Observation of sub-GeV atmospheric gamma rays on GRAINE 2018 balloon experiment and comparison with HKKM calculation

We report a precise measurement of the sub-GeV atmospheric gamma-ray spectrum at balloon altitude on GRAINE 2018 experiment, and comparisons with the predictions calculated by the latest HKKM, which is widely known as a model for atmospheric neutrino flux calculation. Understanding the interactions between cosmic rays and atmospheric nuclei is important for accurate atmospheric neutrino flux calculations. Observation data of sub-GeVatmospheric gamma rays at balloon altitudes are useful for verifying such hadronic interaction models and pion productions in the lowenergy region. In April 2018, we conducted a balloon experiment (GRAINE 2018) in Australia with the aim of detecting and imaging the celestial gamma-ray sources with the nuclear emulsion telescope. Following flight data analysis, we derived an atmospheric gamma-ray spectrum in 0.1-1 GeV region at altitudes of similar to 36 km (residual depth similar to 4 g / cm(2)). The flux around the 1 GeV region is in good agreement with the HKKM prediction and smoothly connects to the multi-GeV observations of past balloon experiments. On the other hand, the flux around 0.1 GeV shows a discrepancy with the prediction. In this presentation, the balloon experiment, flight data analysis, and observation results are described.