The MAPS prototype of the proposed ALICE Forward Calorimeter (FoCal) is the highest granularity electromagnetic calorimeter, with 39 million pixels of $30 times 30 mu$m$^{2}$. Particle showers can be studied with unprecedented detail with this prototype. Electromagnetic showers at energies between 2 GeV and 244 GeV have been studied and compared to Geant4 simulations. Simulation models can be tested in more detail than ever before and the differences observed between FoCal data and Geant4 simulations illustrates that improvements in electromagnetic models are still possible.
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