a newly developed event selection and dataset for both a triggered and an untriggered all-sky search for potential astrophysical neutrino origins. For the first analysis presented in this thesis, gamma-ray bursts (GRBs) are proposed as neutrino emitters. Despite predictions for multi-GeV neutrino emission from collisions of drifting neutrons in sub-photospheric GRB emissions or particle acceleration in internal shocks, the energy range studied in this study had received little attention. I present the results of a search for tens of GeV neutrinos in coincidence with GRBs using IceCube-DeepCore. This analysis was conducted on eight years of IceCube-DeepCore data and in coincidence with 2,268 GRBs. I first look for correlations between neutrino events and GRBs using six overlapping time windows centered on the prompt phase of each GRB. These time windows range from $\pm 5$ s to $\pm 250$ s. Individual $p$-values are combined using a binomial test to look for a potential subgroup of GRBs that could be neutrino sources by identifying an excess of GRBs with $p$-values less than a certain threshold. There is no evidence of neutrino emission in either search. Fermi GBM bn140807500 is the most significant individual burst, with a $p$-value of 0.01 corrected for testing 2268 GRBs. The binomial test does not identify any additional GRBs and yields a $p$-value of 0.65 when all thresholds are considered. This work marks the first attempt using IceCube low-energy neutrinos to search for GRBs. The ongoing IceCube Upgrade is expected to improve angular resolution and energy reconstruction of events, improving the sensitivity of future studies. Astrophysical neutrino sources that cannot be observed using multi-wavelength photons, such as choked GRBs, are candidate targets for the second untriggered analysis shown in this thesis. I expand the previous DeepCore transient half-sky search to an all-sky time-dependent search using the same newly developed dataset used in the above analysis, focusing only on short-lived sources with a flare duration of $10^2$ - $10^5$ seconds. This thesis shows all-sky sensitivities to potential neutrino transients with energies ranging from 10 GeV to 300 GeV. I demonstrate that DeepCore can be used to conduct reliable all-sky searches for short-lived astrophysical sources. IceCube-DeepCore is expected to be used for all-sky searches on larger timescale sources with future extensions to the standard likelihood method explored in this thesis.
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