FoCal: A highly granular digital calorimeter

This contribution discusses the recent progress in the development of a highly granular digital electromagnetic calorimeter proposed as an upgrade to the ALICE detector. This forward electromagnetic calorimeter (FoCal) must be able to discriminate decay photons from direct photons at very high energy, which requires extremely high granularity and a small Molière radius. A dedicated R&D; program is ongoing to develop the technology needed for such a high-granularity device. Within this program we have constructed a prototype of a digital electromagnetic calorimeter based on CMOS monolithic active pixel sensors (MAPS). This prototype has demonstrated the unique capabilities of such a highly granular digital calorimeter, providing unprecedented shower profile measurements and good linearity and energy resolution. The prototype was based on the MIMOSA chip, which is however not fast enough for application in a full detector at the LHC. As a next step, the ALPIDE chip developed for the ALICE Inner Tracker Upgrade is being investigated for performance with high occupancy. This contribution presents results from the current prototype, the performance of the ALPIDE and plans for the next prototype.