Physics of the ALICE Forward Calorimeter upgrade

The ALICE Collaboration proposes to instrument the existing ALICE detector with a forward calorimeter system (FoCal), planned to take data during LHC Run 4 (2029$-$2032). The FoCal detector is a highly-granular Si+W electromagnetic calorimeter combined with a conventional sampling hadronic calorimeter, covering the pseudorapidity interval of $3.4<\eta<5.8$. The FoCal design is optimized to measure isolated photons at most forward rapidity for $p_{\rm T}\gtrsim4$ GeV/$c$. In this note we discuss the scientific potential of FoCal, which will enable broad exploration of gluon dynamics and non-linear QCD evolution at the smallest values of Bjorken $x$ accessible at any current or near-future facility world-wide. FoCal will measure theoretically well-motivated observables in pp and p$-$Pb collisions which are sensitive to the gluon distribution at small $x$ at low to moderate $Q^2$, based on isolated photon, neutral meson, and jet production and correlations in hadronic collisions, and the measurement of vector meson photoproduction in ultra-peripheral collisions. These FoCal measurements will provide incisive tests of the universality of linear and non-linear QCD evolution in different collision systems over an unprecedented kinematic range, in particular when combined with the comprehensive experimental program at the EIC and other forward measurements at RHIC and the LHC. FoCal will also carry out measurements at very forward rapidity in Pb$-$Pb collisions, enabling novel probes of the Quark-Gluon Plasma based on jet quenching phenomena and long-range correlations of neutral pions, jets, and photons.