Extension of the IceTop energy spectrum to 250 TeV and application of the constant intensity cut method to IceTop data

Koirala, Ramesh
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University of Delaware
The all-particle cosmic ray energy spectrum is an important tool to study acceleration and propagation mechanisms of cosmic rays. The main goal of this dissertation is to lower the energy threshold of the all-particle cosmic ray energy spectrum to overlap with the energy spectra from direct measurements. Previous studies with IceTop/IceCube have measured the spectrum from PeV to EeV. Those measurements used events with at least five-stations hit. In this study, events with two or more stations are used to measure the energy spectrum from 250 TeV to 10 PeV. ☐ In addition to lowering the threshold of the cosmic-ray energy spectrum, this thesis also includes a preliminary application of the constant intensity cut method to IceTop data. This method has the potential to improve the reach of IceTop in the EeV range. ☐ To lower the energy threshold of IceTop, a new trigger and filter is developed and is implemented since May 20, 2016. The new trigger and filter uses 4 pairs of nearby infill stations where the separation between stations in each pair is less than 50 m. Data from the entire array are collected for events in which at least one infill pair is hit. Data to calculate the energy spectrum are collected from May 2016 to April 2017 with a livetime (duration) of 330.43 days. A total of 7,442,086 events is used after all quality cuts. ☐ The constant intensity cut (CIC) method is a widely used method to calculate the cosmic ray energy spectrum. The method increases statistics in energy spectrum analysis by including air showers from higher zenith angle. In this dissertation, CIC is used in particular to study atmospheric attenuation and to measure indirectly the energy-dependent average shower maximum ( <Xmax>). The use of CIC method to measure the all-particle cosmic ray spectrum with IceTop in the range of PeV to EeV is discussed. ☐ This dissertation is divided into eight chapters. Chapter 1 is the introduction. Chapter 2 details the IceCube Neutrino Observatory. It describes the Digital Optical Module (DOM) and discusses how DAQ system, trigger/filter, and data transfer operate. IceTop, a surface component of the IceCube Neutrino Observatory, uses various calibration techniques. These calibrations are also discussed in this chapter. Chapter 3 describes the new two-station trigger and filter implemented to collect low energy cosmic rays. Chapter 4 discusses the simulation used for low energy spectrum analysis. Chapter 5 describes the reconstruction of air showers collected using two-station trigger/filter based on machine learning. Chapter 6 reports the result of all-particle cosmic ray spectrum using two-station events. The energy threshold using two-station events is lowered down to ~250 TeV. Chapter 7 details the constant intensity cut (CIC) method. This method is studied to increase the zenith range for energy spectrum and to use a zenith independent energy proxy for all-particle energy spectrum calculations. The final chapter, Chapter 8, summarizes the results and discusses possible future work.