Search for PeV gamma rays and astrophysical neutrinos with icetop and icecube

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University of Delaware
The origin and acceleration process for cosmic rays more energetic than $10^{15}$~eV is not yet completely understood. Since cosmic rays lose directional information due to deflection in interstellar magnetic fields, neutral messengers such as gamma rays and neutrinos are essential to the identification of the accelerators. Measurement of diffuse gamma-ray and neutrino emission from the Galactic plane also provides valuable insight into the cosmic-ray diffusion mechanism. ☐ Located at South Pole, the IceCube Observatory is an astroparticle detector comprised of a cubic kilometer of instrumented ice and a square kilometer surface array, IceTop. IceTop can detect particle air showers from interactions of cosmic rays and gamma rays with energy higher than $10^{15}$~eV. The in-ice array detects TeV muons produced in air showers as well as neutrinos of atmospheric and astrophysical origin. This thesis presents the IceTop log-likelihood ratio (IT-LLHR), a new probabilistic data analysis approach towards the classification of air shower events detected by IceTop. ☐ A search for gamma rays from the direction of the Galactic plane was performed on five years of IceCube air shower data using IT-LLHR in conjunction with in-ice signals from TeV muons. No diffuse PeV emission from the visible part of the Galactic plane was found and an upper-limit on the angular-integrated flux of $2.61\times 10^{-19}$~\si{cm^{-2}\; s^{-1} \;TeV^{-1}} at 2~PeV was placed for an E$^{-3}$ spectral assumption. The IT-LLHR technique was also applied to calculate the efficiency of IceTop in rejecting air shower muons detected in-ice by successfully discriminating between shower-related hits and background hits on the surface. For down-going muon neutrinos with energy greater than 100 TeV and trajectory passing through IceTop, a $10^{-5}$ reduction in the atmospheric background was achieved.