Expression of Interest for an experiment to study charm production with proton and heavy ion beams

We propose an experiment to perform a systematic study of charmonia production in heavy ion collisions at SPS. Taking advantage of significant advances in electromagnetic calorimetry, the measurement of low energy photons from chi_c decays should now be achievable. Together with recent measurements made at RHIC and at LHC on J/psi and upsilon production, such a measurement will offer the opportunity to use quarkonia as a direct test of phase transition and lattice QCD calculations. In a one month data taking one can expect the collection of thousands of chi_c. This new and dedicated experiment is designed to also study, under optimal conditions, Cold Nuclear Matter (CNM) effects in a larger rapidity range than previously explored by the NA50/NA60 experiments. This measurement of nuclear effects in absence of Quark Gluon Plasma formation will provide a clear and unambiguous reference for the study of Hot and Dense Matter (HDM) effects, a reference which is today needed to deduce an unambiguous interpretation of the results already obtained.