First step towards a new proton decay experiment in ice

Grand Unified Theories (GUTs) predict a finite lifetime for the proton. The most recent limit, reported by Super-Kamiokande for a 172.8 kTon×year exposure, on the proton decay partial lifetime (p ? π⁰ + e ⁺) corresponds to 1.01 × 10³⁴ years. In the supersymmetric extensions of SU(5), the lifetime of the proton is expected to be lower than 10³⁶ years. To reach 10³⁶ years sensitivity to proton decay requires a detector with a volume on the megaton scale sensitive to sub-GeV energy. Where and how such a detector might ever be realized remains an open challenge. Installation of massive detectors underground is presently a technological challenge with costly excavation, engineering and installation. We consider here the ice cap at the South Pole which might provide an alternative scenario for a megaton ring imaging Cherenkov experiment in the search for proton decay. The ice, studied by the IceCube Collaboration, is measured to be extremely pure and transparent. Further, IceCube has demonstrated the ability to instrument a detector volume at the gigaton scale on-schedule and on-budget. The 86 strings of IceCube photosensors provide sensitivity to particle interactions with energies between tens of GeV up to extremely high energies. Given the success of the IceCube project in instrumenting the world's largest Cherenkov neutrino detector, and the DeepCore sub-array to extend the reach to low-GeV physics, it seems reasonable to consider the question if the same principle of IceCube, using the Cherenkov medium as the detector support infrastructure, could provide a cost effective and simplified path to instrument megaton scale detectors with sufficient photocathode area to permit a viable proton decay experiment sensitive to 10³⁶ year lifetime and beyond. In this paper we present the very first steps of a developing design study for a proton decay detector to be potentially deployed in the center of IceCube-DeepCore, based on Geant4 and the IceCube software with realistic optical properties of the glacial ice.