First Operation of A Liquid Argon Time Projection Chamber (LArTPC) in the Stratosphere as an Engineering Gamma-Ray and AntiMatter Survey (GRAMS) Balloon Flight (eGRAMS)

Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite experiment utilizing a Liquid Argon Time Projection Chamber (LArTPC), to simultaneously target astrophysical observations of cosmic MeV gamma rays and conduct an indirect dark matter search using antimatter. Although LArTPCs are widely used in particle physics experiments, they have never been operated at balloon altitudes. An engineering balloon flight with a small-scale LArTPC (eGRAMS) was conducted on July 27, 2023, to establish a system for safely operating an LArTPC at balloon altitudes and to obtain cosmic-ray data from the LArTPC. The flight was launched from the Japan Aerospace Exploration Agency’s (JAXA) Taiki Aerospace Research Field in Hokkaido, Japan. The total flight duration was 3 hours and 12 minutes, including a level flight of 44 minutes at a maximum altitude of 28.9 km. The flight system was landed on the sea and successfully recovered. The LArTPC was successfully operated throughout the flight, and about 0.5 million events of cosmic-ray data including muons, protons, and Compton scattering gamma-ray candidates were collected. This pioneering flight demonstrates the feasibility of operating an LArTPC in high-altitude environments, paving the way for future GRAMS missions and advancing our capabilities in MeV gamma-ray astronomy and dark matter research.