Performance studies of an all-layer monolithic pixel vertex detector for the Belle II upgrade.

The Belle II experiment at the SuperKEKB $e^+e^-$ collider has started developing an upgrade program for the time frame of 2026--2027 to improve the detector performance and robustness against beam-induced backgrounds. To replace the current pixel-and-strips VerteX Detector (VXD), the VTX concept has been developed: an all-pixel detector based on Depleted Monolithic Active Pixel Systems (DMAPS) organized in five, or possibly more, barrel layers. To optimize the VTX design, and to compare it to the current VXD, a full-simulation framework has been developed and integrated with the standard Geant4-based Belle II simulation, allowing to directly compare different layouts. This communication will present the VTX detector concept, the simulation framework, and the results obtained from the simulation of benchmark physics channels that include $D$ mesons from $B$ decays. A significant improvement (in terms of reconstruction efficiency, as well as impact parameter and vertex resolution) will be shown, with respect to the current VXD. Also, it will be shown how this simulation work forms the basis for further optimization of the VTX design.