Physics program and performance of the ALICE Forward Calorimeter upgrade (FoCal)

The FoCal is a high-granularity forward calorimeter to be installed as an ALICE upgrade subsystem during the LHC Long Shutdown 3 and take data during the LHC Run 4. The FoCal detector, covering a pseudorapidity interval of $3.2 < \eta < 5.8$, extends the ALICE physics program with the capability to investigate gluon Parton Distribution Functions (PDFs) in the low-$x$ regime, down to $x \approx 10^{-6}$. FoCal measurements will provide experimental constraints for PDFs in a region of phasespace where experimental data is scarce, as well as enable the study of non-linear QCD effects like gluon saturation. The FoCal detector consists of two components. The highly-granular Si+W electromagnetic calorimeter (FoCal-E) with pad and pixel longitudinal and transverse segmented readout layers provides high spatial resolution for discriminating between isolated photons and decay photon pairs. The hadronic calorimeter (FoCal-H) is constructed from copper capillary tubes filled with scintillator fibers and is used for isolation energy measurement and jets.With this detector design, FoCal is capable of measuring direct photons, jets, and the photo-production of vector mesons such as the $J/ \psi$ in p-Pb and Pb-Pb ultra-peripheral collisions. In addition, correlations of different probes can be studied, including $\gamma$-jet, jet-jet and $\pi^0$-$\pi^0$ correlations. These measurements will place stringent constraints to various theoretical models incorporating non-linear QCD effects. This contribution gives an overview of the FoCal physics program. Also, recent experimental results of ever-improving prototypes of the detector, which were tested at the Test Beam facilities of CERN in the years 2021-2023, as well as simulation studies showcasing the robustness of the detector design and its physics potential are presented.