Improved Gamma-Hadron Separation for PeV Photon Searches with IceCube

IceTop, the km² surface array of the IceCube Neutrino Observatory at the South Pole, is sensitive to air showers of all primary particles, including gamma rays. In particular, in the PeV energy range, the combination of IceTop and IceCube’s deep optical detector provides excellent gamma-hadron separation. Almost all air showers induced by cosmic-ray protons and heavier nuclei in this energy range contain high-energy muons detectable by the deep detector, while most photon-induced air showers do not. Therefore, IceCube’s deep detector can be used to strongly suppress hadronic background in photon searches. Furthermore, the lateral distribution of the air-shower signal in IceTop provides additional gamma-hadron separation. In the PeV energy range, the gamma-hadron separation achieved is better than 10⁻³: air-shower events measured with IceCube are suppressed more than 1000 times stronger than photon-induced showers of the same energy simulated with Sibyll 2.3d. This improved gamma-hadron separation in combination with an extension of the energy range to lower energies provides discovery potential for future searches for PeV photon sources in IceCube’s field of view.