Injection-related beam loss in the high-energy ring of SuperKEKB

At SuperKEKB, the maximum achievable beam currents and the luminosity are projected to be constrained by beam injection in the near future. Recent attempts to squeeze the vertical beta function at the IP or increase beam currents have not yielded the expected improvements in luminosity, with poor injection performance in both rings being a major factor. The injection efficiencies are lower than half of the required value and large background signals are observed in the Belle II detector and other beam loss monitors. To avoid a sudden increase in background caused by injection, the Belle II trigger system is vetoed for several tens of milliseconds just after beam injection, leading to significant dead time. These injection issues are expected to become even more severe as luminosity increases, making beam injection one of the major bottleneck to achieving the target luminosity. To investigate and address these issues, detailed injection simulations for the high-energy ring (HER) were performed and compared with dedicated experimental measurements, providing valuable insights into injection-related beam loss mechanisms and offering practical strategies to improve injection efficiency while mitigating injection-induced backgrounds. This paper presents these findings together with their experimental validation.