Letter of Intent: A Forward Calorimeter (FoCal) in the ALICE experiment

We propose a forward electromagnetic and hadronic calorimeter (FoCal) as an upgrade to the ALICE experiment, to be installed during LS3 for data-taking in 2027–2029 at the LHC. The FoCal is a highly granular Si+W electromagnetic calorimeter combined with a conventional sampling hadronic calorimeter covering pseudorapidities of 3.4 < $\eta$ < 5.8. The FoCal provides unique capabilities to measure small-x gluon distributions via prompt photon production and will significantly enhance the scope of ALICE for inclusive and correlation measurements with mesons, photons, and jets to explore the dynamics of hadronic matter at small x down to ~$10^{ -6}$