Performance of the Electromagnetic Pixel Calorimeter Prototype EPICAL-2

The first evaluation of an ultra-high granularity digital electromagnetic calorimeter prototype using 1.0–5.8 GeV/c electrons is presented. The 25 × 10$^{6}$ pixel detector consists of 24 layers of ALPIDE CMOS MAPS sensors, with a pitch of around 30 μm, and has a depth of almost 20 radiation lengths of tungsten absorber. Ultra-thin cables allow for a very compact design. The properties that are critical for physics studies are measured: electromagnetic shower response, energy resolution and linearity. The stochastic energy resolution is comparable with the state-of-the art resolution for a Si-W calorimeter, with data described well by a simulation model using Geant4 and Allpix$^{2}$. The performance achieved makes this technology a good candidate for use in the ALICE FoCal upgrade, and in general demonstrates the strong potential for future applications in high-energy physics.