ALICE Forward Calorimeter upgrade (FoCal): physics program and expected performance

FoCal is a high-granularity forward calorimeter to be installed as an ALICE upgrade during the LHC Long Shutdown 3 for data taking in Run 4. Covering $3.4 < \eta < 5.5$ with full azimuthal acceptance, it will probe QCD at Bjorken-$x$ values down to $\sim 10^{-6}$, where non-linear dynamics are expected to become significant. The detector features a compact silicon–tungsten electromagnetic calorimeter with pad and pixel readout for precise photon measurements, complemented by a hadronic section based on copper capillaries instrumented with scintillating fibers. This design enables measurements of key observables such as isolated photons, jets, ultra-peripheral photo-production of vector mesons, and angular correlations at forward rapidity. Following the completion of its Technical Design Report, the FoCal project is moving into the production phase for installation in 2028. We present an overview of the FoCal physics programme, the detector design, and the expected performance based on recent test-beam results from small-scale prototypes.