Speaker
Description
The success of the Belle II experiment relies for a large part on the very high instantaneous luminosity, close to 8x10^35 cm^-2.s^-1, expected from the SuperKEKB collider. The beam conditions to reach such luminosity levels generate a large rate of background particles in the inner detection layers of Belle II, which exceeds by far the rate of particles stemming from elementary collisions. This beam-induced background creates stringent constraints on the vertex detector, in addition to the requirements coming from physics capability. The SuperKEKB accelerator and Belle II experiment have started full operation in 2019, establishing in 2020 a world record with an instantaneous luminosity of 2.4x10^34 cm^-2.s^-1. The current Belle II vertex detector (VXD), made of a combination of DEPFET pixel sensors and Double-Sided Silicon Strip Detectors (DSSD), has been operating very satisfactorily. While efforts are still ongoing to mitigate beam-induced backgrounds, current prospects for the related occupancy rates in the VXD layers at full luminosity fall close to the acceptable limits of the employed technologies. To reach the nominal luminosity parts of SuperKEKB like the final focusing magnets will be modified with a time frame currently predicted to be around Long Shutdown 2 in 2026. Thus, the Belle II collaboration is considering the possibility to install an upgraded VXD system on the same time scale. Such an upgrade should provide a sufficient safety factor with respect to the background rate expected at the nominal luminosity and possibly enhance performances for tracking and vertexing. Several technologies are under consideration for the upgrade. One approach consists in improving performances of the technologies present in Belle II: faster DEPFET sensors for innermost layers, thinner and more granular DSSDs for the remaining layers. New monolithic technologies for pixel sensors are also under discussion, namely SOI and CMOS. They offer a combination of granularity, speed, low material budget and radiation tolerance matching well Belle II requirements and could be exploited to design a fully pixelated VXD, also benefiting from significant developments made in recent years for other experiments. Following this last concept, both simplified and complete simulations have been conducted to evaluate tracking and vertexing performances with various geometries (e.g. number of layers, addition of disks) and technical specifications (e.g. granularity, speed). This talk will review the context of the proposed VXD upgrade in Belle II, providing some details of the existing technological proposals and discussing performance expectations from simulations.
